Urine Osmolal Gap Calculator
Urine Osmolal Gap Calculator
Estimate urinary ammonium excretion from the difference between measured and calculated urine osmolality — the more robust alternative to the urine anion gap.
Urine Osmolal Gap
Measured − calculated UosmMeasured osmolality 600 mOsm/kg, urine Na 40, K 30, urea 300, glucose 5 mmol/L
Formula
Urine osmolal gap = measured Uosm − calculated Uosm
Urinary ammonium ≈ gap ÷ 2
- ×2 on Na and K
- each cation is accompanied by an anion, so the measured cations are doubled to account for their partners
- urea and glucose
- entered in mmol/L; if the laboratory reports urine urea nitrogen in mg/dL divide by 2.8, and glucose in mg/dL by 18, before entering them
- gap ÷ 2
- ammonium is excreted with an anion, so the unmeasured ammonium salt contributes roughly twice its own concentration to the osmolality
Worked example
Measured osmolality 600 mOsm/kg, urine Na 40, K 30, urea 300, glucose 5 mmol/L
2 × (40 + 30) = 140
Calculated osmolality = 140 + 300 + 5 = 445 mOsm/kg
600 − 445 = 155 mOsm/kg
Estimated ammonium ≈ 155 ÷ 2 ≈ 78 mmol/L → appropriate excretion
Interpreting the gap in a normal anion gap acidosis
| Urine osmolal gap | Estimated ammonium | Interpretation |
|---|---|---|
| < 100 mOsm/kg | < 50 mmol/L | Impaired acid excretion — renal tubular acidosis |
| 100 – 150 mOsm/kg | 50 – 75 mmol/L | Equivocal; interpret with urine pH and potassium |
| > 150 mOsm/kg | > 75 mmol/L | Appropriate excretion — gastrointestinal bicarbonate loss |
Unit conversions before entry
| Analyte | Reported as | To mmol/L |
|---|---|---|
| Urine urea nitrogen | mg/dL | divide by 2.8 to give urea in mmol/L |
| Urine urea (as urea, not nitrogen) | mg/dL | divide by 6.0 |
| Urine glucose | mg/dL | divide by 18 |
| Urine sodium, potassium | mmol/L or mEq/L | numerically identical for these monovalent ions |
What the gap stands in for
The urine osmolal gap is a surrogate for urinary ammonium, which most laboratories cannot measure directly. Ammonium salts are osmotically active but are not among the solutes counted in the calculation, so whatever is measured but not accounted for is largely ammonium and its accompanying anion. Because each ammonium ion is excreted with an anion, the practical rule is that ammonium in mmol/L is roughly half the gap.
The question this answers arises in a normal anion gap, or hyperchloraemic, metabolic acidosis: is the kidney excreting acid as it should? A gap above about 150 mOsm/kg says that it is, which points away from the kidney and towards gastrointestinal bicarbonate loss — diarrhoea, a high-output stoma, or a urinary diversion. A gap below 100 says that it is not, and points to a renal tubular acidosis, after which urine pH and serum potassium help separate type 1, type 2 and type 4.
The urine anion gap answers exactly the same question and is easier to obtain, but it fails whenever unmeasured anions are present in quantity. Ketones, hippurate from toluene sniffing, D-lactate, penicillins and other administered anions all accompany ammonium into the urine and distort the anion gap, which is precisely the situation in which the osmolal gap is preferred. A markedly positive urine anion gap in a patient with ketoacidosis is a classic trap of this kind.
Two practical points. The urea and glucose terms are entered in mmol/L: if the laboratory reports urine urea nitrogen in mg/dL divide by 2.8, and glucose in mg/dL by 18, before entering them, or the calculated osmolality will be badly wrong. The 2.8 is worth being careful with, because it converts urea nitrogen rather than urea — a US report of urine urea means urea nitrogen, but a value genuinely reported as whole urea in mg/dL needs dividing by 6.0 instead. And the gap is only interpretable on a urine that is acidifying in response to a systemic acidosis — mannitol, radiographic contrast, alcohols and other unmeasured osmoles will widen it for reasons that have nothing to do with ammonium.
Frequently asked questions
How is the urine osmolal gap calculated?
Subtract the calculated osmolality from the measured osmolality, where calculated osmolality is 2 × (urine sodium + urine potassium) + urine urea + urine glucose, all in mmol/L. The result approximates twice the urinary ammonium concentration.
What does a low urine osmolal gap mean?
In a normal anion gap metabolic acidosis, a gap below about 100 mOsm/kg indicates that ammonium excretion is impaired, pointing to a renal tubular acidosis rather than gastrointestinal bicarbonate loss.
Why use the osmolal gap instead of the urine anion gap?
The urine anion gap fails when unmeasured anions such as ketones, hippurate, D-lactate or penicillins are present, because they are excreted alongside ammonium and distort the calculation. The osmolal gap is unaffected by which anion accompanies the ammonium.
What units should urea and glucose be entered in?
Millimoles per litre. If the laboratory reports urine urea nitrogen in mg/dL divide by 2.8, and urine glucose in mg/dL by 18, before entering them. The divisor 2.8 converts urea nitrogen, which is what a US report means by urine urea; a value reported as whole urea in mg/dL needs dividing by 6.0.
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References
- Kamel KS, Ethier JH, Richardson RMA, Bear RA, Halperin ML. Urine electrolytes and osmolality: when and how to use them. Am J Nephrol. 1990;10(2):89–102.
- Halperin ML, Goldstein MB, Kamel KS. Fluid, Electrolyte and Acid-Base Physiology: A Problem-Based Approach. 5th ed. Elsevier; 2017.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.