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Urine Osmolal Gap

Urine osmolal gap = measured urine osmolality − [2 × (urine Na + K) + urine urea nitrogen/2.8 + urine glucose/18]; urine NH₄⁺ ≈ gap ÷ 2. In a normal-anion-gap metabolic acidosis a gap below ~150 mOsm/kg means inappropriately low ammonium excretion (renal tubular acidosis); much higher values indicate an appropriate renal response (e.g. diarrhoea).

  • Measured urine osmolality (mOsm/kg)
  • Urine sodium (mmol/L or mEq/L)
  • Urine potassium (mmol/L or mEq/L)
  • Urine urea nitrogen (mg/dL or mmol/L)
  • Urine glucose (mg/dL or mmol/L)
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Interpretation

ResultMeaning
< 150Low NH₄⁺ excretion: RTA likely (in NAGMA)
≥ 150Appropriate NH₄⁺ excretion: GI bicarbonate loss likely (in NAGMA)
> 400Brisk ammoniagenesis

Formula

UOG = measured Uosm − [2 × (UNa + UK) + UUN (mg/dL)/2.8 + Uglucose (mg/dL)/18]. SI: 2(UNa + UK) + Uurea + Uglucose (mmol/L). Urine NH₄⁺ ≈ UOG ÷ 2.

Clinical pearls

  • More reliable than the urine anion gap when unmeasured anions (ketoacids, hippurate from toluene, β-lactams) are in the urine.
  • Only interpretable in a patient who actually has a metabolic acidosis.
  • Thresholds vary between sources (100–150 mOsm/kg); the 150 cut-off corresponds to urine NH₄⁺ ≈ 75 mmol/L.

Examples

urine osmolal gap uosm 600 una 50 uk 30 uun 280urine osm gap: urine osmolality 400, urine Na 60, urine K 40, urine urea nitrogen 336 mg/dL

FAQ

Urine anion gap or urine osmolal gap?

The urine osmolal gap is preferred when the urine contains unmeasured anions such as ketones, or when urine sodium is low, because the urine anion gap then misestimates ammonium.

How do I estimate urine ammonium?

Divide the urine osmolal gap by 2, because ammonium is excreted with an accompanying anion.

Evidence

  1. Kim GH et al. Evaluation of urine acidification by urine anion gap and urine osmolal gap in chronic metabolic acidosis. Am J Kidney Dis 1996;27:42–7.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology