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