Implications for Clinical Performance Measurement of Interlaboratory Variability in Methods for Glycosylated Hemoglobin Testing

In order to evaluate interlaboratory variation in glycosylated hemoglobin (GHb) testing in the context of quality assessment and performance measurement systems, we conducted a telephone survey of laboratories in Washington state. The survey focused on GHb testing methods, analytes measured, reference ranges, awareness of and adherence to the National Glycohemoglobin Standardization Program (NGSP) standards, and recent changes in methods. We contacted 30 (81%) of 37 laboratories known to conduct GHb testing. Among the 25 laboratories using 1 or more test methods in-house, 5 (20%) reported only total GHb levels rather than HbA1C. There was considerable variability in test methods used and in reported high and low reference ranges within and across methods. Eight (30%) of 27 individual methods used by the laboratories were not certified by the NGSP. From January 1997 through August 1998, 11 of 25 laboratories (44%) made changes in testing methods. Despite national efforts to decrease variability in GHb testing methods, substantial variability remained in Washington state in 1997. In addition to hindering management of individual patients, this variability poses challenges for clinical performance measurement systems such as the Health Plan and Employer Data Information Set that attempt to compare the adequacy of glycemic control among health systems. However, the high frequency of recent changes in testing methods may indicate an improving trend in test methods.