Synonyms: PT
Related tests: PTT
Clinical significance
The prothrombin time (PT) is the most widely used coagulation test and explores the extrinsic pathway (factor VII) and the common pathway (II, V, X) of the coagulation cascade. The PT is also widely used to monitor coumarin anticoagulant therapy. The PT is expressed as percentage activity (prothrombin activity) but, to improve standardisation, results are also expressed as a ratio (INR) that normalises the variability of results between laboratories so that they can be compared.
Clinical indications
Assessment of haemostasis; monitoring of anticoagulant therapy.
Sample type
The patient must have a blood sample taken.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.
Notes
For patients on anticoagulant therapy, prothrombin activity lies in the range 18-38%, corresponding to a standardised INR of 2.0-4.0. Many medicines can raise the INR, in particular antibiotics and anti-inflammatory drugs.