Urine Anion Gap
Enter urine electrolyte concentrations (mEq/L) to compute the urine anion gap and assess renal ammonium excretion.
Urine anion gap
5.0 mEq/L
How to Use This Calculator
Collect urine electrolytes
Use a fresh spot urine sample. Measure sodium, potassium, and chloride concentrations in mEq/L.
Calculate the gap
The calculator computes Na⁺ + K⁺ − Cl⁻. A negative value indicates ammonium excretion with unmeasured anions (NH₄Cl).
Interpret in context
Use the result to differentiate renal from extrarenal causes of normal anion gap metabolic acidosis and guide further testing.
Formula
Urine anion gap (mEq/L) = Urine Na⁺ + Urine K⁺ − Urine Cl⁻
Negative values (e.g., < −20) suggest high ammonium excretion (extrarenal bicarbonate loss). Positive values (e.g., > +20) suggest reduced ammonium generation (renal causes).
Full Description
The urine anion gap estimates urinary ammonium excretion indirectly by assuming NH₄⁺ is balanced by unmeasured anions (mostly Cl⁻). In normal anion gap metabolic acidosis, a negative gap indicates intact renal acid excretion (suggesting gastrointestinal bicarbonate loss), whereas a positive gap points to impaired distal acidification. Always correlate with clinical status, urine osmolal gap, and other renal tubular acidosis diagnostics.
Frequently Asked Questions
What is a normal urine anion gap?
In metabolic acidosis, values near zero or negative are expected as kidneys excrete NH₄⁺ with Cl⁻. Positive values indicate renal acidification defect.
How does diuretic use affect interpretation?
Loop or thiazide diuretics increase Na⁺/K⁺ excretion, potentially making the gap positive despite adequate NH₄⁺ excretion. Consider urine osmolal gap instead.
What if chloride is unavailable?
The urine anion gap cannot be calculated without chloride. Obtain a sample or use urine osmolal gap as an alternative.
Is the urine osmolal gap better?
Urine osmolal gap directly estimates NH₄⁺ and may be more reliable in some settings, but requires measured osmolality. Use both when possible.