Urine anion gap

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]

Pathophysiology sample values
BMP/ELECTROLYTES:
Na+ = 140 Cl− = 100 BUN = 20 /
Glu = 150
\
K+ = 4 CO2 = 22 PCr = 1.0
ARTERIAL BLOOD GAS:
HCO3− = 24 paCO2 = 40 paO2 = 95 pH = 7.40
ALVEOLAR GAS:
pACO2 = 36 pAO2 = 105 A-a g = 10
OTHER:
Ca = 9.5 Mg2+ = 2.0 PO4 = 1
CK = 55 BE = −0.36 AG = 16
SERUM OSMOLARITY/RENAL:
PMO = 300 PCO = 295 POG = 5 BUN:Cr = 20
URINALYSIS:
UNa+ = 80 UCl− = 100 UAG = 5 FENa = 0.95
UK+ = 25 USG = 1.01 UCr = 60 UO = 800
PROTEIN/GI/LIVER FUNCTION TESTS:
LDH = 100 TP = 7.6 AST = 25 TBIL = 0.7
ALP = 71 Alb = 4.0 ALT = 40 BC = 0.5
AST/ALT = 0.6 BU = 0.2
AF alb = 3.0 SAAG = 1.0 SOG = 60
CSF:
CSF alb = 30 CSF glu = 60 CSF/S alb = 7.5 CSF/S glu = 0.6

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]

= Na+ + K+ − 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).

References
  1. Kim GH, Han JS, Kim YS, Joo KW, Kim S, Lee JS (January 1996). "Evaluation of urine acidification by urine anion gap and urine osmolal gap in chronic metabolic acidosis". Am. J. Kidney Dis. 27 (1): 42–7. doi:10.1016/S0272-6386(96)90029-3. PMID 8546137.
  2. "Urine Anion Gap: Acid Base Tutorial, University of Connecticut Health Center". Archived from the original on 2008-11-21. Retrieved 2009-04-11.
  3. Julia A. McMillan; Ralph D. Feigin; Catherine DeAngelis; M. Douglas Jones (1 April 2006). Oski's pediatrics: principles & practice. Lippincott Williams & Wilkins. pp. 1–. ISBN 978-0-7817-3894-1. Retrieved 14 November 2010.
  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 (Oct 1986). "The urine anion gap: a clinically useful index of ammonium excretion". The American Journal of the Medical Sciences. 292 (4): 198–202. doi:10.1097/00000441-198610000-00003. PMID 3752165. S2CID 38506494.
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