Warfarin- its management and monitoring and role in stroke risk reduction

Question 1

Atrial fibrillation

Answer 1

Most common abnormal heart rhythm affecting 1-2% population
More common as we get older
1/3 people with AF have a normal heart with no underlying cause

Question 2

Thrombus
Embolism
Thromboembolism

Answer 2

Thrombus- a blood clot that forms in a blood vessel and remains there
Embolism- a blood clot that moves away from the site where it forms to another location in the body
Thromboembolism- a blockage in a blood vessel caused by a blood clot that has travelled from where it was formed elsewhere in the circulation

Question 3

Clotting cascade (simplified)

Answer 3

Damaged blood vessel- injury to vessel lining triggers the release of clotting factors
Formation of platelet plug- vasoconstriction limits blood flow and platelets form a sticky plug
Development of clot- fibrin strands adhere to the plug to form an insoluble clot

Question 4

Warfarin (basics)

Answer 4

Vitamin K antagonist
Used as a rat poison
Dose varies from person to person so requires close monitoring
Not a blood thinner- makes the blood less sticky and therefore less likely to form clots
Used widely and successfully
Actions are reversible- readily available antidote

Question 5

Indication for warfarin

Answer 5

Prophylaxis in AF
Prophylaxis after insertion of a prosthetic heart valve
Prophylaxis of VTE and PE

Question 6

Mode of action of warfarin

Answer 6

Administered orally and has 100% bioavailability and a low volume of distribution
Competitively inhibits vitamin K from diet helping the liver to produce clotting factors, and disrupting the clotting cascade

Question 7

Why does warfarin have a slow onset of action?

Answer 7

Due to the varying half life of each clotting factor, warfarin has a slow onset of action and takes a few days to achieve steady state
It therefore takes a few days for the result to be seen in an INR test (3-5 days)
In the same way, warfarin effects continue for a few days after stopping therapy

Question 8

How long is the half life of warfarin and what are the implications of this?
How are the doses tailored to each individual?

Answer 8

Warfarin has a long half life of approx. 35 hours so only once daily dosing is required. It can be taken at any time of the day with or without food.
Warfarin has a narrow therapeutic window so all warfarin doses need to be tailored to each individual and close monitoring of the anticoagulant effect is required.
This is done by monitoring the patient’s INR

Question 9

INR

Answer 9

International Normalised Ratio
Globally recognised and is used to compare clotting rates
Ratio of the time taken to clot against a control sample

Question 10

Point of care testing (POCT)

Answer 10

Testing at or near where a patient is located
Finger-prick blood sample
Test result straight away
Dosed and managed within minutes
Pharmacist led service
Independent prescribing

Question 11

What are the two types of risk factors for stroke?

Answer 11

Modifiable risk factors

Non-modifiable risk factors

Question 12

Range and target INR

What is the duration of treatment?

Answer 12

The target INR is dependent upon the clinical indication being treated
In AF, the INR target is usually 2.5 within a range of 2.0 to 3.0
The duration of treatment with warfarin can very from 6 weeks to lifelong and is also dependent upon the clinical indication

Question 13

How is warfarin metabolised?

Answer 13

Warfarin is almost entirely removed from the body via the liver by hepatic metabolism. For this reason, variations in age, size, diet and alcohol intake all effect our warfarin requirements.
It is metabolised by the cytochrome P450 system and therefore a plethora of drug interactions can occur.

Question 14

Drug interactions

Answer 14

Corticosteroids
Amiodarone
Levothyroxine
Alcohol
Miconazole
NSAIDs
Herbal preparations such as Devil's claw, St John's Wort, garlic

Question 15

Safer use of oral anticoagulants

Answer 15

Anticoagulants are most frequently associated as causing preventable harm and admission to hospital
Managing the risks can reduce the chance of patients being harmed in the future

Question 16

Criteria relevant to pharmacy on the safer use of oral anticoagulants

Answer 16

Ensure that patients prescribed anticoagulants receive appropriate verbal and written information throughout the course of their treatment
Promote safe practice with prescribers and pharmacists to check that patient’s blood clotting is being monitored regularly and that the INR level is safe before issuing or dispensing repeat prescriptions for oral anticoagulants
Ensure that those dispensing clinically significant interacting medicines for these patients check that additional safety precautions have been taken
Risk assessments should be undertaken on the use of monitored dosage systems for anticoagulants for individual patients

Question 17

Anticoagulation care record book

Answer 17

Patient held record of all their INR results known as the yellow book
Safety booklet includes side effects, dietary advice, colours of tablets, pregnancy, emergency contact information and patient alert card
Must be checked prior to prescribing and dispensing

Question 18

What happens when a stroke is secondary to AF?

Answer 18

A stroke secondary to AF is often severe and results in long term disability
Risk of death from a stroke is double if associated with AF