The anion gap as a prognostic indicator in horses with abdominal pain.

The anion gap was calculated for each of 90 horses with signs of abdominal pain, using laboratory data obtained shortly after admission. The anion gap was defined as the difference between the sodium concentration (mEq/L) and the sum of the chloride and bicarbonate concentrations. To evaluate the use of the anion gap as a prognostic indicator, the survival rates were calculated for horses whose anion gap concentrations were within various ranges. It was found that the probability of survival decreased as the anion gap progressively increased about 20 mEq/L. The survival rates for increasing ranges of anion gap were as follows: less than 20 mEq/L, 81% survival; 20 to 24.9 mEq/L, 47%; greater than or equal to 25 mEq/L, 0%. Though not of diagnostic significance as to the source of abdominal pain, the anion gap is a suitable prognostic indicator.