Sarah Taylor

• Patients who suffer from atrial fibrillation are at increased risk of systemic embolism and stroke. Oral anticoagulant therapy has been reported to reduce the incidence of these complications in patients with non-rheumatic atrial fibrillation.
  
• The target INR will determine the warfarin dose. This varies according to the indication. The indication for treatment should be clearly recorded in the medical notes and in the patient held anticoagulant treatment booklet. The indications and target INRs currently recommended by the British Society for Haematology are;

INR 2-2.5 Prophylaxis of deep-vein thrombosis including surgery on high risk patients
INR 2.5 treatment of deep vein thrombosis and pulmonary embolism (or for recurrence in patients no longer receiving warfarin), atrial fibrillation, cardioversion, dilated cardio-myopathy, mural thrombus following myocardial infarction and rheumatic mitral valve disease
INR 3.5 For recurrent deep vein thrombosis and pulmonary embolism (in patients currently receiving warfarin) and mechanical prosthetic heart valves

• Anticoagulant treatment booklets including sections on monitoring records, information and advice should be given to every patient and are available through community pharmacies.
  
• The plasma half-life of warfarin is 35 hours. Steady state will not be achieved for about one week. Therefore if immediate anticoagulation is required, heparin must be given concurrently.
  
• There are many clinically important drug interactions with warfarin - prescribers are advised to consult the BNF or Summary of product characteristics for full details before prescribing.
  
• Oral anticoagulants should not be used in cerebral thrombosis or peripheral arterial occlusion as first line therapy.
  
• The main adverse effect of warfarin (and all oral anticoagulants) is haemorrhage. The BNF gives details of management of patients depending on their INR result and whether there is minor or major bleeding.
  
• Aspirin is more appropriate for reduction of risk in transient ischaemic attack.
  
• Nicoumalone and phenindione are rarely used but may be considered in warfarin resistant patients.

Formulary Choice - Warfarin

Starting Dose
The usual starting dose of warfarin is 10mg daily for 2 days. The subsequent maintenance dose depends on the prothrombin time, as determined by the International Normalised ratio (INR). A lower induction dose may be required by some patients (See BNF for details)

Monitoring
INR should be measured daily or on alternative days during initial stages of treatment or if the dose is not stable. Once treatment is stabilised, the INR should be measured at least every 12 weeks. An INR within 0.5 units of the target value is generally satisfactory. Larger deviations require dosage adjustments

ADVICE TO PATIENTS

The following advice to patients is also available as a printable PDF document

1. This medicine is used to thin the blood so that clots will not form. There are many medical conditions where this is needed
2. You can take these tablets with or without food. If it upsets your stomach, take it with food
3. You should take this tablet at the same time each day, usually at about 6pm
4. If you miss a dose, take it as soon as possible. If it is almost time for the next dose, do not take the missed one. Do not take an extra dose to make up for it
5. You should tell any doctor, dentist, pharmacist or nurse that you take this medicine. Some medicines that you can buy should not be taken with this medicine. Ask your pharmacist or doctor for advice
6. Tell your doctor immediately if you have any bleeding problems. This includes nosebleeds, coughing up blood, blood in the urine or stool, or unexplained bruising
7. It is important that your blood is tested regularly. Your Doctor will tell you when this should be done. Remember to take your anticoagulant book with you to your appointment and show it to your doctor when you request a repeat prescription.
8. You should read the information leaflet that you have been given with your medicines carefully. This contains full information about side effects etc. If you have any questions or concerns about your medicines, you should discuss them with your doctor or pharmacist. Patient information leaflets are available at www.emc.vhn.net/public

Based on "Advice Tips" published in the Pharmaceutical journal by Pharmacy Practice Consultants.
Available at www.pharmj.com/noticeboard/tips/patients/index.html