The role of haemoglobin A₂testing in the diagnosis of thalassaemias and related haemoglobinopathies

Abstract

The increase in haemoglobin (Hb)A(2) level is the most significant parameter in the identification of beta thalassaemia carriers. However, in some cases the level of HbA(2) is not typically elevated and some difficulties may arise in making the diagnosis. For these reasons the quantification of HbA(2) has to be performed with great accuracy and the results must be interpreted together with other haematological and biochemical evidence. The present document includes comments on the need for accuracy and standardisation, and on the interpretation of the HbA(2) value, reviewing the most crucial aspects related to this test. A practical flow-chart is presented to summarise the significance of HbA(2) estimation in different thalassaemia syndromes and related haemoglobinopathies.

Spectrum of hemoglobinopathies by high performance liquid chromatography with special reference to role of HbA2 levels at tertiary care centre

“…However, in other cases (patients from the Indian subcontinent) who are iron depleted or frankly iron deficient, HbA2 has sometimes been observed to be reduced to the normal range (Barbara J Bain, personal communication). 18 Atul shrivastav et al stated that Iron deficiency may lead a low Hb A2 and hence may mask α-thalassemia trait. 11,19 Out of 73 cases with decreased HbA2 levels only 12 cases showed markedly reduced HbA2 levels (1.4-2%) and altered blood indices.…”

“…This is especially clearly seen in HbH disease, where HbA2 can drop to less than 1%. 18 G B Tan et al studied 25 cases of α-thalassemia trait which showed HbA2 levels 1.4-2.6% and mean of 2.0%.…”

Analytical goals for the determination of HbA2

“…Finally, another approach based on clinical needs has been already reported by our group [1] and is just briefly outlined here. Considering a subject with a true HbA 2 value of 3.6 % , the measurement error should not exceed, in relative units, of more than 7.0 % in order to exclude the possibility of misclassifying him as a β -thalassemia carrier (HbA 2 ≥ 3.8 % ) or as a non-β -thalassemia subject (HbA 2 < 3.3 % ).…”

“…Total error, % Biologic variability 4.5 Opinion of experts > 9.0 Clinical need [1] 7.0 cut-off points and variability of results which will demonstrate improvements necessary in the analytical goals for manufacturers of automated analy zers. To this regard, we have summarized our findings in Table 4 , where a minimal goal for the total error could be set at 6.9 % , a kind of a compromise between the information gained from the study on biological variation, clinical needs and opinion of experts.…”

“…Detection of carriers is easily performed in the classical phenotypes characterized by increased HbA 2 values (4.0 % -6.0 % ) together with reduced mean cell volume (MCV) and mean cell hemoglobin (MCH). However, the existence of atypical β -thalassemia carriers with near normal hematological indices and borderline HbA 2 values (3.3 % -3.8 % ) can make the diagnosis more difficult [1,2] . Borderline HbA 2 levels are generally due to the presence of mild β + mutations or may be the consequence of a number of genetic factors, such as the interaction with other molecular defects ( α -and δ -allele), β -promoter mutations, triplication of the α -globin gene ( α α α / α α ) and KLF1 gene mutations [3,4] .…”