Quality control of electrolyte analyzers. Evaluation of the anion gap average.

The authors have shown previously that averaging at least eight patient anion gaps provides a sensitive technic for the detection of systematic error in electrolyte analysis (Am J Clin Pathol 79:688-696, 1983). They conducted a retrospective and prospective evaluation of this technic on the ASTRA 4. One month of patient and control data were studied retrospectively and showed that 17/71 abnormally low patient anion gap averages were associated with violations in a multi-rule procedure, and 41/71 low averages were associated with violations in cusum, a more sensitive procedure. In the prospective study, a total of 36 runs of eight patient specimens with low anion gap averages (less than 7.5 mmol/L) were reanalyzed after appropriate recalibration and/or maintenance. Thirty-one of the 36 groups had significant changes in either Na (nine groups, delta Na = +1.5 mmol/L), Cl (14 groups; delta Cl = -1.8 mmol/L), or in both Na and Cl (eight groups; delta Na = +1.2 mmol/L; delta Cl = -0.9 mmol/L). Because the average error detected was small, the authors recommend that the average of anion gaps be used as an early indicator of drift. It must be used, however, in conjunction with standard quality control procedures such as the multi-rule approach.