Prothrombin time (PT)





Prothrombin time (PT) measures the integrity of extrinsic pathway

PT measures the clotting time of plasma
... after tissue factors (thromboplastin) and Ca2+ are added to plasma sample


Can be expressed as a ratio, i.e. international normalised ratio (INR)

Use

Useful in:

  • Detecting...
    • Coagulopathies
    • Liver disease
    • DIC
  • Monitoring of warfarin / coumarin Rx

Abnormal results

Raised PT

  • Vitamin K deficiency
  • Warfarin Rx
  • Deficiency of factors
    • Fibrinogen (factor I)
    • Prothrombin (factor II)
    • Factor VII
    • Factor V or X (APTT will also be raised)
  • Liver disease (esp obstructive)
  • DIC



INR

Used to standardise PT between labs

INR = ratio of PT to mean normal PT

(?? Raised to the power of international sensitivity index (ISI) of thromboplastin used.)