Use of 2 years of patient data to estimate intra-laboratory total imprecision of HbA(1c) measured by multiple HPLC analyzers.

BACKGROUND: Analytic imprecision is used to assess the acceptability of HbA(1c) methods performed on a single analyzer. When multiple analyzers are used interchangeably in a laboratory, the analytic imprecision is usually increased and can obscure the detection of a genuine HbA(1c) trend or result in an artefactual patient trend. We have estimated the imprecision of HbA(1c) testing of patient specimens by three HbA(1c) analyzers independent of reference sample analysis.
METHODS: Over 2 years, approximately 150,000 HbA(1c) measurements were obtained from any one of three different Bio-Rad VARIANT II HPLC analyzers operated in a large reference laboratory. We tabulated the HbA(1c) measurements of paired intra-patient blood samples drawn within 30 days of each other. We calculated the standard deviations of duplicates (SDD) of the intra-patient HbA(1c) pairs grouped by the following time intervals: 0-3 days, 4-6 days, 7-9 days, 28-30 days. The SDDs were then regressed against time with extrapolation to zero time representing the random analytic error.
RESULTS: At a mean HbA(1c) of 7.16%, the total analytic imprecision (coefficient of variation [CV]) is 3.6%.
CONCLUSIONS: This variation is remarkably low, given that the HbA(1c) measurements were obtained over a 2-year period on any one of three analyzers and the long-term within-analyzer CV was usually 2.3-3.1% as assessed by reference control analysis. This approach could be extended to all HbA(1c) analyzers since unlike reference control statistics, the patient-derived random error should allow easy comparison of analytic imprecision among different analytical systems.