<prompt> First investigation to be done in a patient with thrombocytopenia: September 2008Platelet countBleeding timeProthrombin timeaPTT  <response>: Ans. A: Platelet count Platelet count is the initial investigation to be done in all the cases of thrombocytopenia, followed by hemoglobin and WBC count. The prothrombin time PT and its derived measures of prothrombin ratio PR and international normalized ratio INR are measures of the extrinsic pathway of coagulation. They are used to determine the clotting tendency of blood, in the measure of warfarin dosage, liver damage, and vitamin K status. The reference range for prothrombin time is usually around 12-15 seconds; the normal range for the INR is 0.8-1.2. PT measures factors II, V, VII, X and fibrinogen. It is used in conjunction with the activated paial thromboplastin time aPTT which measures the intrinsic pathway. 