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 (AG(corr)) could be used as apparent strong ion difference (SID(app)) 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 SID(app) and [Na(+)] – [Cl(−)] and between SIG and AG(corr) 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, SID(app) and SIG were well predicted by [Na(+)] – [Cl(−)] and AG(corr) (R(2) = 0.973 and 0.96, respectively). Accuracy values of [Na(+)] – [Cl(−)] for the identification of SID(app) acidosis (<42.7 mEq/L) and alkalosis (>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 AG(corr) 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 SID(app) and between predicted and measured SIG in the validation group (R(2) = 0.977; bias = 0±1.5 mEq/L and R(2) = 0.96; bias = −0.2±1.8 mEq/L, respectively). CONCLUSIONS: SID(app) and SIG can be substituted by [Na(+)] – [Cl(−)] and by AG(corr) respectively in the diagnosis and management of acid-base disorders in critically ill patients.