Comparison of anion gap and strong ion gap as predictors of unmeasured strong ion concentration in plasma and serum from horses

Abstract

Objective - To compare the accuracy of anion gap (AG) and strong ion gap (SIG) for predicting unmeasured strong ion concentration in plasma and serum from horses. Animals - 6 well-trained Standardbred horses undergoing high-intensity exercise (experimental study) and 78 horses and ponies that underwent IV administration of lactic acid or endotoxin, and endurance, submaximal, or high-intensity exercise. Procedure - Anion gap was calculated as AG = (Na+ + K+)-(Cl-+ HCO-3), and SIG was calculated, using the simplified strong ion model, whereby SIG (mEq/L) = 2.24 × total protein (g/dl)/(1 + 106.65-pH) - AG. The relation between AG or SIG and plasma lactate concentration was evaluated, using linear regression analysis. Results - Linear relations between plasma lactate concentration and AG and SIG were strong for the experimental study (r2 = 0.960 and 0.966, respectively) and the published studies (r2 = 0.914 and 0.925, respectively). The following relations were derived: AG = 1.00 × plasma lactate + 10.5; SIG = 0.99 × plasma lactate + 2.8. An AG > 15 mEq/L indicated an increased unmeasured anion concentration, whereas a SIG