Sodium-chloride difference is not strongly correlated with base excess in chronic kidney disease: an anion gap problem.

Introduction: The prevalence of metabolic acidosis is high in patients with chronic kidney disease (CKD). For the diagnosis, a blood gas analysis is necessary, but not always available. The aim of the study was to evaluate the base excess (BE) of the sodium-chloride difference (BE = Na-Cl-34 mmol/l) as a screening parameter for hyperchloremic metabolic acidosis.

Methods And Statistical Analysis: We retrospectively performed acid-base analyses of 168 non-dialysed patients with CKD according to the physiologic and to the Stewart's approach. We performed linear regression analysis, Bland-Altman plot and receiver operating characteristics (ROC) analysis of BE and BE to evaluate the accuracy of BE predicting the BE. We further investigated possible confounding factors.

Results: The corrected R for the correlation of BE and BE was 0.60 (p < 0.001). The Bland-Altman plot showed a good overall agreement. The bias was negligible, but the 95%-limits of agreement showed a wide interval (10.4 mmol/l). For BE ≤ 2 mmol/l, the ROC analysis yielded an AUC of 0.89 and moderate sensitivity (0.75) and specificity (0.86) for the optimal BE threshold (≤ 2 mmol/l). Subgroup analysis showed similar results. The main factor for the imprecision of BE predicting the BE across all stages of CKD is the variability of the serum anion gap (SAG).

Conclusions: The BE is not an adequate parameter for screening of hyperchloremic acidosis because of the high variability of the SAG. Only, if the BE is ≤ 5 mmol/l, a hyperchloremic acidosis should be suspected. Therefore, a complete blood gas analysis is necessary for the correct diagnosis of acid-base disorders in patients with chronic kidney disease.