Use of Sodium-Chloride Difference and Corrected Anion Gap as Surrogates of Stewart Variables in Critically Ill Patients

Introduction: To investigate whether the difference between sodium and chloride ([Na+] – [Cl−]) and anion gap corrected for albumin and lactate (AGcorr) could be used as apparent strong ion difference (SIDapp) and strong ion gap (SIG) surrogates (respectively) in critically ill patients. Methods: A total of 341 patients were prospectively observed; 161 were allocated to the modeling group, and 180 to the validation group. Simple regression analysis was used to construct a mathematical model between SIDapp and [Na+] – [Cl−] and between SIG and AGcorr in the modeling group. Area under the receiver operating characteristic (ROC) curve was also measured. The mathematical models were tested in the validation group. Results: in the modeling group, SIDapp and SIG were well predicted by [Na+] – [Cl−] and AGcorr (R2 = 0.973 and 0.96, respectively). Accuracy values of [Na+] – [Cl−] for the identification of SIDapp acidosis ( 47.5 mEq/L) were 0.992 (95% confidence interval [CI], 0.963–1) and 0.998 (95%CI, 0.972–1), respectively. The accuracy of AGcorr in revealing SIG acidosis (>8 mEq/L) was 0.974 (95%CI: 0.936–0.993). These results were validated by showing excellent correlations and good agreements between predicted and measured SIDapp and between predicted and measured SIG in the validation group (R2 = 0.977; bias = 0±1.5 mEq/L and R2 = 0.96; bias = −0.2±1.8 mEq/L, respectively). Conclusions: SIDapp and SIG can be substituted by [Na+] – [Cl−] and by AGcorr respectively in the diagnosis and management of acid-base disorders in critically ill patients.