- Use in normal-anion-gap (hyperchloremic) metabolic acidosis to estimate urine ammonium (NH₄⁺) excretion when the urine anion gap is confounded.
- Enter a measured urine osmolality plus urine Na, K, urea, and glucose from the same specimen (pick each unit).
- The tool computes the calculated urine osmolality, the osmolal gap, and an indirect ammonium estimate (≈ half the gap).
- Do not use a calculated urine osmolality as the "measured" input, and do not calculate when exogenous osmoles (mannitol, contrast, alcohols) are suspected.
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When to Use
The urine osmolal gap (UOG) is a second-line surrogate for urine ammonium, useful in normal-anion-gap metabolic acidosis when the urine anion gap fails — for example when ammonium is excreted with a non-chloride anion (ketoanions, hippurate) or urine sodium is low. Ammonium salts are unmeasured osmoles, so a large measured-minus-calculated osmolal gap implies high ammonium output.
Appropriate setting
Confirmed normal-anion-gap metabolic acidosis where you need to know whether the kidney is excreting ammonium appropriately (extrarenal/GI cause) or not (RTA), and the UAG is unreliable.
When not to rely on it
Requires a truly measured urine osmolality. Invalid with exogenous osmoles (mannitol, radiocontrast, ethanol/methanol/ethylene glycol) and unreliable in the same non-steady-state and AKI settings that limit any urine index. Direct urine ammonium, where available, is preferable.
Pearls & Pitfalls
Large gap = high ammonium = appropriate response
A urine ammonium estimate above roughly 200 mmol/L (osmolal gap > ~400 mOsm/kg) in NAGMA fits intact renal acidification — pointing extrarenal (diarrhea, GI bicarbonate loss).
Small gap = low ammonium = impaired response
A urine ammonium estimate below roughly 100 mmol/L (osmolal gap < ~200 mOsm/kg) suggests impaired ammonium excretion — the hallmark of distal RTA or type 4 RTA / hypoaldosteronism.
Critical pitfalls
(1) The ammonium value is an indirect estimate, not a measurement. (2) Use the laboratory-specific calculated-osmolality formula. (3) Exogenous osmoles invalidate the gap. (4) Half-the-gap is a convention, not a constant — treat trends, not decimals.
Why Use It
The urine anion gap assumes ammonium leaves the body as ammonium chloride, so it breaks when ammonium pairs with another anion. The osmolal gap sidesteps that: ammonium and its counter-anion are both unmeasured particles, so they widen the difference between the measured urine osmolality and the osmolality calculated from Na, K, urea, and glucose — regardless of which anion carries the ammonium. It is the right tool exactly where the UAG is least trustworthy.
Urine Osmolal Gap → Ammonium Estimate
Enter a measured urine osmolality and the urine solutes from the same specimen. The ammonium estimate is explicitly indirect.
⚕ Calculated UOsm ≈ 2×(Na+K) + urea + glucose (mmol/L). Estimated NH₄⁺ ≈ gap ÷ 2 — an indirect surrogate only. Invalid with exogenous osmoles or a calculated (not measured) osmolality.
Next Steps
- High ammonium estimate (GI cause suggested): confirm diarrhea / bicarbonate loss; volume and alkali repletion; correct hypokalemia.
- Low ammonium estimate (RTA suggested): separate distal (type 1) from type 4 RTA using serum potassium, urine pH, and the aldosterone axis; start alkali therapy; refer.
- Prefer direct urine ammonium when the laboratory can measure it.
Evidence & References
Formula
| Quantity | Equation |
|---|---|
| Calculated UOsm | 2 × (Urine Na + Urine K) + Urine urea + Urine glucose (mmol/L) |
| Urine osmolal gap | Measured UOsm − Calculated UOsm |
| Estimated urine NH₄⁺ | ≈ Osmolal gap ÷ 2 (indirect) |
| BUN mg/dL → urea mmol/L | × 0.357 |
| Glucose mg/dL → mmol/L | ÷ 18 |
