Predicted effects of hemoglobin A1c assay precision on a patient population distribution of serial hemoglobin A1c difference values

Background: Interpretation of serial measurements of % hemoglobin A1c includes an assessment of differences from preceding values (DHbA1c). We examined predicted effects of different assay precisions on an observed population distribution for DHbA1c. Methods: Primary data were 5260 DHbA1c values from sequential HbA1c measurement pairs obtained within 1 calendar year. Each DHbA1c was replaced by a distribution obtained from sampling each component HbA1c value according to a normal distribution characterized by a fixed coefficient of variation (CV) of either 1%, 3% or 5% (forming data sets A, B and C, respectively). Data sets B and C, with inferior precision, were compared with the reference data set A (highest precision). Results: Using DHbA1c bin widths of 0.5% HbA1c, differences in assay precision caused significant redistribution of numbers within bins. For instance, for CV = 5%, there was a 7.2% decrease in the number of results within the DHbA1c bin = (- 0.5 to ≤ 0.0)% compared with the number for CV = 1%, and a 6.4% increase in numbers of results for DHbA1c > 0.5. Conclusion: Different HbA1c assay CVs can significantly affect the fraction of patients within different clinical categorizations for DHbA1c and consequently may differently influence patient care recommendations.