Bleeding time

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Bleeding time is a medical test done on someone to assess their platelet function.

It involves cutting the underside of the subject's forearm, in an area where there is no hair or visible veins. The cut is of a standardised width and depth, and is done quickly by an automatic device.

A blood pressure cuff is used above the wound, to maintain venous pressure at a special value. The time it takes for bleeding to stop (as thus the time it takes for a platelet plug to form) is measured. Cessation of bleeding can be determined by blotting away the blood every several seconds until the site looks 'glassy'.

Bleeding time is affected by platelet function, certain vascular disorders and von Willebrand Disease, and not by other coagulation factors so in haemophilia it is normal. Diseases that cause prolonged bleeding time include thrombocytopenia and disseminated intravascular coagulation (DIC).

Aspirin and other cyclooxygenase inhibitors can prolong bleeding time significantly. While warfarin and heparin have their major effects on coagulation factors, an increased bleeding time is sometimes seen with use of these medications as well.

People with von Willebrand disease usually experience increased bleeding time, as von Willebrand factor is a platelet agglutination protein, but this is not considered an effective diagnostic test for this condition.

Normal values fall between 2 - 9 minutes depending on the method used.

[edit] Ivy method

The Ivy method is the traditional format for this test. In the Ivy method, a blood pressure cuff is placed on the upper arm and inflated to 40 mmHg. A lancet or scalpel blade is used to make a stab wound on the underside of the forearm.

A standard-sized cut is made (usually using an automatic blade.) The time from when the stab wound is made until all bleeding has stopped is measured and is called the bleeding time. Every 30 seconds, filter paper or a paper towel is used to draw off the blood.

The test is finished when bleeding has stopped completely.