Warfarin Use

What is warfarin?

Warfarin is a prescription medication that interferes with the formation of blood clots. It is also called an anticoagulant. Many people refer to these kinds of medication as blood thinners, although they do not actually cause the blood to become thin.

Warfarin blocks the function of vitamin K in the clotting process. As a result, the clotting mechanism is disrupted and it takes longer for the blood to clot.

Uses of warfarin

In some patients, however, the clotting mechanism may be triggered by a condition that leads to the formation of small clots in the bloodstream. These clots may travel through the bloodstream and become lodged in smaller blood vessels, reducing blood flow to the organs supplied by those vessels. Blockage of blood flow can cause serious problems including stroke (when a blood vessel leading to a portion of the brain is blocked) and heart attack (when a blood vessel leading to the heart is blocked).

Warfarin is prescribed for patients who are at increased risk of developing harmful blood clots. Patients at risk of developing such clots include those with a mechanical heart valve, an irregular heart rhythm called atrial fibrillation, and patients with certain clotting conditions.

Warfarin is also used in patients who have previously developed harmful clots, including patients who have had a stroke, heart attack, a clot which has travelled to the lung (pulmonary embolism), or a blood clot in the leg (deep venous thrombosis or DVT). In addition, warfarin may be used to prevent an existing clot from growing larger.

Monitoring

The goal of warfarin therapy is to decrease the clotting tendency of blood, but not to prevent clotting completely. Therefore, the effect of warfarin on the blood's ability to clot must be carefully monitored with regular blood testing. Based on the results of these tests, the dosage of warfarin is adjusted to maintain the clotting time within a target range.

International Normalised Ratio (INR)

The test most commonly used to measure the effects of warfarin is the prothrombin time (PT). The INR is a way of expressing the PT in a standardised way; this ensures that results obtained by different laboratories can be reliably compared.

Prothrombin time (PT)

The PT is a laboratory test that measures the time it takes for clotting to progress. It is particularly sensitive to the clotting factors affected by warfarin. The PT is also used to calculate the INR value.

The longer it takes for the blood to clot, the higher the PT and INR. The target INR range depends on the clinical situation. In most cases the target range will be two to three, although other ranges may be chosen if there are special circumstances. If at any particular time the INR is below the target range i.e., under-anticoagulation, there is a risk of clotting. If, on the other hand, the INR is above the target range i.e., over-anticoagulation, there is an increased risk of bleeding.

Dosage 

When warfarin is first prescribed, a higher loading dosage may be given so that an effective blood level of the drug is achieved quickly. The loading dosage is then adjusted downward until a maintenance dose is found that maintains the INR within the desired range. The PT and INR are monitored frequently until the maintenance dosage has been determined. Once the patient is on a stable maintenance dose, the PT and INR are monitored less frequently, generally once every two to four weeks.

The warfarin dosage may be adjusted periodically in response to a changing INR or to clinical circumstances that call for an increase or decrease in warfarin therapy. For example, having surgery may require a change in a patient's warfarin regimen. The dosage of warfarin may also be modified if other medication is taken.

Side effects

The major complication associated with warfarin is bleeding due to excessive anticoagulation. Excessive bleeding, or haemorrhaging, can occur from any area of the body, and patients on warfarin should report any falls or accidents, as well as signs or symptoms of bleeding or unusual bruising to their doctor. Signs of unusual bleeding include bleeding from the gums, blood in the urine, bloody or dark stool, a nosebleed, or vomiting blood. Due to the risk of bleeding increasing as the INR rises, it is closely monitored and adjustments are made as needed to maintain the INR within the target range.

Warfarin can also cause skin necrosis or gangrene, showing as dark red or black lesions on the skin. This is a rare complication that may occur during the first few days of warfarin therapy.

When to seek help

Consult your doctor immediately if there are obvious or subtle signs of bleeding, including:

• persistent nausea, gastrointestinal upset, or vomiting blood or other material that looks like coffee grounds;
• headaches, dizziness, or weakness;
• nosebleeds;
• dark red or brown urine;
• blood in the bowel movement or dark-coloured stool;
• pain, discomfort, or swelling, especially after an injury;
• after a serious fall or head injury, even if there are no other symptoms;
• bleeding gums while brushing your teeth;
• swelling or pain at an injection site;
• excessive menstrual bleeding or bleeding between menstrual periods;
• diarrhoea, vomiting, or inability to eat for more than 24 hours; and
• fever (temperature greater than 38º C).

Pregnancy and warfarin

Warfarin is not recommended during pregnancy, especially during the first trimester, due to an increased risk of miscarriage and birth defects. A patient who becomes pregnant or plans to become pregnant while on warfarin therapy should notify their treating doctor immediately.

Breastfeeding

Although warfarin does not pass into breast milk, a woman taking warfarin should first consult her doctor before breastfeeding. Warfarin is considered safe for use by women who are breastfeeding.

Other recommendations

Take warfarin on a schedule

Warfarin should be taken exactly as directed. Do not increase, decrease, or change the dosage schedule unless told to do so by a doctor. If a dose is missed or forgotten, call the prescribing clinician for advice.

If the colour or dosage of the tablet appears different than those taken previously, the patient should immediately notify their pharmacist or doctor.

Reduce the risk of bleeding

There is a tendency to bleed more easily than usual while taking warfarin. Some simple changes can decrease this risk:

• Use a soft bristle toothbrush.
• Floss with waxed floss rather than un-waxed floss.
• Shave with an electric razor rather than a blade.
• Take care when using sharp objects, such as knives and scissors.
• Avoid activities that have a high risk of falling or injury (e.g. contact sports).

Prevent falls

Falling may significantly increase the risk of bleeding. Taking measures to prevent falling is recommended, and could include:

• removing loose rugs and electrical cords or any other loose items in the home that could lead to tripping, slipping, and falling;
• ensuring that there is adequate lighting in all areas inside and around the home, including stairwells and entrance ways;
• trying to avoid walking on ice, wet or polished floors, or other potentially slippery surfaces; and
• avoiding unfamiliar areas outside.

Warfarin and food

Some foods and supplements can interfere with warfarin's effectiveness. After being stabilised on a particular warfarin dosage, consult your doctor before making major dietary changes (e.g. starting a diet to lose weight, starting a nutritional supplement or vitamin).

• Vitamin K: Eating an increased amount of foods rich in vitamin K can lower the prothrombin time and INR, making warfarin less effective, and potentially increasing the risk of blood clots. Patients who take warfarin should aim to eat a relatively similar amount of vitamin K each week. Some foods have a high level of vitamin K, including: kale, broccoli, spinach, collard or turnip greens, lettuce, Brussels sprouts, and cabbage. It is not necessary to avoid these foods. However, the patient should eat a relatively similar amount on a regular basis rather than eating a large serving occasionally.

• Cranberry juice: Some experts have reported that drinking cranberry juice while on warfarin can cause significant over-anticoagulation and bleeding. However, a small study found that drinking one 230ml serving of cranberry juice per day for seven days had no effect on the INR of seven men taking warfarin for atrial fibrillation. It is plausible that larger amounts could have a more significant effect. The best advice is to consume minimal amounts of cranberry juice and to consult your doctor regarding any concerns about a possible interaction.

• Alcohol: Chronic abuse of alcohol affects the body's ability to handle warfarin. Patients on warfarin therapy should try to avoid drinking alcohol. Alcohol should be limited to one serving of alcohol on an occasional basis. In addition, drinking excessive amounts of alcohol can increase the risk of injury, and therefore bleeding.

Warfarin and medication

Medication, herbs and vitamins can interact with warfarin. This interaction may affect the action of warfarin or the other medication. If warfarin is affected, the dosage may need to be adjusted to maintain an optimal coagulation effect. Please inform anyone who prescribes medication to you, that you are on warfarin.

Patients who take warfarin should consult with their clinician before taking any new medication, including over-the-counter (non-prescription) drugs, herbal remedies, vitamins or any other medicinal products.

Some of the most common over-the-counter pain relievers, including paracetamol (such as Panado®), aspirin, and non-steroidal anti-inflammatory drugs (such as ibuprofen and Voltaren®) enhance the anticoagulant effects of warfarin. Vitamin E may increase the anticoagulant effects of warfarin. Consult your doctor before adding or changing a dosage of vitamin E or any other vitamin.

Wear medical identification

People who require long-term warfarin treatment should wear a medic alert bracelet, necklace, or similar identification at all times. In the event of an accident, where the patient is unable to explain his/her condition, this will help emergency personnel provide the appropriate care.

The medical identification should include any major medical conditions and the reason why warfarin is needed (e.g. atrial fibrillation), as well as the name and contact details of an emergency contact person. MedicAlert® South Africa provides a toll-free number that emergency medical workers can call to find out a person's medical history, list of medication, family emergency contact numbers, doctor's name and contact numbers.