Blood platelets and petechiae

Blood platelets and the blood clotting factors have a complementary function in haemostasis.

This is shown clearly by the bleeding time. A small (5 mm) incision in the flexor side of the forearm (Ivy test; first apply a sphygmomanometer and inflate to 40 mm Hg), or a prick in the earlobe (Duke’s test), produce a small amount of bleeding. The time needed to stop the bleeding depends on the speed at which a platelet clot is formed. The bleeding time (normally 3-9 minutes) is longer with severe thrombocytopenia or with impaired blood platelet function. Patients with clotting disorders such as haemophilia do not have any petechiae because platelet adhesion and aggregation are sufficient to close small lesions.

The number of blood platelets is normally much more than is needed for haemostasis (normal platelet count 150,000-450,000/µl). If the platelet count is less than 20,000/µl, the haemorrhaging risk increases considerably. If the count is less than 5,000/µl haemorrhaging will almost certainly occur.

Thrombocytes survive for 10 days on average. Usually a third is sequestered in the sinusoids of the spleen. This percentage increases in case of splenomegaly, especially if this is attributable to a portal hypertension.

Category: Pathology Notes