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Laboratory Calculations


ANION GAP

Definition: The difference in concentration between unmeasured cation and anion equivalents in serum.
Anion gap = Na+ – (Cl + HCO3 )
(The normal anion gap is 10 to 14 mEq/L)
Differential Diagnosis of Increased Anion Gap Acidosis
Organic anions
Lactate (sepsis, hypovolemia, seizures, large tumor burden)
Pyruvate
Uremia
Ketoacidosis (beta-hydroxybutyrate and acetoacetate)
Amino acids and their metabolites
Other organic acids (eg, formate from methanol, glycolate from ethylene glycol)
Inorganic anions
Hyperphosphatemia
Sulfates
Nitrates
Medications and toxins
Penicillins and cephalosporins
Salicylates (including aspirin)
Cyanide
Carbon monoxide
Differential Diagnosis of Decreased Anion Gap
Organic cations
Hypergammaglobulinemia
Inorganic cations
Hyperkalemia
Hypercalcemia
Hypermagnesemia
Medications and toxins
Lithium
Hypoalbuminemia


OSMOLALITY

Definition: The summed concentrations of all osmotically active solute particles
Predicted serum osmolality =
mOsm/L=(2 x serum Na++)+serum glucose
18
+BUN
2.8
The normal range of serum osmolality is 285 to 295 mOsm/L.
Calculated Osm
Note: Osm is a term used to reconcile osmolality and osmolarity
Osmol gap = measured Osm – calculated Osm
0 to +10: Normal
>10: Abnormal
<0: Probable lab or calculation error
Differential Diagnosis of Increased Osmol Gap
(increased by >10 mOsm/L)
Ethanol
Ethylene glycol
Glycerol
Iodine (questionable)
Isopropanol (acetone)
Mannitol
Methanol
Sorbitol


CORRECTED SODIUM FOR HYPERGLYCEMIA

Corrected Na+ = serum Na+ + [1.5 x (glucose – 150 divided by 100)]
Note: Do not correct for glucose <150.

CORRECTED TOTAL SERUM CALCIUM FOR ALBUMIN LEVEL

[(Normal albumin – patient's albumin) x 0.8] + patient's measured total calcium

BICARBONATE DEFICIT

HCO3  deficit = (0.4 x wt in kg) x (HCO3 desired – HCO3  measured)
Note: In clinical practice, the calculated quantity may differ markedly from the actual amount of bicarbonate needed or that which may be safely administered.