What laboratory tests are performed that measure how quickly the blood clots?

Answer 1

How blood clots and how this clotting is measured is a huge subject.Over the years many many different tests have been devised and used to measure how quickly blood clots but these days there are two main tests used to screen for defects in the coagulation (clotting) systems: the Prothrombin Test (PT) and the Activated Partial Thromboplastin Test (APTT) (or variations on these). These test are done on plasma removed from whole blood that has been prevented from clotting by mixing it with trisodium citrate solution (an anticoagulant). The plasma is separated by centrifuging the blood until the cells go to the bottom of the sample tube. This plasma is then removed from the top. Blood clots as a result of a complex interaction (like a chain reaction) between lots of different proteins (clotting factors) in the blood, all finally leading to the production of an insoluble protein called fibrin. This is what makes up the "fabric" of the scab that forms over a wound The two tests look for defects in different parts of the clotting mechanism, traditionally known as the Extrisic and Intrinsic pathways. These two pathways are triggered in different ways depending on what sets off the clotting process. As an example of the difference between these two types of test, if a person was suffering from traditional haemophilia (an inherited deficiency of a clotting protein -or factor - known as Factor VIII) their PT would be normal but their APTT could be markedly prolonged. A deficiency of certain other factors could show up in the opposite way and yet others could affect both tests. As mentioned above, this is a huge subject and the reader is invited to contact the author of this answer through the message board for sources of more detailed information. LenNolanWinner wrote this answer but, for some reason it was attributed to the IP address, not the user despite being logged in! Oh well...