Investigations in alpha thalassaemia

Diagnosis of a alpha thalassaemia is made on the basis of (1,2):

• all affected individuals have a variable degree of anaemia (low Hb), reduced mean corpuscular haemoglobin (MCH/pg), reduced mean corpuscular volume (MCV/fl) and a normal/slightly reduced level of HbA2
  • alpha thalassaemia trait
    • mild to moderate microcytic hypochromic anaemia, carriers (heterozygotes) of alpha thalassaemia, whatever the molecular basis, are clinically asymptomatic
    • diagnosis is often established during a regular health check or during antenatal screening
    • if more severe anaemia in patient with alpha thalassaemia train then generally related to other concomitant disorders
  • HbH disease
    • usually produce less than 30% of the normal amount of alpha globin
    • anaemia (2.6-13.3 g/dl) with variable amounts of HbH (0.8-40%), occasionally accompanied by Hb Bart's in the peripheral blood
    • splenomegaly may be present (which may be severe) and occasionally this is complicated by hypersplenism
    • jaundice may occur
    • children may show growth retardation
    • possible complications include leg ulcers, gall stones, infections, folic acid deficiency and acute haemolytic episodes in response to drugs and infections
      
      
      
• molecular analysis is usually required to confirm the haematological observations (especially in silent alpha-thalassaemia and alpha-thalassaemia trait)
  
  
• predominant features in HbH disease are anaemia with variable amounts of HbH (0.8-40%)
  • type of mutation influences the clinical severity of HbH disease
    
    
• exclusion of iron deficiency, and of anaemia of chronic disease
  • if microcytosis (MCV <80 fL) or hypochromia (MCH <27 pg) is present
    • iron status should be assessed (serum iron, transferrin, transferrin saturation, and ferritin)
    • in HbH disease, the reticulocyte percentage will be elevated (3% to 6% and sometimes higher)
    • blood film should be carefully reviewed for findings consistent with alpha-thalassemia, including microcytosis, hypochromia, increased polychromasia, and target cells
      
      
• prenatal diagnosis is possible - foetal blood sampling or gene probe analysis of amniotic cells

Diagnosis should be suspected by pre-conception and antenatal screening in at risk patients and established by pre-natal or neonate testing. (2)

Iron overload monitoring (2)

• can be assessed by calculation annual transfusions expressed in ml/kg/yr of pure red cells, and average daily transfusional iron loading (expressed as mg/ kg/day) and assuming 1 ml of pure red cells contains 1.08 mg of iron. Patients whose iron loads are more than 0.3 mg/kg/day require higher chelation dosing. (2)
• serum ferritin is a helpful guide. Persistently high levels (>2500μg/l) can lead to cardiac complications. But levels at 500-1500 μg/l over the long term carry a relatively low risk. Although levels can fluctuate and complications present even at this level (2)

Notes:

• inherited as an autosomal recessive disorder characterised by a microcytic hypochromic anaemia, and a clinical phenotype varying from almost asymptomatic to a lethal haemolytic anaemia
• most frequently results from deletion of one or both alpha genes from the chromosome 16 (normal individuals alpha globin synthesis is regulated by four alpha globin genes two on each copy of chromosome 16)
  • can be classified according to its genotype/phenotype correlation
  • normal complement of four functional alphaglobin genes may be decreased by 1, 2, 3 or all 4 copies of the genes, explaining the clinical variation and increasing severity of the disease
  • compound heterozygotes and some homozygotes have a moderate to severe form of alpha thalassaemia called HbH disease
    • predominant features in HbH disease are anaemia with variable amounts of HbH (0.8-40%)
  • Hb Bart's hydrops foetalis is a lethal form in which no alpha-globin is synthesized - presence of Hb Bart's and the total absence of HbF
  • if the level of alpha globin synthesis falls below ~70% of normal, in the foetal period, excess gamma globin chains form Hb Bart's which can be detected on routine Hb analysis
  • in adult life, excess beta globin chains form beta4 tetramers of HbH in the cell and these
    • can be identified by staining the peripheral blood with 1% brilliant cresyl blue, or when present in sufficient quantity by routine Hb analysis

References:

1. Thalassaemia International Federation. Guidelines for the management of alpha-thalassaemia. 2023.
2. Thalassaemia International Federation. A short guide for the management of transfusion-dependent thalassaemia. 2nd ed. 2022 [internet publication].