Clotting Time

 Determination of Whole Blood Clotting Time

When taken out of the body, normal blood clots within few minutes unless prevented by an anticoagulant. The time taken for the whole blood to clot is its clotting time. Two methods are applied for the determination of clotting time

-- Lee-White method 

-- Capillary method.  

 

Lee-White method

Lee-White clotting time is a screening test for determining the coagulation defect. The method is not very sensitive but because of the ease in performing the test, it is used along with a battery of other screening tests. Only severe clotting factor deficiency is recognized by this method. In case .of prolonged clotting time (more than 10 min), the patient should be subjected to more detailed tests for the identification of missing coagulation factors. Presence of a circulatory anticoagulant, like heparin (used as a therapeutic drug), will also cause abnormally long clotting time. This procedure is therefore used to monitor heparin therapy, However, APTT and PT are better tests for this purpose.

Two methods will be described here, venepuncture method and capillary method. The latter is not considered to be as sensitive but it is easy to perform.

Normal range 

  4  to  9 min

Specimen 

The test is done at the side of the patient with freshly drawn blood without any anticoagulant.

Principle

Venous blood is collected in a clean glass tube (acid-washed) without any anticoagulant. The length of time that it takes for the blood to clot or coagulate at 37°C is the clotting time or coagulation time. In case of alternative capillary method, following skin puncture, blood is taken into a plain capillary tube and the stopwatch is started. The formation of fibrin string is noted by breaking the capillary tube at regular interval. The time taken for the first appearance of the fibrin string is noted as the coagulation time.

 

Capillary method

This method is insensitive and unreliable, and should be used only if venous blood cannot be obtained. It is essential to obtain a rapid, free flow of blood, requiring no squeezing or manipulation of the puncture wound. If manipulation is required, the test must be abandoned.

Equipment 

1. Disposable lancet 

2. Capillary tubing (10 to 15 cm length and 1.5 mm diameter) without anticoagulant 

Procedure

- Warm up the finger for skin puncture.

- Make an incision with a sterile disposable lancet to a depth of 3 mm. As soon as the blood is visible, start the stopwatch.

- Wipe off the first drop of blood.

- Hold the capillary over the blood and allow the ensuing drop of blood to flow into capillary tubes held side-by-side, the capillary will fill automatically

- After 4 min, break off the capillary tube, 1 to 2 cm from the end, with the help of a glass tile/manually.

- Repeat breaking at regular time intervals (after every 30 seconds), till fibrin thread appears at the broken end of capillary tube.

   Note: Do not pull away the cut pieces ling apart and bristly.

- Now, after regular intervals break the capillary again and check the fibrin strand.

   Record time interval between pricking finger and first appearance of fibrin thread at the broken ends of capillary tube.

- When a thin string of fibrin can be seen between the broken ends of the capillary tubes, stop the watch and note the time. 

- That is clotting time of blood.

- Don't forget to dispose of the broken tube in the SHARPS CONTAINER.

   Note: When reporting the time, mention that the capillary tube method was used.

 

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