**Question:** Which of the following haemoglobin (Hb) estimation will be diagnostically helpful in a case of beta thalassemia trait?

A. Low Hb level
B. Increased Hb F level
C. Decreased Hb A2 level
D. Increased Hb A2 level

**Core Concept:** Beta thalassemia is a group of inherited blood disorders characterized by reduced or absent synthesis of beta-globin chains, leading to abnormal Hb production and reduced red blood cell (RBC) survival. Beta thalassemia trait (minor) is a heterozygous condition, where only one normal and one mutated beta-globin gene are present.

**Why the Correct Answer is Right:** In beta thalassemia trait, RBCs are slightly decreased due to reduced RBC survival. However, the overall Hb level is normal or slightly reduced. The crucial diagnostic test is to measure the ratio of Hb F (fetal Hb) to Hb A2 (adult Hb A2). In normal individuals, Hb F is < 1% and Hb A2 is 3%), indicating the presence of mutated beta-globin chains.

**Why Each Wrong Option is Inaccurate:**

A. Low Hb level: In beta thalassemia trait, Hb levels are usually normal or slightly reduced.
B. Increased Hb F level: In beta thalassemia trait, Hb F levels are usually normal < 1%. C. Decreased Hb A2 level: In beta thalassemia trait, Hb A2 levels are increased due to the presence of mutated beta-globin chains. D. Increased Hb A2 level: In beta thalassemia trait, Hb A2 levels are usually normal 3%) is due to the presence of mutated beta-globin chains. These mutated chains are incorporated into Hb A2, leading to its increased concentration. This is why the correct answer is D, which states that Hb A2 levels are usually normal or mildly increased in beta thalassemia trait.

**Why Each Wrong Option is Inaccurate:**