Two new alpha chain variants found during glycated hemoglobin screening: Hb Tatras [alpha 7(A5)Lys-->Asn] and HB Lisbon [alpha 23(B4)Glu-->Asp]

Interference of hemoglobin variants in HbA1c quantification

Fasting blood glucose and glycated hemoglobin (HbA_1c) are routine biomarkers to screen and monitor diabetes mellitus. HbA_1c results from glycation at the N-terminus of the β globin chain of tetrameric human hemoglobin. Fasting blood glucose level varies with the nature and amount of food intake, physical exercise, etc., and, accordingly, is a short-term measure of glucose control. In contrast, HbA_1c provides an average measure of glucose control for the long-term (8-12 weeks). Unfortunately, genetic variants of hemoglobin may interfere with HbA_1c quantification using ion exchange chromatography, capillary electrophoresis, immunoassay and boronate affinity chromatography. Mass spectrometry, however, measures total glycation of hemoglobin across both α and β globin chains and correlates well with the ion exchange based method. Additionally, mass spectrometry based quantification is not impacted by the presence of genetic variants of hemoglobin and thus might be a better analytical choice for diabetes mellitus.

Hemoglobin variants and determination of glycated hemoglobin (HbA1c)

Measurement of glycated hemoglobin in diabetic patients is an established procedure for evaluating long-term control of diabetes. The Diabetes Control and Complications Trial (DCCT), as well as the United Kingdom Prospective Diabetes Study (UKPDS), confirmed the direct relationship between the degree of glycemic control as estimated by glycohemoglobin (GHb) determinations and the development and progression of long-term complications in diabetic patients. Samples with known interferences of HbA(1c) determination as hemoglobinopathies are specifically excluded from certification testing and there are no guidelines or requirements for comparability of samples containing hemoglobin (Hb) variants. This paper reviews the interference of Hb variants on determination methods of glycated hemoglobin as they result in false HbA(1c) results.

Hb Peterborough [beta 111(G13)Val-->Phe] in Japan; rapid identification by ESI/MS using proteolytic digests of oxidized globin

A variant hemoglobin was found in a Japanese female whose hemoglobin was studied to clarify the cause of a low Hb A1c value, found during a routine medical examination. The detection and identification of the variant was performed by electrospray ionization mass spectrometry. Its structure was revealed to be the same as Hb Peterborough [beta 111(G13)Val-->Phe]. For sequence determination, oxidized globin as well as non-derivatized globin were cleaved by trypsin and lysyl endopeptidase. An abnormal peptide was found in digests of oxidized globin, as shown by electrospray ionization mass spectrometry. Cysteic acid in oxidized peptides enhanced the abundance of fragment ions in tandem mass spectrometry, which helped to quickly and accurately determine the substitution in beta T-12, a peptide in the core region. Electrospray ionization mass spectrometry analysis of the hemolysate also showed a low level of glycated hemoglobin. The patient's hemolysate showed decreased stability in the isopropanol test. An abnormal band was detected on isoelectrofocusing on the cathodic side of normal Hb A. This is the second report of Hb Peterborough and the first of its occurrence in Japan.