Clinical Relevance:

Warfarin is an oral anti-coagulant. It acts through inhibiting vitamin K dependent carboxylation of clotting factors II, VII, IX and X, all of which are produced in the liver. The response to Warfarin is monitored by measuring the Prothrombin Time (PT).

To standardise the Warfarin effect on coagulation, laboratories report an International Normalised Ratio (INR), which attempts to eradicate the variable potency of different reagents.

Reference Range: N/A
Min. Volume: The volume of blood in coagulation samples must lie within the volume range as indicated by the size of the black fill arrow present on tubes. Volumes above or below the arrow will result in sample rejection to ensure validity of results.
Turnaround Time: Urgent: 1 hour
Routine: 8 hours
Age of Sample: Samples will be rejected if received more than 8 hours after venepuncture.
Specimen Requirements:
  • 1 blue top (sodium citrate) sample.
  • All coagulation tubes must be adequately filled (see above).
  • All coagulation tubes must be mixed several times by gentle inversion immediately after venepuncture. Mixing the sample with the anticoagulant stops the sample clotting within the tube.
  • It is not possible to provide results on haemolysed samples. These will be rejected with the appropriate comment.
  • Sample tubes that have expired cannot be accepted.
Analysing Laboratory: Coagulation (JCUH and FHN)