Skip Navigation LinksCore Service > Lab Test Directory > Haematology > Details

Core Services Test Directory

Go to all departments

Oral Anticoagulant Therapy Monitoring


Oral Anticoagulant Therapy Monitoring
Click here to magnify the image.

Reference Range:

Prophylaxis of DVT including surgery INR 2.0-2.5
Recurrent DVT and PE INR 3.0-4.5
This is not an exhaustive list of all the ranges, If advice required discuss with Anticoagulant nursing team or Haematology consultants
Contact via Hospital Switchboard.

Turnaround Time:

In Patient: 2 hours / GP: 24 hours


Commonly used oral anticoagulant is Warfarin (derived from 4-hydroxy coumarin). Warfarin works by antagonising Vitamin K which is needed for carboxylation of clotting factors whereby preventing the clot formation.
Warfarin has polypharmacy (affected by several other drugs);Potentiating drugs include allopurinol, amiodarone, quinine,tricyclics,thyroxine,anabolic steroids and alcohol;
warfarin antagonising drugs include oral contraceptives, spironolactone, antihistamines, barbiturates and rifampicin.
Laboratory minitoring depends on the Prothrombin Time (PT). and reported as INR (International Normalised Ratio) to accomodate the varaibility in the sensitivity of testing reagent (thromboplastin). The sensitivity of the reagent is labelled with ISI (International Sensitivity Index).
INR = (Prothrombin Time) raised to the power of ISI

Sample Requirements:

Samples must reach the laboratory within 4hrs


Underfilled samples will invalidate results
Haemolysis will invalidate results

Important Information:

All medication must be written on the request form



Transportation and Handling:

Samples which are to be transported by hand or local transport should be dispatched in clearly sealed individual plastic bags within 4 hours of collection.
If it is sent from longer distances, the person sending the specimen must ensure that the appropriate specimen container is used and samples must be spun (following protocol)and plasma should be separated and frozen to -70*C prior to transport. Contact Laboratory if advice required.

Special Precautions:

Do not subject the sample to extreme warm or cold temperatures as it induces heamolysis.

Time limits for requesting additional examinations:

If APTT or other relevant test needs to be added, must request them within 4 hrs