HbA2 is raised in:
Beta thalassemic trait
Sickle cell anemia
Hereditary spherocytosis
Failure to synthesise beta chains (beta-thalassaemia) is the most common type of thalassaemia, most prevalent in the Mediterranean area. Heterozygotes have thalassaemia minor, a condition in which there is usually mild microcytic anaemia and little or no clinical disability, which may be detected only when iron therapy for a mild microcytic anaemia fails. Homozygotes (thalassemia major) either are unable to synthesise haemoglobin A or, at best, produce very little; after the ﬁrst 4–6 months of life, they develop profound transfusion-dependent hypochromic anaemia.
Key Concept: 
In beta thalassemia, HbA2 quantity is raised.
Ref: Davidson Ed 23 Pg 47