What is suggested by a positive urine anion gap (AG) in non-AG metabolic acidosis?

Updated: Oct 10, 2018
  • Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

The major unmeasured urinary anions and cations are HCO3- and NH4+, respectively. HCO3- excretion in healthy subjects is usually negligible, and average daily excretion of NH4+ is approximately 40 mEq/L, which results in a positive or near-zero gap. In the face of metabolic acidosis, the kidneys increase the amount of NH3 synthesized to buffer the excess H+ and NH4 Cl excretion increases. The increased unmeasured NH4+ thus increases the measured anion Cl- in the urine, and the net effect is a negative AG, representing a normal response to systemic acidification. Thus, the finding of a positive urine AG in the face of non-AG metabolic acidosis points toward a renal acidification defect (eg, renal tubular acidosis [RTA]).


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