TRIP Answers

21 February 2008 note: This question is over 2 years old and may differ to any new research.

CKS guidance on Atrial Fibrillation (1) has a section on the initation of antithrombotic therapy. This states:

“How should antithrombotic therapy be initiated?

Both the antithrombotic benefits and the potential bleeding risks of long-term anticoagulation should be explained to and discussed with the patient (D, GPP). Before starting anticoagulation therapy, as part of the clinical assessment, people should be assessed for the risk of bleeding. Particular attention should be paid to people who:

Are over 75 years of age (D).

Are taking antiplatelet drugs (such as aspirin or clopidogrel) or nonsteroidal anti-inflammatory drugs (C).

Are on multiple other drug treatments (polypharmacy) (C).

Have uncontrolled hypertension (C).

Have a history of bleeding (e.g. peptic ulcer or cerebral haemorrhage) (C).

Have a history of poorly controlled anticoagulation therapy (D, GPP).

When starting warfarin in people with AF, there is no need to achieve anticoagulation rapidly; a slow loading regimen is safe and achieves therapeutic coagulation in the majority of people within 3–4 weeks [Baglin et al, 2005].

Warfarin 2 or 3 mg each day is generally an acceptable starting dose. For more information, see How should warfarin be monitored? A low starting dose is often more suitable for elderly people and people at high risk of bleeding.”

In practical prescribing points they note.

“We recommend that people be given a supply of 1-mg tablets on initiation of warfarin, to reduce possible confusion about changing doses. Once a person has been stabilized on warfarin, a combination of tablet strengths may be used to achieve the required dose with the least number of tablets.”

On monitoring they note,

“Initially, the INR should be determined daily or on alternate days (until it is within the therapeutic range on two consecutive occasions), then at longer intervals (depending on the response), and then up to every 12 weeks (if agreed locally).

INR target is 2.5 (range 2.0–3.0).

If the INR is out of the therapeutic range, ask the person about any changes, such as their routine for taking warfarin, alcohol use, or other medicines prescribed or purchased (see What drug interactions can occur with warfarin?). If the INR is high:

Check if the person has any signs of bleeding, as prompt intervention with vitamin K is sometimes needed to reverse the effects (see What adverse signs should people be aware of and what should they do if they have them?). The warfarin dose may need to be (temporarily) stopped or reduced and INR determined after 2 or 3 days to ensure that it is falling.

If the INR is low:

Check whether any doses have been missed. The warfarin dose may need to be (temporarily) increased and sometimes a booster dose may need to be taken — the INR should then be measured 2 or 3 days later to ensure that it is increasing.”

The full document is freely available at http://www.cks.library.nhs.uk/atrial_fibrillation/view_whole_guidance#NodeIdn172245n172301n244060

1. http://www.cks.library.nhs.uk/atrial_fibrillation/view_whole_guidance#NodeIdn172245n172301n244060