Prothrombin time
What it measures, and what changes the number.
Prothrombin time, often shortened to PT, measures how many seconds it takes plasma to clot after a reagent called thromboplastin is added. It checks the proteins of the so-called extrinsic and common clotting pathways, several of which are made by the liver and depend on vitamin K.
It is usually reported with the INR, a standardized version that allows comparison between labs, and in some countries with prothrombin activity. The APTT is often ordered at the same time to look at the other clotting pathway.
Blood is drawn into a citrate tube, which must be filled properly to keep the right ratio of anticoagulant. Clotting in the tube, delays before testing and very high hematocrit can alter the result. The seconds figure depends strongly on the reagent and instrument, which is why the INR exists. Labs may print it as Tiempo de protrombina.
Often read alongside: INR, Prothrombin activity, APTT, APTT ratio, Albumin