Correct interpretation of platelet counts requires blood to be taken by a venepuncture. Arterial blood is commonly taken from an indwelling cannula but is not ideal. Thrombocytopenia is due to decreased platelet production (bone marrow failure, vitamin B12 or folate deficiency), decreased platelet survival (ITP, TTP, infection, hypersplenism, heparin therapy), increased platelet consumption (haemorrhage, DIC) or in vivo aggregation giving an apparent thrombocytopenia; this should be checked on a blood film. Spontaneous bleeding is associated with platelet counts <20 x109/l and platelet cover is required for procedures or traumatic bleeds at counts <50 x109/l.


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