Individuals with thalassaemia trait, thalassaemia minor or thalassaemia carriers, as they are more commonly known, have a mutated gene on only one of the chromosomes inherited from their mother or father. They do not manifest any clinical symptoms and do not require medical treatment or regular medical follow-up. They are generally healthy people.
They do have some changes in their red blood cells, which are generally smaller and may have less haemoglobin content, but not enough to require correction. These small changes are recognised only by special blood tests.
In α- and β-thalassaemia:
- Haemoglobin may be up to 1g/dl lower than the norm for each population.
- Mean corpuscular haemoglobin (MCH) and mean cell volume (MCV) are generally much lower than normal, although in silent α-thalassaemia they may be almost up to normal values. RDW, a measure of anisocytosis (variable sizes of red cells), is raised in all cases. These parameters are measured in automated laboratory instruments.
- These changes are also seen if the red cells are examined under the microscope. They are smaller, paler red cells with varying shapes.
In microscopic examination of red cells and using special stains, inclusion bodies (small dots) may be seen. These are the precipitated H bodies (the extra β-globin chains) and in infants the extra γ-globin chains.
In β–thalassaemia and in sickle cell disease carriers:
The components of the haemoglobin molecule may be separated by special tests (e.g. electrophoresis and HPLC). In a-thalassaemia carriers, because of the reduced production of β-globin chains, there is an attempt by the organism to compensate and produce more γ- or δ-globin chains. HbF (α2γ2) may be raised in some forms. The hallmark of the β- thalassaemia carrier is due to the increased δ-globin chains forming a fraction known as HbA2. This fraction is normally less than 3% of the total haemoglobin, but in carriers it is over 3.5%.
Separation of the fractions of the molecule will also identify globin chain variants, the most common of which are sickle cell (HbS) and HbE (which cause a-thalassaemia syndrome, if co-inherited with β-thalassaemia).
These basic laboratory tests identify carriers. They do not always result in a clear diagnosis and other special tests may be required, such as DNA analysis to clearly identify a carrier, especially where α-thalassaemia is concerned.
Identification of carriers is conducted through surveys in which blood tests are performed on healthy individuals of a population. This will determine their percentage in a given population (epidemiology), but will also identify genetic risk and lead affected couples to counselling (explaining the risk to their offspring and discussing the choices that they have concerning reproduction).