The urine anion gap is calculated using measured ions found in the urine. It is used to aid in the differential diagnosis of metabolic acidosis. Urine anion gap is calculated by subtracting the urine concentration of chloride (anions) from the concentrations of sodium plus potassium (cations).

Formula

Anion Gap = (Na+) + (K+) - (Cl-)
Each of these items are measured in either mEq/L or mmol/L.

Urine anion gap is an artificial and calculated measure that is representative of the unmeasured ions in urine. Usually the most important unmeasured ion in urine is NH ₄ ⁺ .

In normal subjects, the urine anion gap is usually near zero or is positive.

In metabolic acidosis, the excretion of NH ₄ ⁺ (which is excreted with Cl ^- ) should increase markedly if renal acidification is intact. Because of the rise in urinary Cl ^- , the urine anion gap becomes negative, ranging from -20 to more than -50 meq/L. A negative value occurs because the Cl ^- concentration now exceeds the sum total of the cations (Na ⁺ and K ⁺ ).

In renal tubular acidosis, where there is inability of the kidneys to excrete NH ₄ ⁺ , Cl ^- ions will not be increased in urine and the urine anion gap will not be affected and will be positive or zero.

• A positive urine anion gap suggests a low urinary NH ₄ ⁺ ( e.g. renal tubular acidosis)
• A negative urine anion gap suggests a high urinary NH ₄ ⁺ ( e.g. diarrhea)
• Metabolic acidosis
  • Anion Gap < 0 indicates a nonrenal cause
  • Anion Gap > 0 indicates a renal cause

References:

Acid base physiology. The urinary anion gap available at http://www.anaesthesiamcq.com/AcidBaseBook/ab3_4.php . Last accessed 23 March 2009.

Muthukrishnan J, et al. Distal renal tubular acidosis due to primary hyperparathyroidism. Endocr pract. 2008 December;14(9):1133-6.