Use of sodium-chloride difference and corrected anion gap as surrogates of Stewart variables in critically ill patients

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. 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. 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 (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). 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.