Calculated Chemistry Valuescontinued Clinical Chemistry Review 130
VALUE CALCULATION NORMAL RANGE CLINICAL SIGNIFICANCE
Calculated
osmolality
Osmolal gap
Urine-to-serum
osmolality
Formula 1:
1.86 Na+mEq/L +
glucose mg/dL
+
BUN mg/dL
+ 9
18 2.8
Formula 2:
2 Na+mEq/L +
glucose mg/dL
+
BUN mg/dL
20 3
Measured osmolality – calculated osmolality
Urine osmolality
Serum osmolality
275–295 Osm/kg
0–10 mOsm/kg
1–3
Concentration of solute. Electrolytes
contribute most. One of colligative
properties. ↑dehydration, uremia,
uncontrolled diabetes, alcohol or
salicylate intoxication, excessive
electrolyte IVs. ↓excessive water
intake.
Similar to anion gap but based on
osmotically active solute concen-
tration rather than concentration
of ions. >10 indicates abnormal
concentration of unmeasured
substance (ex: isopropanol,
methanol, acetone, ethylene
glycol). Used to Dx poisonings.
↓renal tubular deficiency,
diabetes insipidus.
