see also:

• anticoagulants
• activated partial thromboplastin time (APTT)
• Mx of excessive INR or bleeding on warfarin Rx

• the International Normalised Ratio or INR test measures the overall activity of the extrinsic coagulation pathway.
• performed by adding thromboplastin to an aliquot of the patient's plasma
• length of time taken for the mixture to clot is called the prothrombin time.
• the same test is performed using a normal control plasma and the prothrombin ratio is the derived by dividing the patient clotting time by the control clotting time.
• finally, the INR is calculated using the 'international sensitivity index' of the thromboplastin reagent so that the ratio obtained is independent of the reagent brand or the testing laboratory.
• normal range for INR is 1.0-1.3
• usual target therapeutic INR for warfarin Rx are:
  • deep venous thrombosis (DVT)/pulmonary embolism (PE)/atrial fibrillation: 2.0-3.0
  • prosthetic heart valves: 2.5-3.5

• deficiency of one or more coagulation factors in the extrinsic coagulation pathway
  • this may be caused by either:
    • warfarin Rx
    • malnutrition
    • impaired liver function
    • other causes
• presence of a lupus type coagulation inhibitor - an in vitro phenomenon giving falsely prolonged results in some patients.
• presence of heparin (but not LMW heparin)