Kidney role on acid-base balance: urinary anion-gap versus urinary sodium - chloride difference

Introduction Urinary Anion Gap (uAG = uNa+ + uK+ – uCl-) traditionally has been considered a parameter of kidney response to an acid-base derangement. In Stewart’s approach, however, Na+ and Cl- alone represent the key determinants of plasmatic Strong Ion Difference (SID), since K+ is mainly intra-cellular and its renal excretion is due to multiple factors. Aim We aimed to test the hypothesis that the difference between urinary uNa+ and uCl+ better reflects the renal influence on extra-cellular acid-base status in ICU patients. Methods Fifty-seven patients without kidney disease were enrolled in post-operative ICU. BE variations (ΔBE) were evaluated between admittance and 9 a.m. of the following morning. Urine throughout the study period (18.4±2 hours) was collected for analysis of sodium (uNa+), potassium (uK+) and chloride (uCl-) concentrations. Patients were first divided by uAG quartiles (Q1-4), they were then divided by uNa+ – uCl- difference tertiles. Results uAG was mainly determined by uK+ as shown in Tab.1 (p<0.001). ΔBE did not vary in patients sorted by uAG quartiles (p=0.15). In contrast, patients with low and negative uNa+ – uCl- difference showed a greater ΔBE than patients with uNa+ – uCl- difference > 0 mEq/L. Conclusions Urinary Anion Gap mainly depends on potassium excretion, which does not seem to be directly associated with extra-cellular acid-base balance. These results suggest that urinary Na+ – Cl- difference better reflects the modification occurring in the extra-cellular acid-base equilibrium.