Significance of borderline HbA(2) levels in β thalassemia carrier screening

Increased HbA(2) levels are the characteristic feature of β-thalassemia carriers. A subset of carriers however do not show HbA(2) levels in the typical carrier range (≥ 4.0%) but show borderline HbA(2) levels. As a result, these carriers escape diagnosis and carry the risk of having β-thalassemia major offspring. Borderline HbA(2) values may occur as a consequence of mild β-thalassemia mutations, co-inherited β-thalassemia and α- or δ- thalassemia or iron deficiency anemia. However, there is insufficient knowledge regarding the cause of borderline HbA(2) levels in specific populations. This study aimed to identify the determinants of borderline HbA(2) levels (which we have considered as HbA(2) 3.0–3.9%) in 205 individuals. Primary screening involved detecting the presence of iron deficiency anemia followed by molecular analysis of α, β and δ globin genes. Remarkably, 168 of 205 individuals were positive for a defect. 87% (149/168) of positive individuals were heterozygous for β thalassemia with (59/149) or without (90/149) the presence of co-existing IDA, α or δ gene defects. Notably, 20 of 149 β thalassemia carriers showed HbA(2) < 3.5% and MCV > 80fL. 7 of these 20 carriers were married to carriers of hemoglobinopathies. Our findings describe the genetic basis of borderline HbA(2) levels and emphasize the necessity of a molecular diagnosis in these individuals in the routine clinical setting.