Laboratory features

The laboratory features of ITP include:

• thrombocytopenia:
  • platelet count less than 100 000 per cubic mm
  • no bleeding until less than 50 000 per cubic mm
  • spontaneous bleeds may occur when count between 20 - 50 000 per cubic mm
  • serious bleeding more likely if the count drops below 20 000 per cubic mm
• platelets may be enlarged - megathrombocytes - produced in response to enhanced destruction.
• bone marrow - normal or increased megakaryocytes - there are no other abnormalities. Bone marrow investigation is mandatory if steriods are to be given, to rule out leukaemia.
• PT, PTT and coagulation time are normal
• if haemorrhage occurs then there is:
  • increased WBC with shift to left
  • anaemia - reticulocytes
• tests for platelet associated IgG - 95% sensitivity, 50% specificity for increased platelet IgG due to thrombocytopenia