Several laboratory tests may be used to help detect and diagnose thalassemia:
Complete blood count (CBC). The CBC is an evaluation of the cells in the blood. Among other things, the CBC determines the number of red blood cells present and how much hemoglobin is in them. It evaluates the size and shape of the red blood cells present, reported as the red cell indices. These include the mean corpuscular volume (MCV), a measurement of the size of the red blood cells. A low MCV is often the first indication of thalassemia. If the MCV is low and iron deficiency has been ruled out as a cause, thalassemia should be considered.
Blood smear (also called peripheral smear and manual differential). In this test, a trained laboratory professional examines a thin layer of blood that is treated with a special stain, on a slide, under a microscope. The number and type of white blood cells, red blood cells, and platelets are evaluated to see if they are normal and mature. With thalassemia, the red blood cells often appear smaller than normal (microcytic, low MCV). Red cells may also:
- Be paler than normal (hypochromic)
- Vary in size and shape (anisocytosis and poikilocytosis)
- Be nucleated (normal, mature RBCs do not have a nucleus)
- Have uneven hemoglobin distribution (producing "target cells" that look like a bull's-eye under the microscope)
The greater the percentage of abnormal-looking red blood cells, the greater the likelihood of an underlying disorder and decreased ability of the RBCs to cary oxygen.
Iron studies. These may include: iron, ferritin, unsaturated iron binding capacity (UIBC), total iron binding capacity (TIBC), and percent saturation of transferrin. These tests measure different aspects of the body's iron storage and usage. The tests are ordered to help determine whether an iron deficiency is the cause of a person's anemia. One or more of them may also be ordered to help monitor the degree of iron overload in an individual with thalassemia.
- Alpha thalassemia is sometimes confused with iron deficiency anemia because both disorders have smaller than usual (microcytic) red blood cells. If someone has thalassemia, his or her iron levels are not expected to be low. Iron therapy will not help people with alpha thalassemia and may lead to iron overload, which can cause organ damage over time.
- Erythrocyte porphyrin tests may be used to distinguish an unclear beta thalassemia minor diagnosis from iron deficiency or lead poisoning. Individuals with beta thalassemia will have normal porphyrin levels, but those with the latter conditions will have elevated porphyrin.
Hemoglobinopathy (Hb) evaluation (hemoglobin electrophoresis). This test assess the type and relative amounts of hemoglobin present in red blood cells. Hemoglobin A (Hb A), composed of both alpha and beta globin, is the type of hemoglobin that normally makes up 95% to 98% of hemoglobin in adults. Hemoglobin A2 (HbA2) is usually 2% to 3% of hemoglobin in adults, while hemoglobin F usually makes up less than 2%.
Beta thalassemia upsets the balance of beta and alpha hemoglobin chain formation and causes an increase in those minor hemoglobin components. So individuals with the beta thalassemia major usually have larger percentages of Hb F. Those with beta thalassemia minor usually have elevated fraction of Hb A2. Hb H is a less common form of hemoglobin that may be seen in some cases of alpha thalassemia. Hb S is the hemoglobin more common in people with sickle cell disease.
Hemoglobinopathy (Hb) evaluations are used for state-mandated newborn hemoglobin screening and prenatal screening when parents are at high risk for hemoglobin abnormalities.
DNA analysis. These tests are used to help confirm mutations in the alpha and beta globin-producing genes. DNA testing is not routinely done but can be used to help diagnose thalassemia and to determine carrier status, if indicated.
- For beta thalassemia, the hemoglobin beta gene, HBB, may be analyzed or sequenced to confirm the presence of thalassemia-causing mutations. Genetic tests may also be given for other HBB mutations such as Hb S mutation, which is associated with sickle cell disease. More than 250 mutations have been associated with beta thalassemia, though some cause no signs or symptoms. However, others decrease the amount of beta globin production and some prevent it completely. The presence of one of those mutations confirms a diagnosis of beta thalassemia.
- The primary molecular test available for alpha thalassemia detects common mutations (e.g., deletions) in the two alpha genes HBA1 and HBA2. Each person has two copies of each of these genes, called alleles, in their cells, one from their mother and one from their father. These alleles govern alpha globin production and if mutations lead to functional loss of one or more of alpha genes, alpha thalassemia occurs.
Since having relatives who carry mutations for thalassemia increases a person's risk of carrying the same mutant gene, family studies may be done to evaluate carrier status and the types of mutations present in other family members if deemed necessary by a healthcare practitioner.
Genetic testing of amniotic fluid is used in the rare instances a fetus is at increased risk for thalassemia. This is especially important if both parents likely carry a mutation because that increases the risk that their child may inherit a combination of abnormal genes, causing a more severe form of thalassemia.