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Which of the following is true about prothrombin time estimation (AIIMS Nov 2011)

A Sample should be immediately refrigerated

B Platelet rich plasma

C Processed with in 4 hrs

D Kaolin activation necessary

Ans. C Processed with in 4 hrs

a. 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.

b. This test is also called "ProTime INR" and "INR PT". They are used to determine the clotting tendency of blood, in the measure of warfarindosage, liver damage, and vitamin Kstatus.

c. PT measures factors I, II, V, VII, and X. It is used in conjunction with the activated partial thromboplastin time(aPTT) which measures the intrinsic pathway.

d. This tests the extrinsic system in the coagulation pathway. It involves the addition of brain thromboplastin to plasma, with clot formation after the addition of calcium chloride.


Patient's plasma

a. Blood should be collected into 31.3 g/l trisodium citrate in a concentration of one volume citrate to 9 volumes blood.

b. As soon as possible after collection, the specimen should be centrifuged at about 3000 rpm for ten minutes and the platelet-poor plasma separated into a plastic tube, using a plastic pasteur pipette.

c. This sample must be analysed within four hours after collection, unless it is frozen and kept in the deep-freeze refrigerator until tested at a later date.

Normal control plasma

Freshly collected normal plasma obtained in the same way as that of the patient

Rabbit-brain thromboplastin, stored at 40C

This reagent can be commercially obtained or home made

Calcium chloride solution ,0.025mol/l (i.e. M/40); stored at 40C

In some commercial reagents this has already been added to the thromboplastin


a. Reconstitute a vial of thromboplastin in accordance with the manufacturer's instruction. Leave in water-bath at 370C for 10 minutes.

b. Dispense 0.1 ml into a plastic tube and add 0.1 ml of pre-warmed calcium chloride (or dispense 0.2 ml if the thromboplastin-calcium is already combined)

c. Add 0.1 ml of pre-warmed plasma and start the stop-watch.

d. Tilt the tube gently every other second, keeping it as much as possible under water to maintain the temperature. Record the appearance of a fibrin clot as the end-point.

e. Perform the test on the patient's plasma in duplicate, and also in duplicate on the normal control plasma. Repeat the test if duplicate measurements differ by more than 5%.

f. The normal range should be in the region of about 13-17 seconds. However, this depends on the thromboplastin and should be established by testing a group of healthy subjects whenever a new reagent is introduced

g. The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT or APTT) is a performance indicator measuring the efficacy of both the "intrinsic" (now referred to as the contact activation pathway) and the common coagulation pathways.

h. Apart from detecting abnormalities in blood clotting, it is also used to monitor the treatment effects with heparin, a major anticoagulant.

i. It is used in conjunction with the prothrombin time (PT) which measures the extrinsic pathway. Kaolin cephalin clotting time (KccT) is a historic name for the activated partial thromboplastin time.

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