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Aim: To determine the clotting time of a subject.
Requirements: Fine capillary glass tubes of about 10 mm length, cotton, rectified spirit, lancet, stopwatch.
- Capillary tube method: (Wright’s method): Under sterile precautions make a sufficiently deep prick in the finger tip. Note the time when bleeding starts (start the stop watch). Touch the blood drop at the finger tip using one end of the capillary tube kept tilted downwards. The tube gets easily filled by capillary action. After about two minutes start snapping off small lengths of the tube, at intervals of 15 seconds, each time noting whether the fibrin thread is formed between the snapped ends. Note the time (stop the stop watch) when the fibrin thread is first seen.
- Discussion: Clotting time is the interval between the moment when bleeding starts and the moment when the fibrin thread is first seen. The normal value is 3to 10 minutes. Bleeding time and clotting time are not the same. Bleeding time depends on the integrity of platelets and vessel walls, whereas clotting time depends on the availability of coagulation factors. In coagulation disorders like haemophilia, clotting time is prolonged but bleeding time remains normal. Clotting time is also prolonged in conditions like vitamin K deficiency, liver diseases,disseminated intravascular coagulation, overdosage of anticoagulants etc.
- Modified Lee and White method: Under aseptic precaution, venepuncture is done and one ml. of blood is collected in each 3 small test tubes. Note the time when blood is taken. Keep the test tube in a water bath maintained at 37°c. Tilt the tubes every 30 seconds and see whether the blood is flowing. Repeat this till the tube can be inverted without the blood flowing out. Nore the time. The average value of the results in the 3 test tubes gives the clotting time.
Normal value is2 to 7 minutes