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

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 + 10^6.65-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 < -2 mEq/L indicated an increased unmeasured strong anion concentration. Conclusions and Clinical Relevence - Anion gap and SIG can be used to predict plasma lactate concentration in horses. AG is accurate and clinically useful for estimating unmeasured strong ion concentration in horses with total protein concentrations within or slightly outside reference range, whereas SIG is more accurate in horses with markedly abnormal total protein concentrations and those of various ages and with various concentrations of albumin, globulin, and phosphate.