Thrombocytopenia means a reduction in the platelet count below the normal lower limit (usually 150 x 10⁹/l ). This can have a variety of causes, including a reduction in platelet production, a reduction in platelet survival, and dilution of platelet numbers resulting from the transfusion of platelet-poor blood.

Pseudothrombocytopenia

This can occur when platelets undergo a phenomenon called 'clumping'. In this situation, the platelets stick together, causing a false low reading when passed through an auto-analyser. The condition is caused by the action of ethylenediaminetetraacetic acid (EDTA) used as an anticoagulant. It occurs in about 0.1% of the population but can also be associated with infections with human immunodeficiency virus, rubella, cytomegalovirus, autoimmune disorders, neoplastic diseases, thrombotic disorders and possibly trauma. It is not indicative of a bleeding diathesis or platelet dysfunction. If an abnormally low platelet count is detected in the absence of a suggestive medical history, examination of a peripheral blood smear on a freshly-taken specimen should be performed.¹

Causes^2,3

Disorders affecting platelet production

Congenital

• Megakaryocytic hypoplasia - underdevelopment of megakaryocytes which normally develop in bone marrow and fragment to produce platelets - usually of autoimmune or infectious origin
• Thrombocytopaenia/absent radii (TAR) syndrome - radial aplasia or hypoplasia and thrombocytopenia
• Wiskott-Aldrich syndrome - an X-linked recessive disease characterised by thrombocytopenia,
  lymphopenia and depressed cellular immunity, eczema, malignant lymphoma
• Congenital leukaemia, e.g. Down's syndrome
• Fanconi's anaemia

• Viral infections - e.g. herpes simplex, cytomegalovirus, varicella-zoster, Epstein-Barr, rubella, enterovirus, mumps, hepatitis
• Aplastic anaemia
• Marrow infiltration by a primary neoplasm, e.g. leukaemia or metastatic malignant disease
• Drugs and chemicals - e.g:
  • Quinine
  • Aspirin
  • Antibiotics such as chloramphenicol and sulphonamides
  • Heparin
  • Gold
  • Digoxin
  • Phenylbutazone
  • Indometacin
  • Sulfasalazine
• Alcohol
• Paroxysmal nocturnal haemoglobinuria
• Megaloblastic anaemia
• Myelofibrosis

Decreased platelet survival

• Immune - idiopathic thrombocytopenia purpura, systemic lupus erythematosus, post-transfusion purpura, drug-induced (same drugs as affect platelet production)
• Thrombotic thrombocytopenic purpura
• Haemolytic uraemic syndrome
• Disseminated intravascular coagulation
• Cardiopulmonary bypass
• Hypersplenism
• Strawberry naevi, in Kasabach-Merritt syndrome (cavernous haemangiomata with severe thrombocytopenia, and features of disseminated intravascular coagulation)

Dilutional thrombocytopenia

This is caused by transfusion of large volumes of blood which may be depleted of functioning platelets resulting from prolonged storage.

Presentation, investigations and management

See Thrombocytopathy.