okay, let's see how well i understand it!
prothrombin (coagulation factor ii, a vitamin k-dependent proenzyme that functions in coagulation) time is measured by mixing the patient's blood with thromboplastin (a protease that converts prothrombin to thrombin in the early stages of blood clotting) to see how long it takes the blood to form a clot. there are three commonly used types of thromboplastin used world-wide to run this test... human brain, rabbit brain and recombinant. each is slightly different and therefore will give slightly different results. so a method of "normalizing" the results was necessary to allow for across-the-board accuracy, no matter where the blood is tested. (this is really important in cases where patients are transferred from one facility or state to another, as in the case of trauma patients, transplant recipients and other.) inr (international normalized ratio)
is the solution that was developed by the world health organization. it uses human brain thromboplastin as the benchmark so human brain thromboplastin has an international sensitivity index (isi) or correction factor of 1, rabbit brain thromboplastin has an isi of 2 and recombinant thromboplastin has an isi of 1 as well. the lab knows what type it uses, then applies the isi to the results obtained to come up with the inr. "normal" blood would clot at a given rate (11 to 13.5 seconds, depending on the lab), the value of which would be an inr of 1, regardless of what type of thromboplastin used, because of the correction with the isi. so a value of 1.5 would be 1 1/2 times normal, and so on. does that clear it up for you?
now ptt (partial thromboplastin time) or more likely aptt (activated ptt) is a bit different. in this test a ptt reagent and calcium are added to the specimen and then the timer is started. normal values are 25 to 39 seconds. patients being therapeutically anticoagulated should have ptts 1 1/2 to 2 times the norm, or 37.5 to 78 seconds. so a ptt of 35 is within normal limits and perfect for allowing a subsequent dose, but 100 would be 4 times the lower end of normal and thus too long, and bleeding could be a problem if the heparin is given. make sense?
i learn so much when i try to explain things to someone else! thanks!!