How is the prothrombin time measured?

By William Aird

Blood drawn from a patient into a tube containing the anticoagulant, citrate.
The sample is spun in a centrifuge to separate plasma from red cells and platelets.
Following centrifugation, plasma is on top, red cells on the bottom.
Patient’s platelet-poor plasma is incubated with a negatively charged substance such as silica, phospholipid (PL) and calcium (Ca2+) at 37 degrees Celsius. The activated partial thromboplastin time (aPTT, measured in seconds) is the time it takes for clot to form as monitored by eye or (more commonly) via an automated machine. The human eye and the automated machine cannot distinguish between uncrosslinked and crosslinked fibrin. That is why assays such as the aPTT and prothrombin time (PT) are normal in FXIII deficiency (FXIII is responsible for crosslinking fibrin).