Quality control, quality assessment of laboratory tests, HbA1c.

ABSTRACT

HbA1c level is known to be highly correlated with blood sugar level for a while after the maximum blood sugar level was observed, and is now considered more reliable than blood sugar as a long-term index of blood sugar control in diabetic patients. The Diabetes Control and Complication Trial in United States of America reported that mean HbA1c was the dominant predictor of retinopathy progression and that 10% lower HbA1c (e.g., 8 vs. 7.2 %) is associated with a 43 % to 45 % lower risk of progression of retinopathy. This report indicated that only a 1% difference in HbA1c level has a critical effect on progression of diabetic retinopathy. It also indicates the need for high precision and accuracy of hemoglobin A1c determination for control of diabetes patients. There are various methods to measure HbA1c. but now most laboratories use the high-performance liquid chromatography (HPLC) method; immunoassay is the second most popular, followed by affinity chromatography. It has been reported that each of these methods has its own characteristics, and that their results can vary considerably. In addition, there are two types of HbA1c, one a stable component and the other a labile component, and the measured HbA1c values were influenced by the contents of labile component included in the sample. The Japan Diabetes Society took a leading role and organized a committee on inter-laboratory standardization of HbA1c Determination in 1994. The HPLC method, which eliminates labile components, is used in most laboratories. Also, the number of laboratories using the immunoassay has gradually increased, while use of the other methods has rapidly decreased. In conclusion, since the proposal of a reference method and initiation of supply of reference materials by the Japan Diabetes Society, inter-laboratory differences in measured values have become much smaller, mainly due to the methods used for measurements. We need to continue efforts toward further standardization of HbA1c determination.