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Friday, February 27, 2015
Thrombosis is the medical term for an abnormal blood clot in an artery or vein. The body's ability to form blood clots is its natural defense against bleeding. Clots are formed through a series of chemical reactions between special blood cells (platelets) and proteins (clotting factors) in blood. The platelets and factors work together to regulate the clotting process to start and stop clotting as the body needs it. Sometimes the process does not work correctly, and a clot forms in blood vessels, blocking blood flow to the surrounding tissues. There are two main types of clots. How they effect the body depends on the type and location of the clot:
To treat blood clots and prevent the damage they cause, doctors use anticoagulants, which are commonly called blood thinners, to decrease the clotting power of the blood and prevent growth of a clot. The most common blood thinners used today are heparin, low molecular weight heparin, and warfarin.
Heparin is a strong, fast-acting anticoagulant (blood thinner). It is usually given in the hospital by IV (a small needle inserted in a vein), but it can also be given by an injection under the skin. IV heparin works rapidly; within minutes of receiving it, most patients have excellent anticoagulation that will prevent further clotting. However, patients who get heparin must be monitored every day with a blood test to see if the correct dose is being given. The doctor will adjust the dose of heparin according to the blood test results. Because heparin levels often change in patients, the doctor must check levels frequently. The name of the blood test used to check a patient's heparin level is the activated partial thromboplastin time (aPTT).
For patients who have a new clot, heparin is usually given with another anticoagulant, warfarin (Coumadin®). Warfarin is a pill that patients can take at home for long term anticoagulation. Because it can take 5-7 days (or longer) for the warfarin to take effect, patients will initially take both drugs. Once the warfarin is fully active, the heparin is stopped and the patient can go home from the hospital.
The advantages of heparin are its low cost and fast action (blood can be anticoagulated quickly). The disadvantages of heparin include the need for frequent blood tests to check the levels of anticoagulation and hospitalization to get an IV drug. Patients should expect to be in the hospital 5-10 days to treat a new clot.
The most serious side effect of heparin is bleeding. Other side effects include skin rash, headache, cold symptoms, and stomach upset. A less common side effect is loss of bone strength if patients are on heparin for long periods of time (usually months). This is generally only a problem for pregnant women. A rare side effect of heparin is a condition called Heparin Induced Thrombocytopenia (HIT). HIT is sometimes incorrectly called "heparin allergy." It is an autoimmune process with development of low platelet count. It occurs in a small number of patients - 3-5 % of patients on heparin - but it has very serious symptoms including worsening of clotting and the development of new clots, which can lead to stroke, heart attack, deep vein thrombosis, and death.
Low molecular weight heparins (LMWH) are a fairly new drug class that is similar to heparin but much easier to use. The drugs available in the U.S. are Dalteparin (Fragmin®), Enoxaparin (Lovenox®), and Tinzaparin (Innohep®). Using LMWH has two advantages over heparin:
The side effects of LMWH are very similar to heparin; however, HIT and osteoporosis are much less common. LMWH is more expensive than unfractionated heparin.
Warfarin (Coumadin®) is an anticoagulant pill that is taken by mouth. Patients are given warfarin for different reasons. Some patients may take warfarin for a few weeks; others will need to take warfarin the rest of their lives. The length of treatment depends on the reason why a patient needs anticoagulants.
Warfarin works by slowing down the process in the liver that uses vitamin K to make certain proteins (clotting factors) that cause clotting. Because it may take several days before warfarin becomes completely effective, heparin or LMWH is given concurrently until the warfarin is effective.
As with patients who take heparin, patients taking warfarin need to have their blood tested to see how well the drug is working and to be monitored for safety. This blood test measures how long it takes blood to clot, and is also called a prothrombin time, protime, INR, or clotting time. Because different labs use different methods to measure clotting time, the results of the test can vary. To make sure a doctor can correctly interpret this test, the results are reported with an INR number (International Normalized Ratio) that converts all clotting times to the same number. People who are not taking warfarin have an INR around 1.0 (usually between 0.8 and 1.2). Most patients on warfarin should have an INR between 2 and 3; this is considered their therapeutic range. In some patients, a higher or lower INR range is targeted. If a patient has an INR that is below their therapeutic range, the risk of clotting is higher; if a patient has an INR above the therapeutic range, the risk of bleeding is higher.
When patients first start warfarin, they may get their blood tested two or three times a week. Once patients are on a regular dose of warfarin, they may go as long as 4 weeks between blood tests, but ongoing monitoring of the INR is required as long as the patient remains on warfarin.
Bleeding is the most common side effect of warfarin. Other side effects include headache, rash, hair loss, skin necrosis, purple toe syndrome, and elevated liver enzymes. Sometimes these side effects will go away over time; however, it is important to discuss any of these side effects or unusual symptoms with your health care provider.
Permission to reprint this material has been granted by the National Alliance for Thrombosis and Thrombophilia (http://www.stoptheclot.org/).
Last Reviewed: Nov 17, 2008
Elizabeth A Varga, MS, CGC
Clinical Assistant Professor of Pediatrics
College of Medicine
The Ohio State University