Article contributed by Mr Chai Zheng Hao, a pharmacist at Parkway East Hospital Pharmacy.
What are anti-coagulants?
Anti-coagulants are medications that prevent the formation of clots in an artery, a vein or the heart, and prevent existing clots from enlarging.
What are anti-coagulants used for?
There are various conditions or reasons why anti-coagulant therapy may be prescribed and they include:
There is an increased tendency for blood to clot excessively or too easily in the above conditions. The resultant clots formed may cause poor blood circulation or block a blood vessel, which may lead to a heart attack, stroke, tissue damage or death. Thus, anti-coagulants are prescribed for prevention.
Types of anti-coagulants
How does it work?
Warfarin prevents the activation of vitamin K, an essential step in the blood clotting process.
How is the dosage determined?
The warfarin dosage is based on a blood test called the International Normalised Ratio (INR). The INR test measures how easily blood clots. Your doctor will decide the INR target range for your condition. If the INR result is above your target (too high), it means that the blood takes a longer time to form clots, increasing the risk of bleeding. Conversely, if the INR result is below your target (too low), blood may take a shorter time to form clots, and hence tend to clot more easily. Your doctor may require you to take frequent blood tests at the beginning of the treatment to determine your body’s reaction to warfarin and the suitable dosage for you. Based on your INR results, your doctor will tell you if you need to change your dose and when you will need the next blood test. Once you are on a regular dosage, you do not need to do the blood test as often, unless you get sick, change your medication regimen or change your diet.
Small changes in doses may lead to significant changes in INR and other undesirable side effects. Hence, it is important to take your dosage as prescribed by your doctor to prevent fluctuations in your INR level. Different warfarin manufacturers may have minute differences in their tablet content and there may also be differences in the way the warfarin is absorbed by the body. Hence, it is a good practice to keep to the same brand of warfarin that you are on. The two brands of warfarin that are sold in Singapore are Marevan and Apo-Warfarin. Make it a point to recognise the colours of the warfarin tablets which you are taking.
Below is a chart of the warfarin tablets, showing the different colours and strengths for each brand.
Additional things to note of when taking warfarin
|Food||Amount of Vitamin K Content|
|Cereals & Grain||Bread
|Dairy & Egg||Butter
Novel Oral Anti-Coagulations (NOACs)
NOACs are the newest class of anti-coagulation agents. They work by inhibiting a clotting factor directly. The 3 NOACs in Singapore are:
NOACs are easier to use, as they do not have a small safety window like warfarin and they do not require extensive blood tests. They are easily available as oral tablets.
With the exception of dabigatran, there are no agents for rapid reversal of their effects, in the event of an overdose.
Low-Molecular Weight Heparins and Unfractionated Heparin
These are molecules that inhibit a clotting factor required to commence the clotting mechanism, to stop clots from forming.
This is a popular choice for the in-patient setting, as they take a short time to act. Their effects are easily reversible in the event of an over anti-coagulation and they do not last for a long period of time in our body, making it easy for physicians to control the anti-coagulation effect needed in a short time.
However, they are only available in injectable form. Hence, they are not widely used in the out-patient setting, due to the inconvenience.
Low-molecular weight heparins are available as:
Unfractionated heparin (UFH) was the first anti-coagulant discovered in 1916. The discovery of low molecular weight heparin (LMWH) was based on unfractionated heparin and both heparins are similar in terms of its uses and benefits. However, UFH can only be given intravenously (through a vein directly into the bloodstream) whereas LMWH can be given subcutaneously (under the skin) or in out-of-hospital situations. UFH may require extensive blood tests and closer monitoring of its effects. UFH also tends to be affected by other protein-binding drugs to a greater extent and hence has more drug interactions. However, UFH has been studied more widely and is usually more affordable than LMWH.
What to take note of when taking anticoagulants?
The most common side effect is bleeding from any part of your body. The bleeding can be minor or sometimes serious and dangerous. Hence, it is important to recognise any signs of bleeding in your body. A visit to the doctor to readjust your dosage may be required.
Major signs of bleeding
To avoid the risk of bleeding and bruising:
If your anti-coagulation medication is not adequate, clots will form easily in your body and you may experience signs and symptoms of clotting. This is also dangerous as the blood clots may break off and travel to your lungs, legs and brain, blocking off circulation in the affected areas. A visit to the doctor to readjust your dosage may be required.
Major signs of blood clots
What if I miss a dose?
Keep to your regular timing for medications when taking anti-coagulants. Your body may not have adequate anticoagulants to prevent blood clots from forming, if you miss a dose. Taking doses too near each other or double dosing, will thin your blood and bleeding may occur. The various anti-coagulants have specific instructions on how to manage a missed dose. Make sure to consult your pharmacist or doctor, if you are unsure.
Other things to note