Prothrombin activity
What it measures, and what changes the number.
Prothrombin activity is another way of expressing the result of the prothrombin time, a clotting test that measures how quickly plasma forms a clot after a reagent is added. Instead of seconds, the clotting time is converted into activity relative to a pooled reference plasma, sometimes called the Quick value. This format is common in Spain and other parts of Europe.
It is reported together with the prothrombin time in seconds and the INR, and is often ordered with the APTT, which checks a different part of the clotting system.
The blood must be collected into a citrate tube filled to the correct level, since underfilling changes the ratio of anticoagulant to blood. Delays or a difficult draw can also affect it. Results depend on the reagent used, so values from different labs are not always comparable. It may appear as Actividad de Protrombina.
Often read alongside: Prothrombin time, INR, APTT, APTT ratio