APTT
What it measures, and what changes the number.
APTT (activated partial thromboplastin time) is a clotting test. It measures how many seconds a plasma sample takes to form a clot after reagents are added that start the intrinsic and common parts of the coagulation cascade. It reflects the combined activity of several clotting factors.
It is usually ordered together with prothrombin time and INR, which test a different branch of the cascade, so the two results together help locate where clotting may be slowed. The APTT ratio and D-dimer are sometimes reported alongside.
The sample goes into a citrate (blue top) tube that has to be filled to the correct level, since the balance of blood to anticoagulant changes the result. It is processed fairly quickly, and contamination with heparin from a line can lengthen the time. Reagents differ between labs, so results are best compared within one lab. Reports may list it as Tiempo de Tromboplastina Parcial Activado (APTT), aPTT or PTT.
Often read alongside: Prothrombin time, INR, APTT ratio, D-dimer, Prothrombin activity