A 22 year old man presents with history of bleeding from gums for the last 6 months. On investigation the b was found to be 8.2 gm% TLC 4400/mm, DLC N 64%, L 27%, E 3%, M 6% and platelet count of 20,000 /cu mm. Which one of the following investigation would be most useful in establishing the diagnosis -
Bleeding time and clotting time
Prothrombin time
Partial thromboplastin time
Bone marrow examination