definitions & calculations
Note: Use of equivalents and milliequivalents is valid only for those substances that have fixed ionic valences (e.g. sodium, potassium, calcium, chlorine, magnesium bromine, etc). For substances with variable ionic valences (e.g. phosphorous), a reliable equivalent value cannot be determined. In these instances, one should calculate millimoles (which are fixed and reliable) rather than milliequivalents.
valences and atomic weights of selected ions
Note: The molecular weight of phosphorus only is approximately 31 grams, and sulfur only is 32 grams.
Milliequivalent conversation equations are shown in the table below. Selected approximate milliequivalents and weights of selected ions are also listed below.
definitions and equations
Select abbreviations, definitions, and disorders listed below.
Osmolality: The summed concentrations of all osmotically active solute particles
Normal range: 285-295mOsm/L
Predicted serum osmolality: 2 Na+ + (glucose (mg/dL) / 18) + (BUN (mg/dL) / 2.8)
Differential diagnosis of increased serum osmolal gap (>10mOsm/L): medications and toxins, alcohols (ethanol, methanol, isopropanol, glycerol, ethylene glycol), mannitol.
Osmolal gap = measure Osm - calculated Osm
Normal: 1 to 10
Probably lab or calculation error: <0
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.
Anion gap: The difference in concentration between unmeasured cation and anion equivalents in serum.
Anion gap = Na+ - (Cl + HCO3-)
The normal anion gap is 10-14mEq/L
Differential diagnosis of increased anion gap acidosis: organic anions, lactate (sepsis, hypovolemia, seizures, large tumor burden), pyruvate, uremia, ketoacidosis (hydroxybutyrate and acetoacetate), amino acids and their metabolites, other organic acids, inorganic anions, hyperphosphatemia, sulfates, nitrates.
Differential diagnosis of decreased anion gap: organic cations, hypergammaglobulinemia, inorganic cations, hyperkalemia, hypercalcemia, hypermagnesemia, medications and toxins, lithium, hypoalbuminemia.