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Please enter at least one feature (symptom, sign or investigation result) before performing the calculation.
For example, if chest pain and low oxygen saturations were present, but haemoptysis was absent, the features section should look as follows:
To add a feature that is present, start typing and then click the green arrow.
To add the absence of a feature (i.e. a 'negative' finding), start typing then click the red cross.
If you want to remove a feature from the list simply click the grey cross in the box to the right of the feature.
Warfarin is widely used as an anticoagulant used in the management of atrial fibrillation and venous thromboembolism. Many patients who would previously have been prescribed warfarin are now given direct oral anticoagulants (DOACs).
Mechanism of action
Warfarin is an oral anticoagulant which inhibits epoxide reductase preventing the reduction of vitamin K to its active hydroquinone form, which in turn acts as a cofactor in the carboxylation of clotting factor II, VII, IX and X (mnemonic = 1972) and protein C.
Venous thromboembolism: target INR = 2.5, if recurrent 3.5
Atrial fibrillation, target INR = 2.5
Mechanical heart valves, target INR depends on the valve type and location. Mitral valves generally require a higher INR than aortic valves.
Teratogenic, although can be used in breastfeeding mothers
Skin necrosis: when warfarin is first started biosynthesis of protein C is reduced. This results in a temporary procoagulant state after initially starting warfarin, normally avoided by concurrent heparin administration. Thrombosis may occur in venules leading to skin necrosis
General factors that may potentiate warfarin
P450 enzyme inhibitors (see below)
Drugs which displace warfarin from plasma albumin, e.g. NSAIDs
Inhibit platelet function: NSAIDs
Drugs which either inhibit or induce the P450 system may affect the metabolism of warfarin and hence the INR:
Inducers of the P450 system include - INR will decrease
Inhibitors of the P450 system include - INR will increase
Antiepileptics: phenytoin, carbamazepine
St John's Wort
Chronic alcohol intake
Smoking (affects CYP1A2, reason why smokers require more aminophylline)
Antibiotics: ciprofloxacin, erythromycin
Imidazoles: ketoconazole, fluconazole
SSRIs: fluoxetine, sertraline
Acute alcohol intake
Patients on warfarin are monitored using the INR (international normalised ration), the ratio of the prothrombin time for the patient over the normal prothrombin time. Warfarin has a long half-life and achieving a stable INR may take several days. There a variety of loading regimes and computer software is now often used to alter the dose.