The elevation of the anion gap in steady state chronic kidney disease may be less prominent than generally accepted.

Background: A presumed cause of metabolic acidosis in chronic kidney disease (CKD) is accumulation of unmeasured anions, leading to a high anion gap (AG). In patients with CKD with a high AG, only minor increases are expected. The aim of this study is to evaluate the magnitude of the AG in documented steady state CKD to examine the effect of CKD on a high-AG metabolic acidosis (HAGMA).

Methods: In this cross-sectional study the AG, bicarbonate, and chloride were evaluated in 1045 blood and urine samples of 501 patients with steady state CKD in the outpatient clinic. The influence of phosphate, albumin and potassium on the AG were evaluated.

Results: The mean AG increased from 8.8 mEq/l (±1.57) in CKD stage 1 to 11.2 mEq/l (±2.22) in CKD stage 5 ( < 0.001). Correction for albumin or phosphate did not influence the magnitude of the AG. Correction for potassium did alter the prevalence of HAGMA, but not the severity. [HCO] decreased between CKD stages 1 and 5 by 5.1 mEq/l. The [Cl] increased by 2.6 mEq/l between CKD stages 1 and 5.

Conclusions: The elevation of the AG in patients with steady state CKD is limited and less pronounced than the decrease in [HCO]. Normal AG metabolic acidosis seems to be more important in CKD than HAGMA. The CKD stage and the magnitude of the AG should be taken into account when evaluating a patient with HAGMA. This study suggests that an AG >15 mEq/l is rarely due to renal failure alone.