Labile glycated haemoglobin and carbamylated haemoglobin are still critical points for HbA_1c measurement

Haemoglobin A_1c (HbA_1c) is a key analyte for the monitoring of glycemic balance in diabetic patients and is used for diabetes diagnosis in many countries. The potential interference of carbamylated haemoglobin (cHb) and labile glycated haemoglobin (LA_1c) on HbA_1c assays must remain a matter of vigilance. Such a situation has occurred in our laboratory with a kit replacement on the Bio-Rad Variant™ II testing system, a cation-exchange high performance liquid chromatography (HPLC) system. With this method, LA_1c and cHb coeluted in a same peak which may have different consequences on HbA_1c values.

The influence of increasing LA_1c and cHb values on HbA_1c results was studied with in vitro glycation and carbamylation of samples. Samples from patients with high and normal blood urea concentrations were assayed by HPLC and immunological assay.

We observed that the degree of interference greatly varied depending on the nature of the interfering Hb fractions found under the so-called "LA_1c peak". Thus, we have decided to apply a decision tree using "LA_1c" thresholds depending on: (i) the retention time, (ii) the shape of the peak, (iii) other analytes, like urea. If the peak recognized as "LA_1c" is mainly formed by LA_1c, we consider that there is no interference until 4%. If the peak is mainly formed by cHb, we consider an interference threshold equal to 2%.

This situation reminds that cHb and LA_1c remain critical issues in chromatography-based HbA_1c assays and that adapted criteria must be set up for result interpretation.