A case of α-chain variant hemoglobin (Hb Chad) with falsely high HbA1c levels measured by immunoassay

Variant hemoglobin is often detected during the diagnosis and treatment of diabetes mellitus. We here describe a case of α2-chain variant hemoglobin (Hb Chad) that was identified as a result of differences in HbA1cs values determined by different assays. HbA1c measured by immunoassay was thus falsely high, whereas that measured by high-performance liquid chromatography (HPLC) was slightly low. Sequencing analysis revealed a heterozygous GAG (glutamic acid) → AAG (lysine) mutation at amino acid position 23 of the α2-globin gene. This residue is located at the surface of the α-chain in the crystal structure of hemoglobin. The high HbA1c value determined by immunoassay might have been the result of increased antigenicity of the variant hemoglobin, whereas the low value measured by HPLC reflected differential fractionation of the variant relative to the wild-type protein. Hb Chad has been reported in only three cases to date, and HbA1c was measured for the first time. This is the first case where falsely high HbA1c measured by immunoassay due to increased antigenicity in α-chain variant hemoglobin. This case highlights the importance of comparison with other parameters related to plasma glucose such as glycated albumin if an HbA1c abnormality is suspected.