Sickle-cell anaemia is an autosomal recessive condition that results for synthesis of an abnormal haemoglobin chain termed HbS. It is more common in people of African descent as the heterozygous condition offers some protection against malaria. Around 10% of UK Afro-Caribbean's are carriers of HbS (i.e. heterozygous). Such people are only symptomatic if severely hypoxic.


  • Incidence: 0.50 cases per 100,000 person-years
  • Most commonly see in infants
  • Sex ratio: 1:1
Condition Relative
Beta-thalassaemia trait2.00
Sickle-cell anaemia1
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+


  • polar amino acid glutamate is substituted by non-polar valine in each of the two beta chains (codon 6). This decreases the water solubility of deoxy-Hb
  • in the deoxygenated state the HbS molecules polymerise and cause RBCs to sickle
  • HbAS patients sickle at p02 2.5 - 4 kPa, HbSS patients at p02 5 - 6 kPa
  • sickle cells are fragile and haemolyse; they block small blood vessels and cause infarction

Clinical features


  • the definitive diagnosis of sickle cell disease is by haemoglobin electrophoresis