ABSTRACT
A 6-year-old boy presents with a painful, red, swollen knee. There is no evidence of systemic infection but he has a prolonged APTT with normal factor VIII levels
Differential diagnosis
The clinical history suggests a differential diagnosis of haemarthrosis or septic arthritis. However, the prolonged APTT make this a haemarthrosis. The normal bleeding time and platelet level make this a defect of the clotting cascade, and the APTT and prothrombin time put it in the intrinsic pathway. The most common defect, factor VIII deficiency or haemophilia A, is excluded; the answer is the next most likely, factor IX deficiency or haemophilia B. The lack of a family history is of no significance as one-third of patients carry new mutations.
