Expansion
Reduced production, increased destruction, sequestration or dilution
Expansion
Reduced production
- Marrow failure: aplastic anaemia, leukaemia, myelodysplasia, infiltration
- B12 or folate deficiency
- Alcohol, chemotherapy, radiotherapy
- Viral infection
Increased destruction
- Immune thrombocytopenia (ITP): autoantibody mediated; a diagnosis of exclusion
- Heparin-induced thrombocytopenia: antibodies to platelet factor 4, causing thrombosis rather than bleeding, typically at 5 to 10 days
- Thrombotic thrombocytopenic purpura: ADAMTS13 deficiency, with the pentad of fever, neurological signs, renal impairment, microangiopathic haemolysis and thrombocytopenia
- DIC, antiphospholipid syndrome, drugs, HIV, hepatitis C
Sequestration: hypersplenism from any cause of splenomegaly
Dilution: massive transfusion
Bleeding risk correlates with the count: spontaneous bleeding is unusual above 50, and serious spontaneous bleeding is a risk below 10 to 20.
Note the two conditions where transfusing platelets is harmful: TTP and heparin-induced thrombocytopenia, since both are thrombotic.