Lab

Prothrombin Time

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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. Cirrhosis may be associated with an increased INR/PT, but may also be Hypercoagulable
  • Labs
  • Normal
  1. International Normalized Ratio (INR)
    1. See Warfarin
  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