Performance of hemoglobin A1c assay methods: good enough?

Two very interesting articles on hemoglobin A1c (Hb A1c)2 method performance appear in this issue of Clinical Chemistry . Woodworth et al. (1) use Hb A1c as an example of how the risk of reporting inaccurate results can be estimated to guide individualized QC strategies. They assess the risk of reporting unreliable Hb A1c results with 6 different methods in 4 academic medical centers; 5 are laboratory instruments and 1 is a point-of-care (POC) method. Patient-weighted σ metrics were calculated on the basis of 1–2s QC rules with an average of 1 QC for every 100 samples. The authors found considerable differences in the risk of reporting unreliable results among the methods used at these sites, and that only 1 method, the Capillarys2 Flex Piercing, achieved a patient-weighted σ metric >3 at a total error allowable (TEa) of 6%. This total error limit is also used for College of American Pathologists (CAP) proficiency testing and National Glycohemoglobin Standardization Program (NGSP) certification and is considered optimal for clinical care at this time. This means that for all but the aforementioned method, “maximum QC (three levels, three times per day) should be performed to achieve the necessary error detection.” Of note was that for some methods there was a substantial amount of bias (up to 5.8% relative for values within the reference interval), and CV values were higher than the recommended 2% for either the high or low QC for 3 of the methods studied (2). The maximum number of Hb A1c results out of 100 expected to be unreliable from an out-of-control condition at TEa of 6% ranged from 0.60 to 71.48 (!), and for 2 methods the number of results expected to be unreliable was >19 of 100 even when …