Diagnostic Tests 06.12.2011

7/28/2019 Diagnostic Tests 06.12.2011

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Diagnostic tests


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A diagnostic test is any kind ofmedical test

performed to aid in the diagnosis or detection

ofdisease
http://en.wikipedia.org/wiki/Medical_testhttp://en.wikipedia.org/wiki/Medical_diagnosishttp://en.wikipedia.org/wiki/Diseasehttp://en.wikipedia.org/wiki/Diseasehttp://en.wikipedia.org/wiki/Medical_diagnosishttp://en.wikipedia.org/wiki/Medical_test


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Helps to understand

Patients health

Physiological functioning

Results are reported with a reference range


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Why interpretation?

Appropriateness of patients current drug

therapy

Adequacy of drug response

Efficacy of treatment

Serious drug toxicity


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Medications may alter laboratory test values

through a variety of pharmacological, physical

or chemical mechanisms.

Pharmacologically, a drug may cause a true

change in the concentration of the particular

substance being measured.


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Chemically, a drug or its metabolite may

interfere with the accurate determination of

the desired constituent.

In addition to chemical interferences by drugs,

one should also consider the effects of

anticoagulants and of marked changes in

concentrations of normally occurringsubstances in body fluids which alter

laboratory test values.


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Chemical interferences arise primarily from

lack of specificity of the methods used to

measure desired constituents

Another problem is to determine whether a

reported drug interference was observed at

realistic concentrations which are likely to

occur in serum or urine during a typicaltherapeutic regimen.


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Renal function tests

Serum urea(3-8 mmol/L):

Catabolism of amino acids

Undergoes tubular reabsorption

Vary with diet, rate of protein metabolism, liver

production and GFR.

Upto 10 mmol/L

Non-renal and renal factors influence the increaseand decrease in the levels.


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Uric acid(0.15-0.47 mmol/L):

End product of purine metabolism

Increased rate of formation or reduced excretion

Gouty arthritis


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Drugs interfering with uric acid

measurementInterfering agent

Ascorbic acid

Phenacetin

Caffeine

ChlorineErgothioneine

Salicylates

Levodopa

Methyldopa


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Serum creatinine(55-120 mmol/L or 0.6-1.35

mg/dL):

Product of muscle metabolism

Surrogate marker for GFR

Increase: insult to the kidney

Decrease: no pathological significance

Estimation of GFR


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GFR:

Glomerular filtration rate (GFR) describes the

flow rate of filtered fluid through the kidney

Estimated GFR= (140-age) x weight

Serum creatinine mmol/l x0.84

120-150ml/min

Helps to monitor drug action, transplant

Poses a problem (muscle mass and consumption

of diet)


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Measurement of alternate agent i.e., cystatin

C

Equations are developed considering the

same.


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Drugs interfering with creatinine

measurementInterfering agent Concentration equivalent

Acetoacetic acid. IOO mg/dl of acetoacetic acid =3.4 mg/dl ofcreatinine.

Acetone 500 mg/dl of acetone =1.4

mg/dl of creatinine.

Aminohippurate

Ascorbic acid

Fructose IOOOm g/d1 of glucose

=1 mg/dl of creatinine.

Glucose IOOOm l/d1 of glucose

=0.2 mg/dl of creatinine.

Methyl dopa IO mg/dl of methyldopa = 1.0 mg/dl of creatinine.

Nitrofuran derivatives

Phenolsulphtalein

Pyruvate 10 mgldl of pyruvic acid = 0.3 mgidl of creatinine.

Protein 5 g/d1 of crystalline human serum albumin =2.2 mg/dl

of creatinine.

Sulphbromthalein


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Blood urea nitrogen(2.9-7.1 mmol/L or 8-20

mmol/L):

Alongwith serum creatinine, is very useful

Another prognostic marker for kidney disease

is an elevated level of protein in the urine. The

most sensitive marker of proteinuria is

elevated urine albumin


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Drugs interfering with blood urea nitrogen

measurementInterfering agent

Citrulline

Hydantoin

MethylureaPhenylurea

Sulfonylureas,

Acetone

Chloral hydrate

Creatinine


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Drugs interfering with urobilinogen

measurement

Interfering agent

P-amino salicylate

Chlorpromazine

Bilirubin

Sulphonamides


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SERUM ELECTROLYTES

1. Serum sodium(135-145 mmol/l)

Hyponatraemia: not enough sodium (salt) in the body fluids

Lethargy, nausea, drowsiness and confusion

Pseudohyponatremia, clinical fluid overload, diuretic

therapy, salt wasting nephropathy, The syndrome of

inappropriate antidiuretic hormone hypersecretion

(SIADH)

Acute or chronic


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Hypernatraemia

Excessive loss of hypotonic fluids, severe vomitting

/diarrhea

Diabetes mellitus, acute renal failure, diabetes insipidus Address the underlying dehydration

Causes CNS pathology

Controlled by thirst reflex, ADH, renin-angiotensin-

aldosterone system and renal tubular handling


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2. Serum potassium(3.4-4.8 mmol/l)

Hypokalemia

Vomitting, diarrhea and gastric suction, drug related

Intravenous potassium should be diluted and infused

slowly


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Hyperkalemia

Serious cardiac effects

Administration of intravenous glucose insulin,

treatment with cation exchange resin, hemodialysis Drug related: potassium sparing diuretics, ACE

inhibitors, AT antagonists and NSAIDs

Present in acute renal failure


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3. Serum calcium(2.2-2.5mmol/l or 8.8-10.2

mg/dl)

Ca corr (mg/dl)= {(4-albumin g/dl)x0.8 mg/gl} + Ca

uncorr (mg/dl)

Hypocalcemia : vit D deficiency, hypparathyroidism and

chronic renal failure

Hypercalcemia : overuse of Vit. D or thiazide diruetics.

Constipation, confusion, conjunctivits, drowsiness,

lethargy and polyuria. Most common cause is release of

calcium from bones


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Blood picture

1. Hemoglobin (13.5-17.5 g/dl for men, 12-16

g/dl for women)

Higher in polycythemia rubra vera, patients who

live at high altitude or those with chronic hypoxiclung disease

Anemia :

Types depending on MCV(80-100fl) and MCHC (33-37g/dl)


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Microcytic anemia

Low MCV and/or low MCHC

Microcytic hypochromic anemia is associated with

iron deficiency

Normocytic anemia

MCV and MCHC are within normal ranges

Associated chronic infections, rheumatoid

arthritis, hypothyroidism and malignancy, chronic

renal failure, blood loss


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Macrocytic anemia

High MCV

Liver disease, deficient dietary intake of folic acid

or vit B12


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2. White cell count (4.5-10.5 x 103/mcl)

Luecopenia (neutropenia or lymphopenia)

Elevated levels seen with infections,

corticosteroids and lithium can also increase the

count, eosinophilia (allergic reaction)


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3. Platelet count(150-450 x 103/mcl)

Surrogate marker for inflammatory reaction

Thrombocytopenia : liver disease, infections, drug

induced


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Other blood tests

Erythrocyte sedimentation rate (ESR)

Normal volume is less than 10mm/h

Westergren method

High ESR indicates increased protein level

(inflammatory condition)


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C-reactive protein

Secreted by liver in response to inflammation

Haptoglobin

Functions as an antioxidant

Helps to monitor inflammatory conditions

Coombs test

Anti-human immunoglobulin antibody and

anticomplement antibody autoimmune anemia


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Xanthochromia

Several hours after subarachnoid haemorrhage,

distinguishes from traumatic lumbar puncture

Iron, transferrin and iron binding

Transferrin total iron binding capacity

Ferritin measurement- decrease in iron deficiency

Free protporhyrinincrease in iron deficiency


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Liver function test

Albumin Present in ECF and serum

Important role in binding

Concentration depends on synthesis, volume of

distribution and rate of catabolism Synthesis- fall with increasing severity of liver disease or

malnutrition in response to inflammatory mediators

Volume of distribution increases, causes reduced serum albumin

in case of cirrhosis, fluid retention states

Increased capillary permeability results in movement of albuminfrom serum into interstitial fluid

Other causes include catabolic states- kidney, skin, intestinal wall


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Bilirubin

Elevation of bilirubin above 50micromole/l reveal

presence of jaundice.

Elevation may be due to

Increased production of bilirubin- haemolyis,

ineffective erythropoesis

Impaired transport in hepatocytes- hepatitis or drug

induced

Decreased excretion- drugs, cirrhosis, tumours

Intra-hepatic obstruction- cirrhosis, tumours


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Enzymes

Alanine transaminase


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Ammonia

Indicative of impairment of urea cycle function or

extensive hepatic damage

Hyperammonaemia >60mmol/L Valproic acid can induce encephalopathy

Amylase

Rises within 24 h of pancreatitis, then declines to

normal over 1 week


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Cardiac markers Troponins

Cardiac troponin I(cTn I) and Cardiac troponin

T(cTn T)

Creatine kinase

Increased during shock, myocardial infarction and

muscular dystrophies

Drug induced

Two protein subunits M and B

Lactate dehydrogenase

LD-1-LD-5, increase levels in damage to liver,

skeletal muscle and kidneys, anemia


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Tumor markers

Prostate specific antigen

Human chorionic gonadotropin

Carcinoembyronic antigen

Cancer antigen (CA125 and CA 19)


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Immunoglobulins

Detected by electrophoresis as bands in , ,

regions

Increased levels found in infections chronic

liver disease, autoimmune disease

Lymphatic leukaemia, lymphoma and multiple

myeloma


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Lab Test-Drug Interaction:

Pharmacological Mechanisms

Pharmacologically, a drug may cause a true change in

the concentration of the particular substance being

measured.

Allopurinol, a xanthine oxidase inhibitor, is used toreduce plasma uric acid levels in selected patients.

Incidental pharmacologic effects, other than those

primarily intended, may also occur.


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Antiovulatory drugs, used for contraceptive

purposes, cause an increase in total serum thyroxine-

binding globulin.

This results in an increase in both total serumthyroxine and unsaturated thyroxine-binding

globulin, but no significant change in unbound

(free) thyroxine.

Recognition of these effects is essential in the

interpretation of thyroid function tests.


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Drugs may also cause undesirable toxic or side

effects, as evidenced by changes in liver or renal

function tests.

Thus, in some individuals, phenothiazines willproduce a picture of obstructive jaundice, with

elevation in serum bilirubin and alkaline

phosphatase.

Knowledge of the pharmacologic effects of drugs

helps to explain a number of correct but otherwise

unexpected laboratory results.


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Lab Test-Drug Interaction:

Chemical Mechanisms

Chemically, a drug or its metabolite may interfere

with the accurate determination of the desired

constituent.

Methyldopa, used in the treatment of hypertension,will produce a false positive result for urinary

catecholamines, since it reacts in the method to

produce a green fluorescence similar to that

produced by norepinephrine.

This could lead to an erroneous diagnosis of

pheochromocytoma.


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Radiographic contrast media, containing iodinated

organic compounds, invalidate protein-bound iodine

determinations because of their own high iodine

content. Other drugs can interfere by producing extraneous

color reactions, by chelation, or by inhibition of

enzyme activity.

In uremia, associated high levels of creatinine and

uric acid can result in falsely elevated values for

glucose as measured by alkaline ferricyanide

methods.

Diagnostic Tests 06.12.2011

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