ANTICOAGULATION The objectives of this section are: To be able to write prescriptions according to local anticoagulation guidelines To know how to prescribe unusual dose regimens on a drug chart To know how to initiate anticoagulation therapy To know how to prevent and manage over anticoagulation To be able to demonstrate awareness of possible drug interactions with warfarin To understand the discharge procedure for anticoagulated patients

Warfarin, heparin and related anticoagulants are frequently involved in serious medication errors. Most patients are treated safely with anticoagulants. However, if therapy is not monitored properly, or the patients clinical condition or concurrent drug therapy changes, over- or under- anticoagulation can result with potentially fatal consequences Safe anticoagulation is a multidisciplinary process involving healthcare professionals in both primary and secondary care. INTRODUCTION

A 66 year old man was treated with warfarin for atrial fibrillation. He developed acute arthritis, diagnosed as gout by his general practitioner. He was prescribed the anti-inflammatory drug azapropazone. The dose was subsequently increased in response to an exacerbation of his arthritis. The patient then developed signs of bleeding. The general practitioner arranged for a full blood count, but did not check the INR. Before the results were available the patient suffered a massive intracranial haemorrhage, was admitted to hospital and died. His INR on admission was greater than 10. (from Building a Safer NHS for patients – Improving Medication Safety. Department of Health. 2004) Fatal outcome of azapropazone/warfarin interaction

Drug interactions with warfarin Inhibit warfarin clearance ↑ INR amiodaroneFluconazole 9and other azoles) Ciprofloxacin ( and other quinolones) erythromycin sertralineisoniazidclarithromycin Unknown mechanism ↑ INR metronidazolecitalopramdiltiazemCo-trimoxazole tolterodine

Increase warfarin clearance ↓ INR rifampicincarbamazipin e Unknown mechanism ↓ INR trazadonepenicillins Increased bleeding risk Avoid use with warfarin if possible Antiplatelets aspirinclopidogrelNSAIDS COX II inhibitors Drug interactions with warfarin

The above list contains the drugs that are most likely to interact according to available evidence, however it is prudent to check the INR within 5 days of starting any new drug in a patient who is already on warfarin. For patients who are being loaded on warfarin and who are also taking interacting drugs it is sometimes appropriate to use lower loading doses.

CHECKLIST FOR PATIENTS DISCHARGE Reason for receiving anticoagulant What is warfarin Expected duration of therapy Dosage and compliance Missed dose Possible side effects Drug interactions Diet and alcohol Women of childbearing age Anticoagulation booklets

Summary All patients discharged on warfarin must be referred to an anticoagulation clinic for continued INR monitoring and dosing All patients will need an INR test on the day of discharge (unless previously agreed with anticoagulation clinic) All patients will need to be counselled before discharge. Ensure that the patient at least understands the following: That warfarin comes in 3 different colours/strengths What dose to take (in number of tablets and colours if necessary) and that the dose will be written in their yellow anticoagulation book The date of their next INR test Procedure for their next INR test

PRESCRIBING 1.Should you restart this patients warfarin? Mr X who is 24 hours post-op following TKR. He usually takes warfarin for AF and his INR is now Mr X has attended the anticoagulant OPD this morning to have his INR checked ( he had a PE postoperatively 3 months ago), he has been taking 3mgs of warfarin daily and his INR is now 2.1 is this INR acceptable? And if not what should you do?

1MG 3MG 5MG