Prothrombin Time and International Normalized Ratio (PT/INR)

 Prothrombin is a vitamin K-dependent glycoprotein produced by the liver that is essential for fibrin clot formation. Each laboratory establishes a normal or control value based on the method used to perform the PT test. The PT measures the amount of time it takes in seconds for clot formation, the international normalized ratio (INR) is calculated from a PT result to monitor the effectiveness of warfarin.

Indication for PT and INR

  • Monitor response to warfarin sodium (Coumadin) therapy.
  • Screen for dysfunction of the extrinsic clotting system resulting from vitamin K deficiency disseminated intravascular coagulation or liver disease.

Normal Lab Value for Prothrombin Time (PT)

  • Normal: 11 – 13 seconds
  • Critical value: >20 seconds for persons who do not use anticoagulants.
  • The INR standardizes the PT ratio and is calculated in the laboratory setting by raising the observed PT ratio to the power of the international sensitivity index specific to the thromboplastin reagent used.

Nursing Care for Prothrombin Time

  • If a PT is prescribed, the baseline specimen should be drawn before anticoagulation therapy is started; note the time of collection on the laboratory form.
  • Provide direct pressure to the venipuncture site for 3 to 5 minutes.
  • Concurrent warfarin therapy with heparin therapy can lengthen the PT for up to 5 hours after dosing.
  • Diets high in green leafy vegetables can increase the absorption of vitamin K, which shortens the PT.
  • Orally administered anticoagulation therapy usually maintains the PT at 1.5 to 2 times the laboratory control value.
  • Initiate bleeding precautions, if the PT value is longer than 30 seconds in a client receiving warfarin therapy.