Tests for Bleeding disorders
Clot retraction time:
It is a very simple test to have idea about the platelets as the platelets are responsible for retraction'of the newly formed clot. The Ca2 released from the activated platelets along with the thrombin initiates the process. Platelets attach to the adjacent fibrin threads and draw them together. The contractile proteins like thrombosthenin, actin, myosin, etc. in platelets are responsible for the process. Normally retraction starts on an average by 30 minutes and the serum is squeezed out. When platelet count is inadequate the time is prolonged or no retraction occurs.
Platelet count:
Platelet is counted directly from a sample of blood in countingchamber after dilution with platelet fluid. It canalso be counted indirectly from the RBC count after findingthe ratio of platelets to RBC from a blood film.
Coagulation time (CT):
It is the time taken by blood to coagulate outside the body. It depends on many factors involved in coagulation and also on container used. In ordinary tests, in laboratory, glass capillary tube is used and is called capillary tube method. In this method the normal value is 3 to 5 minutes. Different other methods are also used which give different results. In siliconised (silicon coated) tubes it takes longer time to coagulate. A prolonged CT helps to suspect but not to pinpoint the diagnosis. Coagulation time is prolonged in haemophilia, but is normal in purpura.
Bleeding time (BT):
It is the time by which bleeding stops from an injury caused by the puncture of a sharp needle. It can be performed on ear lobes or on finger tips. It measures the time between the injury and the temporary haemostasis (platelet plug formation and vasospasm). Normal value is 2 to 4 minute. It should be less than 8 min. Bleeding time is prolonged in purpura, but normal in haemophilia.
Note: Both BT and CT are very simple to perform but are very important. These tests routinely performed before surgery, help to detect bleeding disorders and sometimes save the patient's life.
Prothrombin time (PT):
It is the time taken for coagulation of a sample of recalcified plasma (plasma + CaC1,) when mixed with tissue thromboplastin. Normal time is 11 to 16 seconds (upper limit 25 seconds). It is used as a guide during anticoagulant therapy and the time is kept between 25 to 30 seconds. It is a non- specific test. PT is affected not only by the prothrombin concentration but also due to deficiency of other factors of the extrinsic system except calcium ion and thromboplastin, i.e., 1. V, VIl and X. It gives an indirect idea about the prothrombin content of blood. Blood collection for this test is done with great care to prevent mixture with tissue extract.
The plasma for PT is obtained from a citrated sample which is then mixed with thromboplastin solution. The latter is made from thromboplastin tablet available in the market which also contains plenty of Ca2* (CaCI,). Before mixing, both the solutions are incubated at 37°C.