Diagnosis of mixed acid-base disorders in diabetic ketoacidosis

In diabetic ketoacidosis, a mixed acid-base disorder is suggested when the anion gap increase (ΔAG) does not equal the bicarbonate decrease (ΔHCO₃), or when the ΔAG/ΔHCO₃ ratio does not equal 1.0. It is widely assumed that ΔAG/ΔHCO₃ is significantly different from 1.0 when it is less than 0.8 or greater than 1.2. The validity of these ratio limits were examined by analyzing a normal control group of 68 subjects and 27 diabetic ketoacidosis admissions that had no evidence of mixed disorders. In the 27 ketoacidosis admissions, regression analysis showed that ΔAG was predicted to equal ΔHCO₃, as expected in pure anion gap acidosis: ΔAG = 1.0ΔHCO₃ (r = 0.744, p < 0.001). It was found that ΔAG is significantly different from ΔHCO₃ when they differ by more than 8 mEq/L, and equivalently, ΔAG/ΔHCO₃ is significantly different from 1.0 when it is less than (1.0 - 8/ΔHCO₃) or greater than (1.0 + 8/ΔHCO₃). These criteria from regression analysis suggested that 4% of the 27 pure anion gap acidoses, and 3% of the control group, had mixed disorders. In contrast, the ratio limits of 0.8 and 1.2 suggested 56% of the pure anion gap acidoses, and 94% of the control group, had mixed disorders. It was concluded that mixed disorders are overdiagnosed by the ratio limits of 0.8 and 1.2. Mixed disorders are more accurately detected by noting whether ΔAG and ΔHCO₃ differ by more than 8 mEq/L.