Diagnosis and Classification of DiabetesMellitusAMERICAN DIABETES ASSOCIATION

DEFINITION ANDDESCRIPTION OF DIABETESMELLITUSdDiabetes is a group ofmetabolic diseases characterized by hy-perglycemia resulting from defects in in-sulin secretion, insulin action, or both.The chronic hyperglycemia of diabetes isassociated with long-term damage, dys-function, and failure of different organs,especially the eyes, kidneys, nerves, heart,and blood vessels.

Several pathogenic processes are in-volved in the development of diabetes.These range from autoimmune destruc-tion of the b-cells of the pancreas withconsequent insulin deficiency to abnor-malities that result in resistance to insulinaction. The basis of the abnormalities incarbohydrate, fat, and protein metabo-lism in diabetes is deficient action of in-sulin on target tissues. Deficient insulinaction results from inadequate insulin se-cretion and/or diminished tissue respon-ses to insulin at one or more points in thecomplex pathways of hormone action.Impairment of insulin secretion and de-fects in insulin action frequently coexist inthe same patient, and it is often unclearwhich abnormality, if either alone, is theprimary cause of the hyperglycemia.

Symptoms ofmarked hyperglycemia in-cludepolyuria,polydipsia,weight loss,some-times with polyphagia, and blurred vision.Impairment of growth and susceptibility tocertain infections may also accompanychronic hyperglycemia. Acute, life-threaten-ing consequences of uncontrolled diabetesare hyperglycemia with ketoacidosis or thenonketotic hyperosmolar syndrome.

Long-term complications of diabetesinclude retinopathy with potential lossof vision; nephropathy leading to renalfailure; peripheral neuropathy with riskof foot ulcers, amputations, and Charcotjoints; and autonomic neuropathy caus-ing gastrointestinal, genitourinary, and

cardiovascular symptoms and sexual dys-function. Patients with diabetes have anincreased incidence of atherosclerotic car-diovascular, peripheral arterial, and cere-brovascular disease. Hypertension andabnormalities of lipoprotein metabolismare often found in people with diabetes.

The vast majority of cases of diabetesfall into two broad etiopathogenetic cate-gories (discussed in greater detail below).In one category, type 1 diabetes, the causeis an absolute deficiency of insulin secre-tion. Individuals at increased risk of de-veloping this type of diabetes can often beidentified by serological evidence of anautoimmune pathologic process occurringin the pancreatic islets and by geneticmarkers. In the other, much more preva-lent category, type 2 diabetes, the cause is acombination of resistance to insulin actionand an inadequate compensatory insulinsecretory response. In the latter category, adegree of hyperglycemia sufficient to causepathologic and functional changes in var-ious target tissues, but without clinicalsymptoms, may be present for a longperiod of time before diabetes is detected.During this asymptomatic period, it ispossible to demonstrate an abnormality incarbohydrate metabolism by measurementof plasma glucose in the fasting state orafter a challenge with an oral glucose loador by A1C.

The degree of hyperglycemia (if any)may change over time, depending on theextent of the underlying disease process(Fig. 1). A disease process may be presentbut may not have progressed far enoughto cause hyperglycemia. The same diseaseprocess can cause impaired fasting glu-cose (IFG) and/or impaired glucose toler-ance (IGT) without fulfilling the criteriafor the diagnosis of diabetes. In some indi-viduals with diabetes, adequate glycemiccontrol can be achieved with weight reduc-tion, exercise, and/or oral glucose-lowering

agents. These individuals therefore donot require insulin. Other individualswho have some residual insulin secretionbut require exogenous insulin for ade-quate glycemic control can survive with-out it. Individuals with extensive b-celldestruction and therefore no residual in-sulin secretion require insulin for survival.The severity of the metabolic abnormalitycan progress, regress, or stay the same.Thus, the degree of hyperglycemia reflectsthe severity of the underlying metabolicprocess and its treatment more than thenature of the process itself.

CLASSIFICATION OFDIABETES MELLITUS ANDOTHER CATEGORIESOF GLUCOSEREGULATIONdAssigning a type ofdiabetes to an individual often dependson the circumstances present at the timeof diagnosis, and many diabetic individ-uals do not easily fit into a single class. Forexample, a person diagnosed with gesta-tional diabetes mellitus (GDM) may con-tinue to be hyperglycemic after deliveryand may be determined to have, in fact,type 2 diabetes. Alternatively, a personwho acquires diabetes because of largedoses of exogenous steroids may becomenormoglycemic once the glucocorticoidsare discontinued, but then may developdiabetes many years later after recurrentepisodes of pancreatitis. Another examplewould be a person treated with thiazideswho develops diabetes years later. Becausethiazides in themselves seldomcause severehyperglycemia, such individuals probablyhave type 2 diabetes that is exacerbated bythe drug. Thus, for the clinician andpatient,it is less important to label the particulartype of diabetes than it is to understand thepathogenesis of the hyperglycemia and totreat it effectively.

Type 1 diabetes (b-cell destruction,usually leading to absolute insulindeficiency)Immune-mediated diabetes. This formof diabetes, which accounts for only5–10% of those with diabetes, previouslyencompassed by the terms insulin-dependent diabetes or juvenile-onset di-abetes, results from a cellular-mediatedautoimmune destruction of the b-cells of

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Updated Fall 2012.DOI: 10.2337/dc13-S067© 2013 by the American Diabetes Association. Readers may use this article as long as the work is properly

cited, the use is educational and not for profit, and thework is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ for details.

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the pancreas. Markers of the immune de-struction of the b-cell include islet cell au-toantibodies, autoantibodies to insulin,autoantibodies toGAD (GAD65), and auto-antibodies to the tyrosine phosphatasesIA-2 and IA-2b. One and usually more ofthese autoantibodies are present in 85–90% of individuals when fasting hyper-glycemia is initially detected. Also, thedisease has strong HLA associations,with linkage to the DQA and DQB genes,and it is influenced by the DRB genes.These HLA-DR/DQ alleles can be eitherpredisposing or protective.

In this form of diabetes, the rate ofb-cell destruction is quite variable, beingrapid in some individuals (mainly infantsand children) and slow in others (mainlyadults). Some patients, particularly chil-dren and adolescents, may present withketoacidosis as the first manifestation ofthe disease. Others have modest fastinghyperglycemia that can rapidly changeto severe hyperglycemia and/or ketoaci-dosis in the presence of infection or otherstress. Still others, particularly adults,may retain residual b-cell function suffi-cient to prevent ketoacidosis for manyyears; such individuals eventually be-come dependent on insulin for survivaland are at risk for ketoacidosis. At thislatter stage of the disease, there is littleor no insulin secretion, as manifested bylow or undetectable levels of plasmaC-peptide. Immune-mediated diabetes

commonly occurs in childhood and ado-lescence, but it can occur at any age, evenin the 8th and 9th decades of life.

Autoimmune destruction of b-cellshas multiple genetic predispositions andis also related to environmental factorsthat are still poorly defined. Although pa-tients are rarely obese when they presentwith this type of diabetes, the presence ofobesity is not incompatible with the diag-nosis. These patients are also prone toother autoimmune disorders such asGraves’ disease, Hashimoto’s thyroiditis,Addison’s disease, vitiligo, celiac sprue,autoimmune hepatitis, myasthenia gravis,and pernicious anemia.Idiopathic diabetes. Some forms of type1 diabetes have no known etiologies.Some of these patients have permanentinsulinopenia and are prone to ketoaci-dosis, but have no evidence of autoim-munity. Although only a minority ofpatients with type 1 diabetes fall into thiscategory, of those who do, most are ofAfrican or Asian ancestry. Individualswith this form of diabetes suffer fromepisodic ketoacidosis and exhibit vary-ing degrees of insulin deficiency be-tween episodes. This form of diabetesis strongly inherited, lacks immunolog-ical evidence for b-cell autoimmunity,and is not HLA associated. An absoluterequirement for insulin replacementtherapy in affected patients may comeand go.

Type 2 diabetes (ranging frompredominantly insulin resistancewith relative insulin deficiency topredominantly an insulin secretorydefect with insulin resistance)This form of diabetes, which accounts for;90–95% of those with diabetes, previ-ously referred to as non–insulin-depen-dent diabetes, type 2 diabetes, or adult-onset diabetes, encompasses individualswho have insulin resistance and usuallyhave relative (rather than absolute) insu-lin deficiency At least initially, and oftenthroughout their lifetime, these individu-als do not need insulin treatment to sur-vive. There are probably many differentcauses of this form of diabetes. Althoughthe specific etiologies are not known, au-toimmune destruction of b-cells does notoccur, and patients do not have any of theother causes of diabetes listed above orbelow.

Most patients with this form of di-abetes are obese, and obesity itself causessome degree of insulin resistance. Patientswho are not obese by traditional weightcriteria may have an increased percentageof body fat distributed predominantly inthe abdominal region. Ketoacidosis sel-dom occurs spontaneously in this type ofdiabetes; when seen, it usually arises inassociation with the stress of anotherillness such as infection. This form ofdiabetes frequently goes undiagnosed formany years because the hyperglycemia

Figure 1dDisorders of glycemia: etiologic types and stages. *Even after presenting in ketoacidosis, these patients can briefly return to normo-glycemia without requiring continuous therapy (i.e., “honeymoon” remission); **in rare instances, patients in these categories (e.g., Vacor toxicity,type 1 diabetes presenting in pregnancy) may require insulin for survival.

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develops gradually and at earlier stages isoften not severe enough for the patient tonotice any of the classic symptoms ofdiabetes. Nevertheless, such patients areat increased risk of developing macro-vascular and microvascular complica-tions. Whereas patients with this form ofdiabetes may have insulin levels thatappear normal or elevated, the higherblood glucose levels in these diabeticpatients would be expected to result ineven higher insulin values had their b-cellfunction been normal. Thus, insulin se-cretion is defective in these patients andinsufficient to compensate for insulin re-sistance. Insulin resistance may improvewith weight reduction and/or pharmaco-logical treatment of hyperglycemia but isseldom restored to normal. The risk ofdeveloping this form of diabetes increaseswith age, obesity, and lack of physical ac-tivity. It occurs more frequently in womenwith prior GDM and in individuals withhypertension or dyslipidemia, and its fre-quency varies in different racial/ethnic sub-groups. It is often associated with a stronggenetic predisposition, more so than is theautoimmune form of type 1 diabetes. How-ever, the genetics of this form of diabetesare complex and not fully defined.

Other specific types of diabetesGenetic defects of the b-cell. Severalforms of diabetes are associated withmonogenetic defects in b-cell function.These forms of diabetes are frequentlycharacterized by onset of hyperglycemiaat an early age (generally before age 25years). They are referred to as maturity-onset diabetes of the young (MODY) andare characterized by impaired insulin se-cretion with minimal or no defects in in-sulin action. They are inherited in anautosomal dominant pattern. Abnormali-ties at six genetic loci on different chro-mosomes have been identified to date.The most common form is associatedwith mutations on chromosome 12 in ahepatic transcription factor referred to ashepatocyte nuclear factor (HNF)-1a. Asecond form is associated with mutationsin the glucokinase gene on chromosome7p and results in a defective glucokinasemolecule. Glucokinase converts glucoseto glucose-6-phosphate, the metabolismof which, in turn, stimulates insulin secre-tion by the b-cell. Thus, glucokinaseserves as the “glucose sensor” for theb-cell. Because of defects in the glucoki-nase gene, increased plasma levels of glu-cose are necessary to elicit normal levelsof insulin secretion. The less common

forms result frommutations in other tran-scription factors, including HNF-4a,HNF-1b, insulin promoter factor (IPF)-1, and NeuroD1.

Diabetes diagnosed in the first 6months of life has been shown not to betypical autoimmune type 1 diabetes. Thisso-called neonatal diabetes can either betransient or permanent. The most com-mon genetic defect causing transientdisease is a defect on ZAC/HYAMI im-printing, whereas permanent neonataldiabetes is most commonly a defect in thegene encoding the Kir6.2 subunit of theb-cell KATP channel. Diagnosing the latterhas implications, since such children canbe well managed with sulfonylureas.

Point mutations in mitochondrialDNA have been found to be associatedwith diabetes and deafness The mostcommon mutation occurs at position3,243 in the tRNA leucine gene, leadingto an A-to-G transition. An identicallesion occurs in the MELAS syndrome(mitochondrial myopathy, encephalop-athy, lactic acidosis, and stroke-like syn-drome); however, diabetes is not partof this syndrome, suggesting differentphenotypic expressions of this geneticlesion.

Genetic abnormalities that result inthe inability to convert proinsulin to in-sulin have been identified in a few fami-lies, and such traits are inherited in anautosomal dominant pattern. The resul-tant glucose intolerance is mild. Similarly,the production of mutant insulin mole-cules with resultant impaired receptorbinding has also been identified in a fewfamilies and is associated with an autoso-mal inheritance and only mildly impairedor even normal glucose metabolism.Genetic defects in insulin action. Thereare unusual causes of diabetes that resultfrom genetically determined abnormali-ties of insulin action. The metabolic ab-normalities associated with mutations ofthe insulin receptor may range fromhyperinsulinemia and modest hyperglyce-mia to severe diabetes. Some individualswith these mutations may have acanthosisnigricans. Women may be virilized andhave enlarged, cystic ovaries. In the past,this syndrome was termed type A insulinresistance. Leprechaunism and the Rabson-Mendenhall syndrome are two pediatricsyndromes that have mutations in theinsulin receptor gene with subsequentalterations in insulin receptor functionand extreme insulin resistance. The formerhas characteristic facial features and isusually fatal in infancy, while the latter is

associated with abnormalities of teeth andnails and pineal gland hyperplasia.

Alterations in the structure and func-tion of the insulin receptor cannot bedemonstrated in patients with insulin-resistant lipoatrophic diabetes. Therefore,it is assumed that the lesion(s) must residein the postreceptor signal transductionpathways.Diseases of the exocrine pancreas. Anyprocess that diffusely injures the pancreascan cause diabetes. Acquired processesinclude pancreatitis, trauma, infection, pan-createctomy, and pancreatic carcinoma.With the exception of that caused bycancer, damage to the pancreas must beextensive for diabetes to occur; adreno-carcinomas that involve only a smallportion of the pancreas have been associ-ated with diabetes. This implies a mech-anism other than simple reduction inb-cell mass. If extensive enough, cystic fi-brosis and hemochromatosis will alsodamage b-cells and impair insulin secre-tion. Fibrocalculous pancreatopathy maybe accompanied by abdominal pain radi-ating to the back and pancreatic calcifica-tions identified on X-ray examination.Pancreatic fibrosis and calcium stonesin the exocrine ducts have been found atautopsy.Endocrinopathies. Several hormones(e.g., growth hormone, cortisol, gluca-gon, epinephrine) antagonize insulin ac-tion. Excess amounts of these hormones(e.g., acromegaly, Cushing’s syndrome,glucagonoma, pheochromocytoma, re-spectively) can cause diabetes. This gen-erally occurs in individuals withpreexisting defects in insulin secretion,and hyperglycemia typically resolveswhen the hormone excess is resolved.

Somatostatinomas, and aldostero-noma-induced hypokalemia, can causediabetes, at least in part, by inhibitinginsulin secretion. Hyperglycemia gener-ally resolves after successful removal ofthe tumor.Drug- or chemical-induced diabetes.Many drugs can impair insulin secretion.These drugs may not cause diabetes bythemselves, but they may precipitate di-abetes in individuals with insulin resis-tance. In such cases, the classification isunclear because the sequence or relativeimportance of b-cell dysfunction and in-sulin resistance is unknown. Certain tox-ins such as Vacor (a rat poison) andintravenous pentamidine can perma-nently destroy pancreatic b-cells. Suchdrug reactions fortunately are rare. Thereare also many drugs and hormones that

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can impair insulin action. Examples in-clude nicotinic acid and glucocorticoids.Patients receiving a-interferon have beenreported to develop diabetes associatedwith islet cell antibodies and, in certaininstances, severe insulin deficiency. Thelist shown in Table 1 is not all-inclusive,but reflects the more commonly recog-nized drug-, hormone-, or toxin-inducedforms of diabetes.Infections. Certain viruses have been as-sociated with b-cell destruction. Diabetesoccurs in patients with congenital rubella,although most of these patients have HLAand immune markers characteristic of type1 diabetes. In addition, coxsackievirus B,cytomegalovirus, adenovirus, and mumpshave been implicated in inducing certaincases of the disease.Uncommon forms of immune-mediateddiabetes. In this category, there are twoknown conditions, and others are likelyto occur. The stiff-man syndrome is anautoimmune disorder of the central ner-vous system characterized by stiffness ofthe axial muscles with painful spasms.Patients usually have high titers of theGAD autoantibodies, and approximatelyone-third will develop diabetes.

Anti-insulin receptor antibodies cancause diabetes by binding to the insulinreceptor, thereby blocking the binding ofinsulin to its receptor in target tissues.However, in some cases, these antibodiescan act as an insulin agonist after binding tothe receptor and can thereby cause hypo-glycemia. Anti-insulin receptor antibodiesare occasionally found in patients withsystemic lupus erythematosus and otherautoimmune diseases. As in other states ofextreme insulin resistance, patients withanti-insulin receptor antibodies often haveacanthosis nigricans. In the past, this syn-dromewas termed type B insulin resistance.Other genetic syndromes sometimesassociated with diabetes. Many geneticsyndromes are accompanied by an in-creased incidence of diabetes. These in-clude the chromosomal abnormalities ofDown syndrome, Klinefelter syndrome,and Turner syndrome. Wolfram syn-drome is an autosomal recessive disordercharacterized by insulin-deficient diabe-tes and the absence of b-cells at autopsy.Additional manifestations include diabetesinsipidus, hypogonadism, optic atrophy,and neural deafness. Other syndromes arelisted in Table 1.

GDMFor many years, GDM has been defined asany degree of glucose intolerance with

Table 1dEtiologic classification of diabetes mellitus

I. Type 1 diabetes (b-cell destruction, usually leading to absolute insulin deficiency)A. Immune mediatedB. Idiopathic

II. Type 2 diabetes (may range from predominantly insulin resistance with relative insulin deficiencyto a predominantly secretory defect with insulin resistance)

III. Other specific typesA. Genetic defects of b-cell function

1. MODY 3 (Chromosome 12, HNF-1a)2. MODY 1 (Chromosome 20, HNF-4a)3. MODY 2 (Chromosome 7, glucokinase)4. Other very rare forms of MODY (e.g., MODY 4: Chromosome 13, insulin promoter factor-1;

MODY 6: Chromosome 2, NeuroD1; MODY 7: Chromosome 9, carboxyl ester lipase)5. Transient neonatal diabetes (most commonly ZAC/HYAMI imprinting defect on 6q24)6. Permanent neonatal diabetes (most commonly KCNJ11 gene encoding Kir6.2 subunit of b-cell

KATP channel)7. Mitochondrial DNA8. Others

B. Genetic defects in insulin action1. Type A insulin resistance2. Leprechaunism3. Rabson-Mendenhall syndrome4. Lipoatrophic diabetes5. Others

C. Diseases of the exocrine pancreas1. Pancreatitis2. Trauma/pancreatectomy3. Neoplasia4. Cystic fibrosis5. Hemochromatosis6. Fibrocalculous pancreatopathy7. Others

D. Endocrinopathies1. Acromegaly2. Cushing’s syndrome3. Glucagonoma4. Pheochromocytoma5. Hyperthyroidism6. Somatostatinoma7. Aldosteronoma8. Others

E. Drug or chemical induced1. Vacor2. Pentamidine3. Nicotinic acid4. Glucocorticoids5. Thyroid hormone6. Diazoxide7. b-Adrenergic agonists8. Thiazides9. Dilantin

10. g-Interferon11. Others

F. Infections1. Congenital rubella2. Cytomegalovirus3. Others

G. Uncommon forms of immune-mediated diabetes1. “Stiff-man” syndrome2. Anti-insulin receptor antibodies3. Others

H. Other genetic syndromes sometimes associated with diabetes1. Down syndrome2. Klinefelter syndrome3. Turner syndrome4. Wolfram syndrome5. Friedreich ataxia6. Huntington chorea7. Laurence-Moon-Biedl syndrome8. Myotonic dystrophy9. Porphyria

10. Prader-Willi syndrome11. Others

IV. Gestational diabetes mellitus

Patients with any form of diabetes may require insulin treatment at some stage of their disease. Such use ofinsulin does not, of itself, classify the patient.

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onset or first recognition during preg-nancy. Although most cases resolve withdelivery, the definition applied whetheror not the condition persisted after preg-nancy and did not exclude the possibilitythat unrecognized glucose intolerancemay have antedated or begun concomi-tantly with the pregnancy. This definitionfacilitated a uniform strategy for detectionand classification of GDM, but its limi-tations were recognized for many years.As the ongoing epidemic of obesity anddiabetes has led to more type 2 diabetes inwomen of childbearing age, the number ofpregnant womenwith undiagnosed type 2diabetes has increased.

After deliberations in 2008–2009, theInternational Association of Diabetes andPregnancy Study Groups (IADPSG), aninternational consensus group with rep-resentatives from multiple obstetrical anddiabetes organizations, including theAmerican Diabetes Association (ADA),recommended that high-risk womenfound to have diabetes at their initial pre-natal visit, using standard criteria (Table3), receive a diagnosis of overt, not gesta-tional, diabetes. Approximately 7% of allpregnancies (ranging from 1 to 14%, de-pending on the population studied andthe diagnostic tests employed) are com-plicated by GDM, resulting in more than200,000 cases annually.

CATEGORIES OF INCREASEDRISK FOR DIABETESdIn 1997 and2003, the Expert Committee on Diagno-sis and Classification of Diabetes Mellitus(1,2) recognized an intermediate group ofindividuals whose glucose levels do notmeet criteria for diabetes, yet are higherthan those considered normal. These peo-ple were defined as having impaired fast-ing glucose (IFG) [fasting plasma glucose(FPG) levels 100 mg/dl (5.6 mmol/l) to125mg/dl (6.9 mmol/l)], or impaired glu-cose tolerance (IGT) [2-h values in theoral glucose tolerance test (OGTT) of140 mg/dl (7.8 mmol/l) to 199 mg/dl(11.0 mmol/l)].

Individuals with IFG and/or IGT havebeen referred to as having prediabetes,indicating the relatively high risk for thefuture development of diabetes. IFG andIGT should not be viewed as clinicalentities in their own right but rather riskfactors for diabetes as well as cardiovas-cular disease. They can be observed asintermediate stages in any of the diseaseprocesses listed in Table 1. IFG and IGTare associated with obesity (especially ab-dominal or visceral obesity), dyslipidemia

with high triglycerides and/or low HDLcholesterol, and hypertension. Structuredlifestyle intervention, aimed at increasingphysical activity and producing 5–10%loss of body weight, and certain pharma-cological agents have been demonstratedto prevent or delay the development ofdiabetes in people with IGT; the potentialimpact of such interventions to reducemortality or the incidence of cardiovascu-lar disease has not been demonstrated todate. It should be noted that the 2003ADA Expert Committee report reducedthe lower FPG cut point to define IFGfrom 110 mg/dl (6.1 mmol/l) to 100 mg/dl(5.6 mmol/l), in part to ensure that prev-alence of IFG was similar to that of IGT.However, the World Health Organization(WHO) and many other diabetes organi-zations did not adopt this change in thedefinition of IFG.

As A1C is used more commonly todiagnose diabetes in individuals with riskfactors, it will also identify those at higherrisk for developing diabetes in the future.When recommending the use of the A1Cto diagnose diabetes in its 2009 report,the International Expert Committee (3)stressed the continuum of risk for diabe-tes with all glycemicmeasures and did notformally identify an equivalent intermedi-ate category for A1C. The group did notethat those with A1C levels above the lab-oratory “normal” range but below the di-agnostic cut point for diabetes (6.0 to,6.5%) are at very high risk of develop-ing diabetes. Indeed, incidence of diabe-tes in people with A1C levels in this rangeis more than 10 times that of people withlower levels (4–7). However, the 6.0 to,6.5% range fails to identify a substantialnumber of patients who have IFG and/orIGT. Prospective studies indicate thatpeople within the A1C range of 5.5–6.0% have a 5-year cumulative incidenceof diabetes that ranges from 12 to 25%(4–7), which is appreciably (three- toeightfold) higher than incidence in theU.S. population as a whole (8). Analysesof nationally representative data from theNational Health and Nutrition Examina-tion Survey (NHANES) indicate that theA1C value that most accurately identifiespeople with IFG or IGT falls between 5.5and 6.0%. In addition, linear regressionanalyses of these data indicate that amongthe nondiabetic adult population, an FPGof 110 mg/dl (6.1 mmol/l) correspondsto an A1C of 5.6%, while an FPG of 100mg/dl (5.6 mmol/l) corresponds to anA1C of 5.4% (R.T. Ackerman, personalcommunication). Finally, evidence from

the Diabetes Prevention Program (DPP),wherein the mean A1C was 5.9% (SD0.5%), indicates that preventive interven-tions are effective in groups of people withA1C levels both below and above 5.9%(9). For these reasons, the most appropri-ate A1C level above which to initiate pre-ventive interventions is likely to besomewhere in the range of 5.5–6%.

As was the case with FPG and 2-h PG,defining a lower limit of an intermediatecategory of A1C is somewhat arbitrary,as the risk of diabetes with any measureor surrogate of glycemia is a continuum,extending well into the normal ranges. Tomaximize equity and efficiency of pre-ventive interventions, such an A1C cutpoint should balance the costs of “falsenegatives” (failing to identify those whoare going to develop diabetes) againstthe costs of “false positives” (falsely iden-tifying and then spending intervention re-sources on those who were not going todevelop diabetes anyway).

As is the case with the glucose mea-sures, several prospective studies that usedA1C to predict the progression to diabetesdemonstrated a strong, continuous asso-ciation between A1C and subsequent di-abetes. In a systematic review of 44,203individuals from 16 cohort studies witha follow-up interval averaging 5.6 years(range 2.8–12 years), those with an A1Cbetween 5.5 and 6.0% had a substantiallyincreased risk of diabetes with 5-yearincidences ranging from 9 to 25%. AnA1C range of 6.0–6.5% had a 5-year riskof developing diabetes between 25 and50% and relative risk 20 times highercompared with an A1C of 5.0% (10). Ina community-based study of black andwhite adults without diabetes, baselineA1C was a stronger predictor of subse-quent diabetes and cardiovascular eventsthan was fasting glucose (11). Other anal-yses suggest that an A1C of 5.7% is asso-ciated with similar diabetes risk to thehigh-risk participants in the DPP (12).Hence, it is reasonable to consider anA1C range of 5.7–6.4% as identifying in-dividuals with high risk for future diabe-tes, to whom the term prediabetes may beapplied.

Individuals with an A1C of 5.7–6.4%should be informed of their increased riskfor diabetes as well as cardiovascular dis-ease and counseled about effective strate-gies, such as weight loss and physicalactivity, to lower their risks. As with glu-cose measurements, the continuum of riskis curvilinear, so that as A1C rises, the riskof diabetes rises disproportionately.

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Accordingly, interventions should bemost intensive and follow-up should beparticularly vigilant for those with A1Clevels above 6.0%, who should be consid-ered to be at very high risk. However, justas an individual with a fasting glucose of98 mg/dl (5.4 mmol/l) may not be at neg-ligible risk for diabetes, individuals withA1C levels below 5.7% may still be atrisk, depending on level of A1C and pres-ence of other risk factors, such as obesityand family history.

Table 2 summarizes the categories ofincreased risk for diabetes. Evaluation ofpatients at risk should incorporate aglobal risk factor assessment for both di-abetes and cardiovascular disease. Screen-ing for and counseling about risk ofdiabetes should always be in the prag-matic context of the patient’s comorbidi-ties, life expectancy, personal capacity toengage in lifestyle change, and overallhealth goals.

DIAGNOSTIC CRITERIA FORDIABETES MELLITUSdFor deca-des, the diagnosis of diabetes has beenbased on glucose criteria, either the FPGor the 75-g OGTT. In 1997, the firstExpert Committee on the Diagnosis andClassification of Diabetes Mellitus revisedthe diagnostic criteria, using the observedassociation between FPG levels and pres-ence of retinopathy as the key factor withwhich to identify threshold glucose level.The Committee examined data from threecross-sectional epidemiologic studies thatassessed retinopathy with fundus pho-tography or direct ophthalmoscopy andmeasured glycemia as FPG, 2-h PG, andA1C. These studies demonstrated glyce-mic levels below which there was littleprevalent retinopathy and above whichthe prevalence of retinopathy increased inan apparently linear fashion. The decilesof the three measures at which retinopa-thy began to increase were the same foreach measure within each population.Moreover, the glycemic values above

which retinopathy increased were similaramong the populations. These analysesconfirmed the long-standing diagnostic 2-hPG value of $200 mg/dl (11.1 mmol/l).However, the older FPG diagnostic cutpoint of 140mg/dl (7.8mmol/l) was notedto identify far fewer individuals with dia-betes than the 2-h PG cut point. The FPGdiagnostic cut point was reduced to$126 mg/dl (7.0 mmol/l).

A1C is a widely used marker ofchronic glycemia, reflecting averageblood glucose levels over a 2- to 3-monthperiod of time. The test plays a critical rolein the management of the patient withdiabetes, since it correlates well with bothmicrovascular and, to a lesser extent,macrovascular complications and iswidely used as the standard biomarkerfor the adequacy of glycemic manage-ment. Prior Expert Committees have notrecommended use of the A1C for diag-nosis of diabetes, in part due to lack ofstandardization of the assay. However,A1C assays are now highly standardizedso that their results can be uniformlyapplied both temporally and across pop-ulations. In their recent report (3), an In-ternational Expert Committee, after anextensive review of both established andemerging epidemiological evidence, rec-ommended the use of the A1C test to di-agnose diabetes, with a threshold of$6.5%, and ADA affirms this decision.The diagnostic A1C cut point of 6.5% isassociated with an inflection point for ret-inopathy prevalence, as are the diagnosticthresholds for FPG and 2-h PG (3). Thediagnostic test should be performedusing a method that is certified by the Na-tional Glycohemoglobin StandardizationProgram (NGSP) and standardized ortraceable to the Diabetes Control andComplications Trial reference assay.Point-of-care A1C assays are not suffi-ciently accurate at this time to use for di-agnostic purposes.

There is an inherent logic to using amore chronic versus an acute marker ofdysglycemia, particularly since the A1C isalready widely familiar to clinicians as amarker of glycemic control. Moreover,the A1C has several advantages to theFPG, including greater convenience,since fasting is not required, evidence tosuggest greater preanalytical stability, andless day-to-day perturbations during pe-riods of stress and illness. These advan-tages, however, must be balanced bygreater cost, the limited availability ofA1C testing in certain regions of thedeveloping world, and the incomplete

correlation between A1C and averageglucose in certain individuals. In addi-tion, the A1C can be misleading in pa-tients with certain forms of anemia andhemoglobinopathies, which may alsohave unique ethnic or geographic distri-butions. For patients with a hemoglobin-opathy but normal red cell turnover, suchas sickle cell trait, an A1C assay withoutinterference from abnormal hemoglobinsshould be used (an updated list is avail-able at http://www.ngsp.org/interf.asp).For conditions with abnormal red cellturnover, such as anemias from hemolysisand iron deficiency, the diagnosis ofdiabetes must employ glucose criteriaexclusively.

The established glucose criteria forthe diagnosis of diabetes remain valid.These include the FPG and 2-h PG.Additionally, patients with severe hyper-glycemia such as those who present withsevere classic hyperglycemic symptomsor hyperglycemic crisis can continue to bediagnosed when a random (or casual)plasma glucose of $200 mg/dl (11.1mmol/l) is found. It is likely that in suchcases the health care professional wouldalso measure an A1C test as part of theinitial assessment of the severity of the di-abetes and that it would (in most cases) beabove the diagnostic cut point for diabe-tes. However, in rapidly evolving diabe-tes, such as the development of type 1diabetes in some children, A1C may notbe significantly elevated despite frankdiabetes.

Just as there is less than 100% con-cordance between the FPG and 2-h PGtests, there is not full concordance be-tween A1C and either glucose-basedtest. Analyses of NHANES data indicatethat, assuming universal screening of theundiagnosed, the A1C cut point of$6.5% identifies one-third fewer casesof undiagnosed diabetes than a fastingglucose cut point of $126 mg/dl (7.0mmol/l) (www.cdc.gov/diabetes/pubs/factsheet11/tables1_2.htm). However, inpractice, a large portion of the populationwith type 2 diabetes remains unaware oftheir condition. Thus, it is conceivablethat the lower sensitivity of A1C at thedesignated cut point will be offset by thetest’s greater practicality, and that widerapplication of a more convenient test(A1C) may actually increase the numberof diagnoses made.

Further research is needed to bettercharacterize those patients whose glyce-mic status might be categorized differ-ently by two different tests (e.g., FPG and

Table 2dCategories of increased risk fordiabetes (prediabetes)*

FPG 100mg/dl (5.6 mmol/l) to 125 mg/dl (6.9mmol/l) [IFG]

2-h PG in the 75-g OGTT 140 mg/dl (7.8mmol/l) to 199 mg/dl (11.0 mmol/l) [IGT]

A1C 5.7–6.4%*For all three tests, risk is continuous, extendingbelow the lower limit of the range and becomingdisproportionately greater at higher ends of therange.

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A1C), obtained in close temporal approx-imation. Such discordancemay arise frommeasurement variability, change overtime, or because A1C, FPG, and post-challenge glucose each measure differentphysiological processes. In the setting ofan elevated A1C but “nondiabetic” FPG,the likelihood of greater postprandial glu-cose levels or increased glycation ratesfor a given degree of hyperglycemia maybe present. In the opposite scenario (highFPG yet A1C below the diabetes cutpoint), augmented hepatic glucose pro-duction or reduced glycation rates maybe present.

As with most diagnostic tests, a testresult diagnostic of diabetes should berepeated to rule out laboratory error,unless the diagnosis is clear on clinicalgrounds, such as a patient with classicsymptoms of hyperglycemia or hypergly-cemic crisis. It is preferable that the sametest be repeated for confirmation, sincethere will be a greater likelihood of con-currence in this case. For example, if theA1C is 7.0% and a repeat result is 6.8%,the diagnosis of diabetes is confirmed.However, there are scenarios in which re-sults of two different tests (e.g., FPG andA1C) are available for the same patient. Inthis situation, if the two different tests areboth above the diagnostic thresholds, thediagnosis of diabetes is confirmed.

On the other hand, when two differ-ent tests are available in an individual andthe results are discordant, the test whoseresult is above the diagnostic cut pointshould be repeated, and the diagnosis ismade on the basis of the confirmed test.That is, if a patient meets the diabetescriterion of the A1C (two results $6.5%)but not the FPG (,126 mg/dl or 7.0mmol/l), or vice versa, that personshould be considered to have diabetes. Ad-mittedly, in most circumstance the

“nondiabetic” test is likely to be in a rangevery close to the threshold that defines di-abetes.

Since there is preanalytic and analyticvariability of all the tests, it is also possiblethat when a test whose result was abovethe diagnostic threshold is repeated, thesecond value will be below the diagnosticcut point. This is least likely for A1C,somewhat more likely for FPG, and mostlikely for the 2-h PG. Barring a laboratoryerror, such patients are likely to have testresults near the margins of the thresholdfor a diagnosis. The healthcare profes-sional might opt to follow the patientclosely and repeat the testing in 3–6months.

The decision about which test to useto assess a specific patient for diabetesshould be at the discretion of the healthcare professional, taking into account theavailability and practicality of testing anindividual patient or groups of patients.Perhaps more important than which di-agnostic test is used, is that the testing fordiabetes be performed when indicated.There is discouraging evidence indicatingthat many at-risk patients still do not re-ceive adequate testing and counseling forthis increasingly common disease, or for itsfrequently accompanying cardiovascularrisk factors. The current diagnostic criteriafor diabetes are summarized in Table 3.

Diagnosis of GDMGDM carries risks for the mother andneonate. The Hyperglycemia and AdversePregnancy Outcomes (HAPO) study(13), a large-scale (;25,000 pregnantwomen) multinational epidemiologicstudy, demonstrated that risk of adversematernal, fetal, and neonatal outcomescontinuously increased as a function ofmaternal glycemia at 24–28 weeks, evenwithin ranges previously considered

normal for pregnancy. For most compli-cations, there was no threshold for risk.These results have led to careful reconsid-eration of the diagnostic criteria for GDM.After deliberations in 2008–2009, theIADPSG, an international consensusgroup with representatives from multipleobstetrical and diabetes organizations, in-cluding ADA, developed revised recom-mendations for diagnosing GDM. Thegroup recommended that all women notknown to have diabetes undergo a 75-gOGTT at 24–28 weeks of gestation. Ad-ditionally, the group developed diagnos-tic cut points for the fasting, 1-h, and 2-hplasma glucose measurements that con-veyed an odds ratio for adverse outcomesof at least 1.75 compared with womenwith mean glucose levels in the HAPOstudy. Current screening and diagnosticstrategies, based on the IADPSG state-ment (14), are outlined in Table 4.

These new criteria will significantlyincrease the prevalence of GDM, primar-ily because only one abnormal value, nottwo, is sufficient to make the diagnosis.The ADA recognizes the anticipated sig-nificant increase in the incidence of GDMto be diagnosed by these criteria and issensitive to concerns about the “medical-ization” of pregnancies previously catego-rized as normal. These diagnostic criteriachanges are being made in the context ofworrisome worldwide increases in obe-sity and diabetes rates, with the intent ofoptimizing gestational outcomes forwomen and their babies.

Admittedly, there are few data fromrandomized clinical trials regarding ther-apeutic interventions in women who willnow be diagnosed with GDM based ononly one blood glucose value above thespecified cut points (in contrast to theolder criteria that stipulated at least two

Table 3dCriteria for the diagnosis of diabetes

A1C$6.5%. The test should be performed in a laboratory using a method that is NGSP certifiedand standardized to the DCCT assay.*

ORFPG $126 mg/dl (7.0 mmol/l). Fasting is defined as no caloric intake for at least 8 h.*

OR2-h plasma glucose$200mg/dl (11.1mmol/l) during anOGTT. The test should be performed asdescribed by theWorld Health Organization, using a glucose load containing the equivalent of75 g anhydrous glucose dissolved in water.*

ORIn a patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasmaglucose $200 mg/dl (11.1 mmol/l).

*In the absence of unequivocal hyperglycemia, criteria 1–3 should be confirmed by repeat testing.

Table 4dScreening for and diagnosis ofGDM

Perform a 75-g OGTT, with plasma glucosemeasurement fasting and at 1 and 2 h, at24–28 weeks of gestation in women notpreviously diagnosed with overt diabetes.

The OGTT should be performed in themorning after an overnight fast of at least8 h.

The diagnosis of GDM ismade when any of thefollowing plasma glucose values areexceeded:

c Fasting: $92 mg/dl (5.1 mmol/l)c 1 h: $180 mg/dl (10.0 mmol/l)c 2 h: $153 mg/dl (8.5 mmol/l)

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abnormal values). Expected benefits totheir pregnancies and offspring is inferredfrom intervention trials that focused onwomen with more mild hyperglycemiathan identified using older GDM diag-nostic criteria and that found modestbenefits (15,16). The frequency of theirfollow-up and blood glucose monitoringis not yet clear but likely to be less inten-sive than women diagnosed by the oldercriteria. Additional well-designed clinicalstudies are needed to determine the op-timal intensity of monitoring and treat-ment of women with GDM diagnosedby the new criteria (that would nothave met the prior definition of GDM).It is important to note that 80–90% ofwomen in both of the mild GDM studies(whose glucose values overlapped withthe thresholds recommended herein)could be managed with lifestyle therapyalone.

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