Laboratory Test Reference Guide

([SOD] + [POT]) - [CHL], All units are in mmol/L. When considering Renal Tubular Acidosis, the AG serves as an indirect measure of ammonium ion (NH4+) excretion. A negative AG indicates increased NH4+ excretion due to the relative increase in the measured counter ion (chloride). In the presence of systemic acidosis, a negative AG indicates enhanced ammoniagenesis (pRTA) whilst a positive AG suggests altered distal urinary acidification (distal or type IV RTA). Note: The reliability of this test is reduced if the urine pH is >6.5 and/or other unmeasured ions (eg drugs, ketones) are present.

Reference: "Renal Tubular Acidosis" - Ann' Clin Biochem 1999; 36: 416.