Urine anion gap explained

In clinical chemistry, the urine anion gap is calculated using measured ions found in the urine.^[1] It is used to aid in the differential diagnosis of metabolic acidosis.^[2]

The term "anion gap" without qualification usually implies serum anion gap. The "urine anion gap" is a different measure, principally used to determine whether the kidneys are capable of appropriately acidifying urine.

Calculation

Urine anion gap is calculated by subtracting the urine concentration of chloride (anions) from the concentrations of sodium plus potassium (cations):^[3]

$$\backslash text\; =\; [\backslash ce\{Na^2+\}]\; +\; [\backslash ce\{K+\}]\; -\; [\backslash ce\{Cl-\}]$$

where the concentrations are expressed in units of milliequivalents/liter (mEq/L).

In contrast to the serum anion gap equation, the bicarbonate is excluded. This is because urine is acidic, so the bicarbonate level would be negligible.^[4]

Uses

Determining the cause of a metabolic acidosis that lacks a serum anion gap often depends on determining whether the kidney is appropriately excreting acid. The 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 NH₄⁺ since it is the most important form of acid excretion by the kidney.^[5] Urine NH₄⁺ is difficult to measure directly, but its excretion is usually accompanied by the anion chloride. A negative urine anion gap can be used as evidence of increased NH₄⁺ excretion. In a metabolic acidosis without a serum anion gap:

• A positive urine anion gap suggests a low urinary NH₄⁺ (e.g. renal tubular acidosis).
• A negative urine anion gap suggests a high urinary NH₄⁺ (e.g. diarrhea).

Notes and References

1. Kim GH, Han JS, Kim YS, Joo KW, Kim S, Lee JS . Evaluation of urine acidification by urine anion gap and urine osmolal gap in chronic metabolic acidosis . Am. J. Kidney Dis. . 27 . 1 . 42–7 . January 1996 . 8546137 . 10.1016/S0272-6386(96)90029-3.
2. Web site: Urine Anion Gap: Acid Base Tutorial, University of Connecticut Health Center . 2009-04-11 . https://web.archive.org/web/20081121214116/http://fitsweb.uchc.edu/student/selectives/TimurGraham/Urine_Anion_Gap.html . 2008-11-21 . dead .
3. Book: Julia A. McMillan. Ralph D. Feigin. Catherine DeAngelis. M. Douglas Jones. Oski's pediatrics: principles & practice. 14 November 2010. 1 April 2006. Lippincott Williams & Wilkins. 978-0-7817-3894-1. 1–.
4. Batlle DC, Hizon M, Cohen E, Gutterman C, Gupta R. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med. 1988;318(10):594-9.
5. Goldstein. MB. Bear, R . Richardson, RM . Marsden, PA . Halperin, ML . The urine anion gap: a clinically useful index of ammonium excretion.. The American Journal of the Medical Sciences. Oct 1986. 292. 4. 198–202. 3752165 . 10.1097/00000441-198610000-00003. 38506494.