Determination of Clotting Time (CT)

               The Clotting Time is the time taken by blood to coagulate outside the body. It depends on many factors involved in coagulation. There are two methods applied for the determination of Clotting time – Venipuncture method (Lee-White method) and Capillary method. Capillary method is commonly followed in the laboratory of developing countries.

Clinical Significance :

Only severe clotting factor deficiency is recognized by this method. Therefore, the test is of little value as a screening procedure. In case of prolonged Clotting time (> 10 minutes), the patient should be subjected to more detailed test for the identification of the missing coagulating factor.

Normal value :

4 to 9 minutes.

 

Determination of Clotting Time

by Capillary method

Principle

A standard incision is made in the skin of the patient and blood is collected in a Capillary tube. Stop-watch is started. The formation of fibrin string is noted by breaking the capillary tube at regular intervals. The time is noted at first appearance of the fibrin string.

Requirements :

  • Doctor’s alcohol (70% ethanol).:
  • Sterile lancet or needle (24G).
  • Capillary tube (Contain no anticoagulant)
  • Stop watch
  • Cotton etc.

Procedure :

  1. Gently clean the finger tip with an alcohol sponge (cotton with alcohol). Warm up the finger for skin puncture.
  2. Make an incision with a sterile lancet or needle, to a depth of 3mm. When the blood is visible, start the Stop-watch.
  3. Wipe off the first drop of blood and allow the ensuring drop of blood to flow into the capillary tube.
  4. After 2 minutes, break the capillary tube, 1 to 2 cm from the end of the capillary tube.
  5. When a thin fibrin string is appear in between the broken ends of the capillary tube, stop the Stop-watch and note the time.
  6. Reporting the time with using method.

Source of error :

Starting the Stop-watch and stopping the Stop-watch must be maintained if the starting or stopping time is not maintain; the Clotting time will not true.

 

Determination of Clotting Time

by Venipuncture method (Lee-White method)

Venipuncture method is more reliable and sensitive then the capillary method. It requires special arrangements for Venipuncture and temperature controlled water-bath. Venipuncture method is also known as Lee-White method.

Principle :

Venous blood is collected in a clean glass tube (acid washed) with out any anticoagulant. The length of time that is taken for the blood to clot or coagulate at 37°C is the clotting time.

Requirements :

  • Water-bath (temperature holding 37°C)
  • Disposable plastic syringe with 24G needle.
  • Tourniquet
  • Test tube with test tube rack.
  • Doctor’s alcohol (70% ethanol).:
  • Stop watch
  • Cotton

Procedure :

  1. Collected over 2 ml of venous blood by routine Venipuncture method with a disposable plastic syringe with 24G needle and tourniquet. Start the Stop-watch.
  2. Remove the needle from the syringe and transfer 1.0 ml of blood in each two test tube.
  3. Plug the test tube with non-absorbance cotton and place them in the water bath at 37°C.
  4. After 3 minutes remove the first tube from the water bath and tilt the tube gently to an angle of 45°to see the formation of clot. If the blood has not clotted, return the tube in the water bath and examine it at 30 seconds intervals to see the formation of clot.
  5. When the blood is clot, the tube can be tilted through an angle of 90° with out spilling the contents. As soon as the blood clotted, immediately examine the second tube. The blood of second tube usually clots just before the blood in the first tube.
  6. Stop the stop-watch and note the time required for coagulation of blood in second tube.

Precautions :

  • Collect venous blood by using proper technique.
  • Transfer exact 1.0 ml of blood in each of two test tubes.
  • Control the water-bath’s temperature at 37°C (fixed).
  • No air bubble formation in the syringe and test tubes.
  • Start the stop watch at proper time.
  • Always used clean and dry glass test tube and plastic test tube.