Prothrombin Time


Prothrombin Time, ProTime, Pro-Time, PT, International Normalized Ratio, INR

  • Background
  1. Evaluates the Extrinsic Clotting Pathway
  2. Plasma collected in a tube containing citrate, which binds Calcium and prevents initial clotting
  3. Both Calcium and Tissue Thromboplastin added to plasma
  4. Tissue Thromboplastin activates the Extrinsic Clotting Pathway which is fast (compared with Intrinsic Clotting Pathway)
  • Precautions
  1. International Normalized Ratio (INR) and ProTime (PT) were developed only to monitor Warfarin
    1. All Clotting Factors except Factor 8 and vWF are synthesized in hepatocytes
    2. In Cirrhosis and hepatic failure, INR is a marker of decreased hepatic synthetic function
    3. Cirrhosis may be associated with an increased INR/PT, but may also be Hypercoagulable
    4. Elevated INR in Cirrhosis does not reflect bleeding risk
  • Labs
  • Normal
  1. International Normalized Ratio (INR)
    1. See Warfarin (includes INR targets per condition)
    2. See Warfarin Protocol (includes INR initiation protocols and maintenance dosing)
    3. See Warfarin Protocol for the Perioperative Period (includes bridging, and perioperative INR targets)
  2. ProTime (replaced by INR)
    1. Range: 10-12 sec (11-15 sec in some assays)
  • Causes
  • Increased
  • Causes
  • Decreased
  1. Vitamin K supplementation
  2. Thrombophlebitis
  3. Medications
    1. Glutethimide
    2. Estrogens
    3. Griseofulvin
    4. Diphenhydramine