Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1094 page)

   Ethanol; an especially large osmolal gap with a low or only moderately elevated ethanol level should raise the possibility of another low molecular weight toxin (e.g., methanol).

   Methanol

   Isopropyl alcohol

   Mannitol (osmolal gap can be used to detect accumulation of infused mannitol in serum)

   Ethylene glycol, acetone, ketoacidosis, and paraldehyde result in relatively small osmolal gaps, even at lethal levels

   Severely ill patients, especially those in shock, acidosis (lactic, diabetic, alcoholic), renal failure.

   Limitations

   Laboratory analytic error

   Random error from all measurements could add or subtract ≤15 mOsm/kg

   Use of incorrect blood collection tubes

OSMOLALITY, SERUM AND URINE

   Definition

   Osmolality refers to the osmotic concentration of a fluid. The osmolality of serum, urine, or any other body fluid depends on the number of active ions or molecules in a solution and yield important information about a patient’s ability to maintain a normal fluid balance status. Osmolality is measured with an osmometer by freezing point depression or vapor pressure elevation techniques, or it can be calculated from a formula.

   Osmolarity is the osmotic concentration of solution expressed as osmoles of solute per liter of solution or the property of solution that depends on the concentration of solute per unit of total volume of solvent.