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This measures the time taken for plasma to clot when thrombin is added. It is
a function of the final phase of coagulation.
 Materials
 Patient's plasma
As for the prothrombin time
(#15), blood should be collected into 31.3 g/l trisodium
citrate in a concentration of one volume citrate to nine volumes blood. 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. 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
Barbitone buffered
saline, pH 7.4
Thrombin
This is available commercially as a freeze-dried
material. It must be reconstituted with saline to 100 NIH
units/ml, dispensed in 1-ml aliquots and stored at - 200C. For
use a vial is thawed and diluted with barbitone-buffered
saline, pH 7.4 to obtain a normal clotting time of about 17 seconds – usually
about 7-8 units/ml.
Method
1. Add
0.1 ml buffered saline to 0.1 ml normal plasma and leave in the water-bath at
370C for four minutes.
2. Add
0.1 ml thrombin, and mix by shaking and simultaneously start the stop-watch.
3. Measure
the clotting time.
4. Repeat
with the patient's plasma in duplicate followed by a second sample of the
normal plasma.
5. Express
results as the mean values for the patient and normal. Repeat the test if
duplicate measurements differ by more than 5%.
A patient's Thrombin Time should be within two seconds
of the control. Times of 20 seconds and over are definitely abnormal.
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