A comparative evaluation of glycosylated haemoglobin assays: feasibility of references and standards

Four assays, high pressure liquid chromatography, colorimetric with thiobarbituric acid, affinity columns, and microcolumn cation exchange were compared for ability to discriminate between samples taken from diabetic and normal subjects; correlation with each other; stability over time at different temperatures; and reproducibility between laboratories. The most discriminatory (10 samples from a diabetic and 10 samples from a normal group) was the microcolumn cation exchange method (t = 5.25; p less than 0.001), but all were significantly different at p less than 0.005. The intra-assay coefficient of variation was 1%-6%, except for the affinity column method which was 13% in normal subjects. High pressure liquid chromatography was used as a reference and the other assays correlated well (r = 0.93-0.99). Storage at -80 degrees C, -20 degrees C, 4 degrees C, and 24 degrees C showed marked differences. The thiobarbituric acid method results were stable except for 24 degrees C. Microcolumn cation exchange was labile under all conditions. Affinity column was stable for up to 15 days, only if samples were stored as whole blood. High pressure liquid chromatography showed an increase in haemoglobin A1a + b and a decrease in the haemoglobin A1c. Haemoglobin A1c was reproducible for 4 days when stored at 4 degrees C and up to 11 days when stored at -80 degrees C. Samples exchanged between centres at 4 degrees C and performed within 5 days by high pressure liquid chromatography for haemoglobin A1 and haemoglobin A1c correlated well (r = 0.98 and 0.99). Samples exchanged between centres after storage (up to 40 days -80 degrees C) correlated (r = 0.99) by the thiobarbituric acid method.(ABSTRACT TRUNCATED AT 250 WORDS)