Prothrombin is a protein produced by the liver. It is an important clotting factor.
The test measures blood clotting time and is often used to monitor the effect of blood thinners such as warfarin or to evaluate liver disease. It reflects the extrinsic pathway and the common coagulation pathway.
The result is often expressed as the international normalised ratio which expresses the prothrombin time as a ratio against the standard ‘normal’ prothrombin time.