Prothrombin Time (PT)

Measures the effectiveness of the EXTRINSIC SYSTEM and COMMON PATHWAY. The time for fibrin formation is measured at 37°C, in the presence of a very high concentration of tissue factor (thromboplastin), on RECALCIFICATION by adding calcium chloride to the test sample. Thromboplastin is obtained from a brain extract, e.g. rabbit(UK), bovine brain(France) - this species difference in the source of thromboplastin can lead to small variations in PT.

To standardise thromboplastin the International Normalised Ratio (INR) is used. This is important for patients who are prescribed oral anticoagulant drugs where the PT is used to control their treatment.

Normal PT: 10 - 14 seconds.

Glossary

International Normalised Ratio (INR) - is based on the ratio of patient PT to a mean normal PT which is then corrected for any variation in thromboplastin. Thromboplastin is calibrated against a standard thromboplastin supplied by the World Health Organisation.