Calculated Uosm = 2×(UNa + UK) + UUrea/2.8 + UGlucose/18 Osmolal Gap = Measured Uosm − Calculated Uosm Est. NH₄⁺ ≈ Osmolal Gap / 2
Normal gap: <10–15 mOsm/kg
Clinical Use in Acidosis: Urinary osmolal gap correlates with NH₄⁺ excretion. Gap >400 mOsm/kg indicates robust renal acid excretion (appropriate response). Low gap in acidosis suggests impaired NH₄⁺ excretion (distal RTA or type IV RTA). Complements the urine anion gap when NH₄Cl is excreted without Cl (e.g., with ketones or other anions present).
