The protime measures the extrinsic pathway. Protime is a historic term. We use citrated plasma (not serum!) Citrate chelates the calcium Hemolyzed or partially clotted samples are useless. We restore calcium to an aliquot of the plasma and Add tissue thromboplastin as the trigger. Record time it takes for the sample to clot. Usually about 11 seconds. Results expressed as INR: ratio of your patient's result and the simultaneously run control. Expected INR = 1 Mr. Bullroar's value is substantially prolonged. What vitamin might be low?