30th April 2024

A 10-year-old boy presents with a history of recurrent, prolonged nosebleeds and excessive bruising.

A 10-year-old boy presents with a history of recurrent, prolonged nosebleeds and excessive bruising. He reports a recent episode of knee swelling and pain after minor trauma. His maternal uncle was diagnosed with a bleeding disorder.

Investigations:

Haemoglobin: 130 g/L (115–155)

White cell count: 6.2 × 10^9/L (4.5–13.5)

Platelets: 210 × 10^9/L (150–400)

Prothrombin time (PT): Normal

Activated partial thromboplastin time (aPTT): Prolonged

Which initial investigation is most likely to help establish a diagnosis?