Wide variation in serum anion gap measurements by chemistry analyzers

The traditional anion gap [AG = Na - Cl - (total CO₂)] mean value of 12 mean value of 12 mEq/L was established during the 1970s with analyzer methods that are no longer used widely: No studies have systematically compared mean AG values from analyzers in current use. We used data from the healthy subjects obtained from 27 clinical laboratories, 5 manufacturers, and 8 publications to compute mean AG values from 1970s analyzers and 8 current analyzers. We also compared mean AG values by evaluating Na, Cl, and total CO₂ data from the College of American Pathologists Chemistry Surveys (1990- 1996). Data from healthy subjects showed that overall mean AG values of the 9 analyzers ranged from 5.9 to 12.4 mEq/L. The pooled (ie, average) AG SD was 2.3 mEq/L. We then used the data of the Surveys and the mean value from 1 analyzer to compute predicted mean values for the other 7 current analyzers. Almost all mean AG values predicted from the Surveys agreed (within 1.5 mEq/L) with mean values from healthy subjects. These results show that mean values of analyzers vary widely, indicating that analytic bias strongly influences the AG. The results should be a useful guide for the AG measurements that can be expected from different analyzers.