Urine anion gap

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	Mg^2+ = 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 = 0.4

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]

= 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.

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.^[4] 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". 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. 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.