Anticoagulant medicines work by interrupting part of the process that is involved in the formation of blood clots. This means that blood clots are less likely to form where they are not needed, but can still form when they are.
Warfarin is a commonly prescribed anticoagulant medicine that is taken orally (swallowed) in tablet form. Warfarin interferes with your body's natural chemical processes by targeting a substance called vitamin K.
Vitamin K has an essential role to play in the production of prothrombin, which is a protein found in the blood. Prothrombin is a clotting factor, which plays an important part in the process of the formation of clots. If the production of vitamin K is slowed down, the production of prothrombin is also slowed. This means that it will take longer for blood clots to form.
Heparin occurs naturally in the body, but it can also be extracted and purified to be used as an anticoagulant medicine. It can be given to you by a healthcare professional, either as an injection or through a drip into a vein. Alternatively, you can be trained to administer the medication yourself.
Heparin interacts with the enzyme, thrombin. During the clotting process, thrombin acts upon a protein called fibrinogen to turn it into fibrin, which then forms the clot. Heparin can slow down the effect of the thrombin, so that it takes longer for fibrinogen to turn into fibrin. This means it takes longer for clots to form.
Different types of heparin medication include:
- enoxaparin (Clexane),
- tinzaparin (Innohep), and
- dalteparin (Fragmin).
How well your anticoagulant medicine is working is measured using the international normalisation ratio (INR).
International normalisation ratio
The INR is a way of measuring how fast your blood clots. As INR is an internationally recognised test, it can be used by healthcare professionals around the world.
During the test, a sample of your blood is taken and a chemical is added to it. The chemical starts a chain of chemical reactions that should make the blood in the sample clot (thicken).
During the clotting process, a protein that is found in the blood, called prothrombin, turns into the enzyme called thrombin. The time that it takes the prothrombin to turn into thrombin is called the prothrombin time ('pro-time', or PT). This is measured in seconds.
Your PT is compared to the PT of someone who is not taking anticoagulants. This then gives your INR.
For people who are on anticoagulant medicines, the aim is usually an INR of between 2.5-3.5, depending on what the anticoagulant medicines are being taken for (however, in some people, it can range from between 2-4.5).
This means that your PT will be between 2.5-3.5 times longer compared with the PT of someone who is not taking anticoagulant medicines. People who are not taking anticoagulant medicines usually have an INR of between 0.8-1.2.
Once your anticoagulant medicines begin to work, your INR should start to increase.
While you are taking anticoagulant medicines, your INR will be regularly tested by your GP, your pharmacist, or by a nurse at your local hospital. This is to make sure that your dose is correct.
If your INR is too high, blood clots will not form quickly enough and you may experience bruising or you may be at increased risk of bleeding. In this case, your dose may need to be reduced.
If your INR is too low, your medication is not working sufficiently, which means that clots could still form unnecessarily and block a blood vessel. In this case, your dose may need to be increased.
When you first start taking warfarin, your INR will be tested within the first 2-4 days of starting treatment. Depending on your reading, further tests will be carried out 1-2 times a week. If your INR stabilises within the correct range, testing may become less frequent.
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
To haemorrhage means to bleed or lose blood.
Anticoagulant is a substance that stops blood from clotting (prevents coagulation). For example, warfarin.