Which of the following haemoglobin (Hb) estimation will be diagnostically helpful in the case of beta thalassemia trait –
Hemoglobin A2

In hemoglobin A2 the polypeptide consists of two a and two δ chains i.e. the β chain of the normal adult hemoglobin is replaced by the δ chain.
In normal adults, hemoglobin A2 is present in trace amounts. It accounts for 1.5% to 3.5% of the normal adult hemoglobin.

The clinical implication of HbA2 
HbA2 levels have special application in the diagnosis of β thalassemia trait. 

Elevation of HbA2 is present in these cases even through the peripheral blood smear is normal.
Microcytosis and other morphological characteristics of β-thalassemia trait are also seen it iron deficiency but unlike β thalassemia trait HbA2 level is decreased.
Hb–F
Hb1–C
Hb–A2
Hb–H