Urine osmolal gap (urinary NH4+ estimate)
Urine osmolal gap (Halperin 1988): the urinary ammonium estimate that separates extrarenal from renal non-anion-gap acidosis.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Measured urine osmolality
- 600 mOsm/kg
- Urine sodium
- 40 mEq/L
- Urine potassium
- 20 mEq/L
- Urine urea nitrogen
- 280 mg/dL
- Urine glucose
- 0 mg/dL
Result: Urine osmolal gap 380 mOsm/kg (calculated osm 220; ~NH4+ 190 mEq/L): a wide gap, consistent with intact urinary ammonium excretion (extrarenal cause, e.g. diarrhea).
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Reading
- in a non-anion-gap acidosis, a wide gap (intact NH4+ excretion) points to an extrarenal cause such as diarrhea; a narrow gap points to renal tubular acidosis.
What you enter
- Measured urine osmolality
- Urine sodium
- Urine potassium
- Urine urea nitrogen
- Urine glucose
How this is calculated
- Formula
- calculated urine osm = 2 x (Na + K) + urea nitrogen(mg/dL)/2.8 + glucose(mg/dL)/18; gap = measured - calculated; gap/2 ~ urinary NH4+.
Halperin ML, Goldstein MB, Stinebaugh BJ, Jungas RL. The urine osmolal gap: a clue to estimate urine ammonium in "hybrid" types of metabolic acidosis. Clin Invest Med. 1988;11(3):198-202. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.