Beta-thalassaemia trait (thalassaemia minor)

What it is

Beta thalassemia trait, also called beta thalassemia minor, is the mild end of beta thalassemia. Its genotype pairs the normal gene A with one beta or beta+ gene, while beta thalassemia major carries two of them, beta0/beta0 or beta0/beta+. The adult hemoglobin, HbA, stays close to normal in the trait, so the condition is most often found on a hemoglobin study done for some other reason, such as premarital screening, and not because a patient is ill. It is named alongside beta thalassemia intermedia, beta thalassemia major and hemoglobin H disease when the cause of an anemia is being chosen.

How it is diagnosed

The study is HPLC, and it gives HbA, HbA2, HbF, HbS and HbC. In beta thalassemia trait HbA runs 90-95%, HbA2 is above 3.5%, HbF is 1-3%, and HbS and HbC are absent. HbA2 above 3.5 is by itself immediately beta thalassemia trait, but the same table puts three other conditions above that line, and the HbS is what sorts them out. Beta thalassemia major runs HbA2 above 5 with HbA 0 and HbF 90-95%. Sickle-beta0 thalassemia runs HbA2 above 3.5 with HbA 0, HbF 2-15% and HbS 80-90%. Sickle-beta+ thalassemia runs HbA2 above 3.5 with HbA 3-30%, HbF 2-10% and HbS 65-90%. In the trait itself HbS is 0, so a raised HbA2 with any HbS at all is one of the sickle-beta forms and not the trait. A worked example from a premarital screen: HbA 92%, HbF 1% and HbA2 5.2% is beta thalassemia trait. The minor form is given in round numbers as HbA about 93%, HbS 0%, HbF about 2% and HbA2 about 5%. The normal AA pattern is HbA 95-98%, HbA2 2-3% and HbF under 2%, and in beta thalassemia major HbA is absent while HbF is very high. Two short rules sort the two groups on hemoglobin electrophoresis: HbH and Hgb Barts are alpha thalassemia, while a raised HbA2 with a raised HbF is beta thalassemia.