HbG-Coushatta: an unexpected discovery during HbA1c measurement

Hemoglobin (Hb) A1c is widely used as the gold standard for blood glucose level monitoring in diabetic patients. Various methods are being used in clinical laboratories for HbA1c measurement. However, the presence of Hb variants may interfere with HbA1c measurement.

The HbA1c assay was performed routinely in a 55-year old woman with type 2 diabetes and hyperlipidemia that was prescribed with a therapy of oral anti-diabetic medication. The test results of CE-HPLC showed that there was an Hb variant which interfered with the quantitation of HbA1c. The patient Hb was profiled through capillary electrophoresis and DNA sequencing of the Hb β-chain gene (HBB) was performed to define its genotype. Capillary electrophoresis and tandem HPLC-capillary electrophoresis (LC/CE) were used to determine the HbA1c of this sample.

Hemoglobin electrophoresis showed that proportions of HbA2 and HbA were 2.6% and 54.1%, respectively. An Hb variant was confirmed by the presence of an abnormal peak in the HbD zone, with a proportion of 43.3%. Further DNA sequencing of the Hb β-chain gene (HBB) revealed that the variant was HbG Coushatta. The measurement of HbA1c through capillary electrophoresis yielded a result of 7.7% (60.6 mmol/mol). Tandem HPLC-capillary electrophoresis (LC/CE), reported an HbA1c value of 6.7% (49.7 mmol/mol). Integrals from capillary electrophoresis suggested that glycation rate of HbG Coushatta may be less than HbA. This difference could affect the IFCC reference method determination of HbA1c.

Elimination of Hb variant interference is critical to achieve an accurate result of HbA1c. High resolution or low interference methods might be preferred in the regions with high prevalence of Hb variants. And staffs in clinical laboratory should pay more attention to analyze raw data of the test and find problems of the analysis.