The use of sodium-chloride difference and chloride-sodium ratio as strong ion difference surrogates in the evaluation of metabolic acidosis in critically ill patients

PurposeInorganic apparent strong ion difference (SIDai) improves chloride-associated acidosis recognition in dysnatremic patients. We investigated whether the difference between sodium and chloride (Na+-Cl−) or the ratio between chloride and sodium (Cl−/Na+) could be used as SIDai surrogates in mixed and dysnatremic patients.Patients and MethodsTwo arterial blood samples were collected from 128 patients. Physicochemical analytical approach was used. Correlation, agreement, accuracy, sensitivity, and specificity were measured to examine whether Na+-Cl− and Cl−/Na+ could be used instead of SIDai in the diagnosis of acidosis.ResultsNa+-Cl− and Cl−/Na+ were well correlated with SIDai (R = 0.987, P < 0.001 and R = 0.959, P < 0.001, respectively). Bias between Na+-Cl− and SIDai was high (6.384 with a limit of agreement of 4.463-8.305 mEq/L). Accuracy values for the identification of SIDai acidosis (<38.9 mEq/L) were 0.989 (95% confidence interval [CI], 0.980-0.998) for Na+-Cl− and 0.974 (95% CI, 0.959-0.989) for Cl−/Na+. Receiver operator characteristic curve showed that values revealing SIDai acidosis were less than 32.5 mEq/L for Na+-Cl− and more than 0.764 for Cl−/Na+ with sensitivities of 94.0% and 92.0% and specificities of 97.0% and 90.0%, respectively. Na+-Cl− was a reliable SIDai surrogate in dysnatremic patients.ConclusionsNa+-Cl− and Cl−/Na+ are good tools to disclose SIDai acidosis. In patients with dysnatremia, Na+-Cl− is an accurate tool to diagnose SIDai acidosis.