This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'True regarding prothrombin time measurement? \nSelect and explain. \nA. Platelet rich plasma is required, B. Activate with kaolin, C. Should be measured within 2 hours, D. Immediate refrigeration to preserve coagulation factor viability', 'role': 'user'}, {'content': "Answer: C \nExplanation: Major screening tests of coagulation\xa0\n\nIn all of these tests, blood is collected with a calcium-chelating anticoagulant (usually citrate) and then centrifuged to remove the erythrocytes, leukocyte and platelets.\nThe remaining platelet-poor plasma (PPP) serves as the substrate for these tests.\nDepending upon the test, the phospholipid is added to the PPP to substitute for platelets and an activating agent is added.\nThe PPP is then recalcified and the time needed for a clot to form is measured.\nThe sample to be kept at room temperature if it is to be used for prothrombin time, Lupus anticoagulant or factor VII assays and to be kept at 4° C for other assays.\nOnce blood is drawn for testing, the tests should be completed within a time frame of 30 minutes to 2 hours.\xa0\n\nMeasurement of prothrombin time\n\nThe PT measures the function of the extrinsic and common coagulation pathway. It is commonly used to monitor oral anticoagulant therapy.\nMethod:- A source of phospholipid and tissue thromboplastin (rabbit or human brain extract) is added to platelet poor plasma, and Ca2+ is added to initiate coagulation. The time for the appearance of fibrin strands (PT) is measured and is compared with the control.\nNormal PT is 11- 13 seconds.\n\nMeasurement of Partial thromboplastin time (PTT) and activated partial thromboplastin time (APTT or aPTT)\n\nThe PTT and APTT are used to measure the function of intrinsic and common coagulation pathway. PTT and APTT are used to monitor heparin therapy.\nPTT method:- Phospholipid is added to platelet poor plasma taken from blood that is anticoagulated with citrate. Clotting is then initiated by the addition of Ca2+. The time to the formation of first fibrin strands is PTT.\nAPTT method:- The APTT is performed in the same manner as PTT except for the addition of an activating agent such as kaolin or ellagic acid before recalcification.\nNormal PTT is 28-35 seconds.\xa0\nMeasurement of thrombin time\nIt tests the conversion of fibrinogen to fibrin, i.e., it screens thrombin-fibrinogen reaction.\nMethod:- Thrombin is added to the patient's platelet poor plasma and the time to fibrin strand formation is measured\nNormal TT is 15- 20 seconds.\nOther screening tests\nThese are:-\n\n\nFibrinogen assay\nPlatelet function tests ( bleeding time)\nScreening for factor XIII\nOther individual factor assay", 'role': 'assistant'}]