Background:
HbA1c is used routinely to monitor long-term glycemic control in diabetes patients. Like all laboratory tests, HbA1c measurement is subject to interferences and its accuracy can be affected adversely by presence of Hb variants, which may be an incidental discovery during HbA1c analysis.
Objective:
To report a case of a 65 year old patient with clinically silent Hb variant, which hampered the software's ability to calculate HbA1c concentration.
Case summary:
In the reported patient, HbA1c test was performed using high performance liquid chromatography (HPLC) by Bio-Rad D-10 device, Dual Kit/A1c method. Due to visual inspection of the chromatogram, an unknown hemoglobin fraction (5.6%, RT 1.48) was demonstrated (a).
In regard to acceptance/ rejection criteria, the sample was flagged by lab staff as having an "atypical profile" and no HbA1c result was clinically reportable. A new sample produced a similar pattern.
The sample was also tested using: Bio-Rad Variant II, Short program (b) and Bio-Rad Variant II Turbo 2.0 (c) : a slight "bump" was observed on the HbA0 peak and an unknown peak ( 37% ) was eluted after HbA0 peak, in accordance.
We checked the patient's history in our lab, and curiously, a HbA1c analysis performed one year ago in the same analytic device, showed normal Hb-HPLC pattern.
A sequence analysis of the patient's hemoglobin genes was performed in the Hemoglobinopathy Reference Laboratory (Oakland, CA), which demonstrated the presence of a beta globin variant, Hb Sitia {β128(H6)Ala>Val}. Hb Sitia is a mildly unstable variant and was first reported in Greece*. The mutated residue is located in an area of high affinity dimerization ɑ1ß1 interface in the hemoglobin tetramer. Mutations that disrupt ɑ1ß1 dimerization increase the concentration of free monomers, which are unstable.
Discussion: The majority of human hemoglobin mutants, which are not associated with any hematologic or clinical phenotype, were discovered as an incidental finding. However, measurement procedures differ in the ability to detect the presence of rare Hb variants and to quantify HbA1c in patients who harbor such variants. It is important to keep in mind that there are a variety of limitations of the HbA1c assay methods used.
* Papassotiriou I., et.el. Hemoglobin, 25(1), 45-56 (2001)