Evaluation of acid-base disorders in dogs and cats presenting to an emergency room. Part 2: comparison of anion gap, strong ion gap, and semiquantitative analysis.

OBJECTIVE: To compare the diagnostic performance of the anion gap (AG) with 2 physicochemical approaches to identify unmeasured anions.

DESIGN: Prospective cohort study.

SETTING: University teaching hospital.

ANIMALS: Eighty-four dogs and 14 cats presenting to a university teaching hospital emergency room.

INTERVENTIONS: All dogs and cats in which venous blood samples for acid-base, lactate, and serum biochemical analysis were all collected within 60 minutes of each other, over a 5-month enrollment period. Unmeasured anions were quantified using each of three approaches: the anion gap (AG), strong ion gap (SIG), and a semiquantitative approach (XA).

MEASUREMENTS AND MAIN RESULTS: An increased AG metabolic acidosis was evident in 34/98 of cases. The Stewart approach identified an increased SIG acidosis in 49/98 of cases. There was a strong correlation between SIG and AG (r = 0.89; P < 0.001). The semiquantitative approach identified increased unmeasured anions in 68/98 of cases. There was a moderate correlation between AG and XA (r = 0.68; P < 0.001) and a slightly stronger correlation between SIG and XA (r = 0.75; P < 0.001). Plasma lactate concentrations and AG were poorly correlated (r = 0.22; P = 0.029) and there was no correlation between lactate concentrations and BE (r = 0.19; P = 0.069).

CONCLUSIONS: Unmeasured anions occurred commonly in this sample of small animal emergency room patients and physiochemical approaches identified more animals with unmeasured anions than the traditional AG calculation. Further studies are needed to determine if the results of the physicochemical approach improves clinical management and warrants the associated increases in cost and complexity.