Lab Medicine Conference :

Serum Electrolytes & Chemistries

Serum Sodium Concentrations

ƒ 60 % of sodium is in extracellular fluid (ECF) & 40 % is inactive in bone

ƒ Normal value is 135 to 145 meq/Lƒ In normal patient, rarely varies by > 2

meq/Lƒ Sx occur if level < 130 or > 150

–Patient at risk for seizures or coma if < 115

ƒ Onset & degree of Sx related to rapidity of change in concentration

ƒ Renin - angiotensin - aldosterone and ADH are main modulators of sodium concentration

Serum Sodium Measurement

ƒ Sample can be in either red top tube (no preservative) or green top (lithium heparinate) & measurement is run on serum after the specimen clots

ƒ Measurement methods :–Flame photometry–Ion selective electrode

ƒ Electrical potential of electrode proportional to activity of sodium ions in solution

ƒ Results are 2 % higher than by flame photometry

Causes of Pseudohyponatremia(Artifactual Hyponatremia)

ƒ Due to % of specimen volume taken up by non-water components & measurement technique using set specimen volume

ƒ Is problem only with flame photometry method

ƒ Occurs in marked hyperlipidemia & hyperproteinemias (protein > 11 g/dl)

ƒ Also hyperglycemia causes hyponatremia (1.6 meq/L decrease per 100 mg/dl increase in glucose)

Causes of Hyponatremia with Low Total Body Sodium (Hypovolemia)

ƒ Extrarenal losses–Vomiting / NG suction–Diarrhea–Third space losses–GI fistulas

ƒ Renal losses–Excess diuretic use–Salt - losing nephropathies–Osmotic diuretics–Adrenal insufficiency

Causes of Hyponatremia with Normal Total Body Sodium

ƒ SIADHƒ "Sick cell" syndromeƒ Hypothyroidismƒ Meds & drugs (see next slide)

Drugs & Meds Causing Hyponatremia (most via SIADH)

ƒ Phenothiazinesƒ Carbamazepineƒ Chlorpropamideƒ Narcoticsƒ Barbituratesƒ Acetominophenƒ Indomethacin

ƒ Tolbutamideƒ Clofibrateƒ Cyclophosphami

deƒ Vincristineƒ Isoproterenolƒ Nicotine

Causes of SIADHƒ CNS disease

–Head trauma–Intracranial bleed–Meningitis–Encephalitis–Brain abscess–Brain tumor–Stroke–Porphyria

ƒ Pulmonary disease–Pneumonia–Tuberculosis–Mycoses–Lung abscess–Mechanical ventilation

ƒ Carcinomasƒ Painƒ Stress

Causes of Hyponatremia with High Total Body Sodium

ƒ Cardiac failureƒ Cirrhosisƒ Nephrotic syndromeƒ Acute & chronic renal failure

Differential Diagnosis of Hyponatremia Medical Condition

BUN Hematocrit Urine Sodium Urine Osmolality

Extrarenal fluid loss

High High < 10 meq/L Hypertonic

Renal sodiumwasting (loss)

Mildly elevated

Normal or high

> 20 meq/L Variable

SIADH < 10 Normal or low > 20 meq/L Inappropriate hypertonic

Water intoxication

Normal or decreased

Normal or low Variable < 100 mOsmol / kg

"Sick cell" syndrome

Normal Normal Variable Variable

Cirrhosis or heart failure

Variable Normal < 10 meq/L Hypertonic

Renal failure

High Normal or low > 20 meq/L Isosthenuric

Causes of Hypernatremia with Low Total Body Sodium

ƒ Diarrheaƒ Profuse sweatingƒ Osmotic diuresis

–Mannitol–Hyperglycemia

Other Causes of Hypernatremia

ƒ With normal total body sodium :–Central diabetes insipidus–Nephrogenic diabetes insipidus

ƒ With high total body sodium :–Exogenously administered sodium bicarbonate–Hypertonic dialysate for renal dialysis–Saline - induced abortions–Ingestion of excessive salt tablets–Drowning in the Dead Sea

Serum Potassium (K+) Concentrations

ƒ K+ is major intracellular cationƒ Total body amount about 50

meq/kgƒ Normal ECF level is 3.6 to 5.0

meq/Lƒ K+ influences water distribution

between intracellular & extracellular fluid

ƒ Also is obligate activator of many enzymes

ƒ Serum level is 0.5 meq/L higher than in whole blood or plasma

K+ Turnover and Renal Handling

ƒ Usual intake is 50 to 150 meq per day

ƒ Usual minimum daily losses are 10 meq each in stool, sweat, & obligatory renal

ƒ Acidosis shifts K+ from intracellular to extracellular fluid–Each pH change by 0.1 changes K+ by 0.6 meq/L in opposite direction

ƒ Secreted & reabsorbed in distal nephron

Methods of K+ Concentration Measurement

ƒ Flame photometry–Dual channel type measures Na & K

ƒ Ion selective electrode–Uses cyclic antibiotic valinomycin (a selective K+ binder) on the liquid ion exchange membrane

ƒ Nuclear magnetic resonance can measure intracellular K+

Causes of Pseudohyperkalemia(Artifactual)

ƒ Traumatic hemolysis from mechanical forces of blood aspiration

ƒ Extreme leucocytosis (> 600,000 cells/mm3)

ƒ Platelet counts > 1,000,000 / mm3ƒ Forearm exercise prior to blood

draw (causes a 20 % increase)ƒ Use of cork instead of rubber

stopper on collection tubes

Causes of Hypokalemiaƒ Inadequate intakeƒ GI losses

–Vomiting, NG suction–Diarrhea, laxative abuse–Villous adenoma–Ureteroenterostomy

ƒ Renal losses–Diuretics–Osmotic diuretics

ƒ Iatrogenicƒ Glucose in DKA

ƒ Hyperaldosteronismƒ Glucocorticoid excessƒ Bartter's Syndromeƒ Licorice ingestionƒ Renal tubular acidosisƒ Renal artery stenosisƒ Postobstructive diuresis

ƒ Alkalosisƒ Familial hypokalemic

paralysisƒ DKA treatment with

insulinƒ Artifactual / lab error

Causes of Hyperkalemiaƒ Excessive intakeƒ Transfusion of old bloodƒ Acute or chronic renal

failureƒ Potassium-sparing

diureticsƒ Hypoaldosteronismƒ Adrenal failureƒ Sickle cell diseaseƒ S. L. E.ƒ Obstructive uropathyƒ Amyloidosisƒ Renal transplant

ƒ Acidosisƒ Rhabdomyolysisƒ Tumor lysis

syndromeƒ Hyperkalemic

periodic paralysisƒ Digitalisƒ Succinylcholineƒ Glucagonƒ Arginineƒ Artifactual

EKG Findings with Altered K+ Levels

ƒ Hypokalemia (< 3.0 meq/L)–Flattening & inversion of T waves–U waves–PAC's, PVC's, 1st & 2nd degree block

ƒ Hyperkalemia (> 6.5 meq/L)–High peaked T waves (in all leads)–Prolonged QRS & PR intervals–Wide complex tachycardias–Sine wave

Bicarbonate (HCO3-) Background Physiology

ƒ 80 % of carbon dioxide transported from tissues to lungs as plasma bicarbonate

ƒ 15 % of CO2 transported as carbamino groups in RBC's & 5 % is dissolved as CO2 or H2CO3 (carbonic acid)

ƒ Carbonic anhydrase catalyzes CO2 + H2O to H2CO3, which then dissociates to H+ and HCO3- –HCO3- is exchanged from RBC's for chloride (Cl-)

ƒ Acute rise in bicarbonate causes biphasic pulmonary response : first hyper- , then hypo- ventilation

Renal Handling of Bicarbonate

ƒ 80 to 90 % of filtered HCO3- is reabsorbed in proximal tubule

ƒ Also reabsorbed from distal tubuleƒ In systemic acidosis, normally no

HCO3- will appear in final urineƒ Kidney is very efficient at

eliminating acute bicarb loads

Bicarbonate Measurement

ƒ The value reported on ABG's is usually just calculated

ƒ Total CO2 content = sum of dissolved CO2 + total carbonic acid (H2CO3) + bicarbonate (HCO3-) + carbamino CO2

ƒ 95 % of total CO2 content contrbuted by HCO3-

ƒ Bicarb level acts as measure of blood buffering capacity

Reference Ranges for Total CO2 Concentration

Specimen Type Reference Range meq/L Reference Range mMol/L

Venous blood, whole 22 to 26 22 to 26

Arterial blood, whole 19 to 24 19 to 24

Venous plasma, serum 23 to 29 23 to 29

Capillary plasma, premie 14 to 27 14 to 27

Cap. plasma, newborn 13 to 22 13 to 22

Capillary plasma, infant 20 to 28 20 to 28

Capillary plasma, child 20 to 28 20 to 28

Capillary plasma, adult 22 to 28 22 to 28

Umbilical cord blood 14 to 22 14 to 22

Potentiometric Method for Total CO2 Measurement

ƒ Measures electrical potential difference between 2 identical ion-selective electrodes in contact with sample solution & reference solution

ƒ Interfered with by Hypaque (radiographic contrast media) or the anticoagulant sodium fluoride

Colorimetric Method for Total CO2 Measurement

ƒ All CO2 forms are converted to HCO3- via an alkaline buffer

ƒ HCO3- then reacts with phosphoenolpyruvate to form oxaloacetate & inorganic phosphate

ƒ Reduction of oxaloacetate to maleate then causes decrease in NADH measureable by change in absorbance at 340 nm, & this is proportional to bicarb in specimen

ƒ Interfered with by fluoride, EDTA, or mercury

Beckman / ASTRA Method for Total CO2 Measurement

ƒ Electrode records rate of pH change of reference bicarb solution as CO2 diffuses thru it from specimen

ƒ pH change is directly proportional to CO2 concentration in specimen

ƒ False elevations occur from increased temperature or WBC's in sample

Anion Gap Measurement

ƒ Anion gap = portion of anions not directly measured by standard tests

ƒ Defined as serum Na+ concentration minus the sum of the serum bicarb and chloride concentrations

ƒ Normal anion gap is 8 to 16 meq/L

Differential Diagnosis of Metabolic Acidosis

ƒ Normal anion gap–GI loss of bicarb–Renal loss of bicarb–Addition of hydrochloric acid

ƒ Elevated anion gap–MUDPILES

Differential Diagnosis of Metabolic Alkalosis

ƒ Saline responsive (urine Cl- < 10 meq/L)–GI losses - vomiting, NG suction, diarrhea–Diuretics–Alkali administration–Posthypercapnic–Non-reabsorbable anion

ƒ Saline resistant (urine Cl- > 10 meq/L)–Mineralocorticoid excess :

ƒ Hyperaldosteronism, Cushing syndromeƒ Licorice ingestion, Bartter's syndrome

–Severe hypokalemia

Physiologic Roles of Chloride Anion

ƒ Important determinant of urine concentration

ƒ Major determinant of ECF volume (with Na+)

ƒ Factor in acid-base and K+ balanceƒ Allows calculation of the anion gapƒ The major anion in the ECFƒ 90 % excreted in urine; some in sweat

& stoolƒ Reabsorbed in nephron

Lab Measurement of Chloride

ƒ Normal range is 98 to 106 meq/Lƒ Pseudohypochloremia caused by :

–Hemolysis–Acute dilutional states

ƒ Pseudohyperchloremia caused by :–Hyperproteinemia–Increased bromide or iodide (suspect with very small or negative anion gap)

Lab Methods for Chloride Measurement

ƒ Spectrophotometric–Chloride reacted with mercury, thiocyanate, & ferric ion to form a red complex measured at 525 nm–Is temperature sensitive

ƒ Ion selective electrodeƒ Beckman / ASTRA

–Columetric generation of silver ions to form silver chloride ; when all Cl- is titrated, free silver ions detected amperometrically–Is method least influenced by other halides

Causes of Hyperchloremia

ƒ Hypoalbuminemiaƒ Bromism, Iodidismƒ Unmeasured non-Na+

cationsƒ GI losses of bicarb

–Diarrhea–GI tract fistulas–Ureterosigmoidostomy–Ileal loop conduit–CaCl2 or MgCl2 ingestion–Cholestyramine ingestion

ƒ Renal losses of bicarb–Renal tubular acidosis–Hypoaldosteronism–Hyperparathyroidism–Carbonic anhydrase inhibitors

ƒ Miscellaneous–Dilutional acidosis–Hyperalimentation–Sulfur ingestion–Compounds with Cl- anion–Chronic respiratory acidosis

Causes of Hypochloremiaƒ NaCl Responsive (urine

chloride < 80 meq/L) :–Vomiting, NG suction–Villous adenoma–Diuretics –Cystic fibrosis–Rapid correction of chronic hypercapnia

ƒ Massive blood transfusionƒ Non-parathyroid

hypercalcemiaƒ Large doses of carbenicillin

or penicillin

ƒ NaCl resistant (urine Cl- > 20 meq/L) :–Excess mineralocorticoid–Hyperaldosteronism–Cushing's Syndrome–Bartter's syndrome–Licorice ingestion–Severe K+ depletion

ƒ Milk-alkali syndromeƒ Alkali administration

Indications to Obtain Serum Electrolytes

ƒ Uncertain hydration statusƒ Suspected adrenal insufficiencyƒ Suspected new onset renal dysfunctionƒ Side effects of meds (digoxin, ACE

inhibitors, diuretics, etc.)ƒ Suspected SIADH (also need urine Na+)ƒ Suspected acid-base disorderƒ New ( only if prolonged postictal or

abnormal mental status) or atypical seizures

Serum Glucose Levelsƒ At birth is 70 to 80 % of maternal level

–Range 30 to 100, mean 50 mg/dl in first few hours–Range 65 to 80 in first few days–By 4 to 10 days, same as in adults

ƒ Normal adult range is 45 to 130 mg/dl (fasting)–Although some are symptomatic at levels below 70 mg/dl

Plasma Versus Whole Blood Glucose Levels

ƒ Plasma contains 15 % more water than whole blood

ƒ So plasma or serum glucose is > whole blood value by factor of 1.15

ƒ Deproteinization of plasma yields increased solute concentration with 5 % increase in measured glucose

ƒ Capillary or arterial values are 10 to 20 mg/dl higher than venous (this effect negligible with fasting)

Artifactual Causes of Hypoglycemia

ƒ Glycolysis from erythrocytes & leucocytes can cause glucose to decrease by 18 % in 30 minutes (even faster in hyperleucocytotic states)

ƒ Methods to prevent glycolysis in sample :–Rapid centrifugation–Add glycolysis inhibitor such as fluoride (however this can inhibit enzymes on test strips)–Rapid cooling on ice

Methods of Glucose Measurement in the Lab

ƒ Spectrophotometric–o-toluidine method being phased out since may be carcinogen

ƒ Enzymatic (uses hexokinase or glucose oxidase)–Glucose oxidized to gluconic acid & H2O2–Peroxidase then oxidizes a chromogen in proportion to the specimen glucose concentration–Or, polarographic method measures O2 consumption in oxidizing the glucose (rate of consumption is proportional to glucose concentration)

Reagent Strip Systems for Rapid Glucose Estimation

ƒ Strips are impregnated with glucose oxidase, peroxidase, & chromogen ; dye response is specific for glucose

ƒ Some use visual comparison to reference color chart & others use reflectance meter

ƒ Good correlation with lab measurement except at very low or very high values

ƒ Some are affected by low or high hematocrits

ƒ Commonly used brands include Dextrostix, Chemstrip bG, Visidex, Stat-Tek, & Glucoscan

Lab Charges at H.M.C. for Electrolytes and Glucose

Test Routine Stat

Electrolyte Panel (Na, K, CO2, Cl)

$ 12 $ 35

Any single electrolyteSingle lyte, nonblood

$ 17 $ 24

$ 26 $ 35

Glucose $ 19 $ 29

Calcium Ionized calcium

$ 19 $ 28

$ 29

Osmolality $ 24 $ 36

Indications to Obtain Serum Glucose Level

ƒ Stat bedside test should be done for almost all patients with altered mental status or focal neurologic signs

ƒ Suspected poorly controlled diabetesƒ New onset glucosuriaƒ NOT needed for simple clear-cut insulin

induced hypoglycemia (initial & followup bedside tests are sufficient if initial low vaue increases with Rx)

ƒ Recurrent infections

Calcium Physiologyƒ Most abundant mineral in body

–1 to 2 grams–99 % in bone

ƒ 40 to 45 % of serum calcium bound to plasma proteins, mostly albumin

ƒ 5 to 10 % is in non-ionized complexes (citrate, phosphate)

ƒ 40 to 50 % is free ionized–the physiologically active form–involved in osteogenesis & osteolysis

Serum Calcium Concentrations

ƒ Normal : 8.5 to 10.5 mg/dlƒ Lower in elderly & pregnancyƒ Venous stasis (prolonged tourniquet) raises

value 10 %ƒ For change in serum protein by 1 gm/dl, serum

calcium changes by 0.8 mg %ƒ Normal ionized calcium range : 1.0 to 1.38

mmol/L or 4 to 4.8 mg %ƒ Acidosis increases ionized fraction

–pH change by 0.1 causes change in calcium of 0.17 mg %

Serum Calcium Measurement

ƒ Most common methods use :–Calcium dialyzed into acid solution of the dye o-cresolphthalein complexone–Base then added & absorption at 570 nm measured

ƒ Automated analyzers : multiple different brands–Only 20 microliters of blood needed–Only takes 10 minutes or less

ƒ Many other methods can be used : precipitation, photometry, polarographic, atomic absorption spectroscopy

Measurement of Serum Ionized Calcium

ƒ Calcium ion selective electrode used–Membrane impregnated with liquid ion-exchange & is saturated with calcium–When serum is pumped thru the electrode, it causes a potential difference between the serum ionized calcium & the liquid ion-exchanger–The potential difference is proportional to the serum ionized calcium concentration–Precision of this method is +/- 2 %

Costs of Serum Calcium Tests

ƒ Serum calcium at H.M.C. : $15.00

ƒ Ionized calcium : $25.00ƒ As part of SMA-12 : $22.50ƒ Increase charge by 50 % for stat

Indications to Obtain a Serum Calcium

ƒ Altered mental status in an ill-appearing patient

ƒ Clinical signs of tetany (for confirmation)

ƒ Recurrent nephrolithiasisƒ Known or suspected malignancies

involving bone

Serum Osmolalityƒ Free permeability of biologic membranes to

water results in osmolal equilibrium throughout body

ƒ Extracellular fluid osmolality equilibrates with intracellular fluid

ƒ Normal serum osmolality is 285 to 295 mosmol/kg

ƒ Osmoreceptors in hypothalamus sense change of less than 2 %–ADH (causes resorption of water in distal nephron) secreted to preserve osmolal homeostasis

Effects of Adding Solute Particlesto a Solution (continuous linear effects)

ƒ Adding one mole (6.02 x 10 particles) to pure water causes :–Osmotic pressure increased–Freezing point decreased (1.86 C)–Vapor pressure decreased (0.3 mm Hg)–Boiling point increased (1.86 C)

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Osmolality Measurementƒ Osmotic coefficients are correction factors that

index degree of variance of solutes from ideal calculated activity

ƒ Osmolality then is measure of concentration of particles in solution–Defined as : 1 osmolal solution contains 1 mole of solute particles per kg of solvent

ƒ Osmolarity is measure of concentration of particles per volume–1 osmolar solution contains 1 mole solute particles per liter of solvent

ƒ Osmolality = osmolarity only when solvent is pure water

Types of Osmometers in Use

ƒ Vapor pressure osmometer–Can accurately analyze grossly lipemic specimens–But is insensitive to volatile solutes–Not preferred for most E.D. needs

ƒ Freezing point osmometer–More useful for E.D.

Principles of Freezing Point Osmometers

ƒ Freezing point = unique temperature at which solid & liquid phases of a substance exist at equilibrium

ƒ Supercooling is the tendency of a solution to remain liquid despite cooling to temp. below its freezing point (by stirring, etc.)

ƒ The energy required to preserve the liquid supercooled state can be measured as the heat generated once crystallization occurs–This is the "heat of fusion" & is measured by the freezing point osmometer–Thermistor coupled to transducer & galvanometer measures liberated heat of fusion when stirring wire rod is stopped

Freezing Point Osmometer

ƒ Can measure samples as small as 0.2 ml

ƒ Error from hemolysis of sample is small

ƒ Sample tube must be covered at all times to prevent loss of volatile CO2

ƒ Measurements still reliable even if delayed, if sample is covered & refrigerated

Reported Measurement Units Requiring Standardization for Osmolality Determinations

ƒ Nonelectrolytes (glucose, alcohol, etc.)–Reported in mg / deciliter–Osmolal contribution is then the reported lab value divided by 1/10 of the gross molecular weight of the compound

ƒ Electrolytes (elemental ions)–Reported as meq / liter–Reported number requires no modification for osmolal contribution

Errors in Osmolality Assumptionsƒ Normal serum is 93 % water

–Lipids & proteins comprise the remaining 7 %

ƒ So osmolarity of serum does not exactly equal osmolality

ƒ Hyperlipidemia to a degree sufficient to cause lactescence requires concurrent vapor pressure measurement for accuracy

ƒ Serum protein content changes (e.g., multiple myeloma) rarely affect calculation significantly

Standard Formula for Calculated Serum Osmolality

ƒ Derived from a study comparing 13 different equations using measurements of different serum components in 705 inpatients

ƒ Calculated osmolality = 2 X Na + glucose/18 + BUN/2.8

The Osmolal Gapƒ Represents measured osmolality (by

lab) > calculated osmolality (by formula)

ƒ Represents unmeasured solutes which often could be toxins

ƒ Always clinically significant if > 20 mosmol/kg

ƒ Should always be determined for suspected non-ethanol alcohol ingestions or other osmotically active suspected ingestants

Expected Osmolal Contributions by Different Alcoholsƒ Ethanol

–measured value in mg/dl divided by 4.6

ƒ Ethylene glycol–measured value in mg/dl divided by 6

ƒ Isopropanol–measured value in mg/dl divided by 6.2

ƒ Methanol–measured value in mg/dl divided by 3.2

ƒ Add these values to the standard calculated osmolality equation

Other Substances Which Can Cause An Osmolal Gap

ƒ Glycerol, mannitol–Iatrogenic for diuresis

ƒ Radiographic contrast mediaƒ Iodine, bromine

–Low anion gap

ƒ Very rarely from high dose antibiotics or other meds