Urine anion and osmolality gaps in the comprehensive evaluation of acid base balance across different levels of kidney function.

Ammonium excretion is often estimated using urinary anion gap (UAG) and osmolality gap (UOG) when direct measurement is unavailable, particularly in patients with impaired kidney function. We aimed to examine how UAG and UOG vary across different levels of kidney function and their relationships with acid-base parameters. This retrospective cohort study included 531 outpatients categorized by estimated glomerular filtration rate (eGFR: ≥ 90, 60-89, 45-59, 30-44, and < 30 mL/min/1.73 m). UAG values increased significantly with declining eGFR (p = 0.002), whereas UOG showed no clear trend (p = 0.303). UAG correlated positively with serum pH (ρ = 0.135, p = 0.002) and urinary pH (ρ = 0.333, p < 0.001). UOG was strongly inversely correlated with urinary pH (ρ = -0.512, p < 0.001). Sensitivity analyses stratified by serum anion gap and bicarbonate substantiated UAG's relationship with eGFR. Multivariate analyses confirmed UAG's association with eGFR, serum pH, and urinary pH, whereas UOG was predominantly influenced by urinary pH and minimally by eGFR. Although UOG has traditionally been considered superior, our findings suggest that UAG demonstrated more consistent associations with kidney function parameters. However, direct NH₄⁺ measurements remain necessary for definitive clinical assessment.