Heparin Induced Thrombocytopenia (HIT)

What Is HIT?
Heparin is a blood thinner commonly used for the prevention and treatment of blood clots and for heart and blood vessel procedures. The primary complication or side effect of heparin use is bleeding due to excessive blood thinning. One other important complication of heparin is a reaction called heparin-induced thrombocytopenia, or HIT. HIT is a severe and potentially life-threatening reaction to heparin or its cousin, low-molecular-weight heparin that can result in blood clots, and occurs in 3-5% of patients who are exposed to heparin but in only about 1% of those exposed to low molecular weight heparin. Patients who are at a particularly high risk of developing HIT include those receiving heparin with openheart or other surgical procedures.

In HIT, heparin causes the blood cells involved in clotting, called the platelets, to become abnormally activated, which can predispose to blood clot formation. Up to three quarters of patients who develop the HIT reaction to heparin can have a blood clot. Clotting can occur anywhere in the body such as in the veins of the arms or legs (DVT), or in the veins of the heart and lungs as a pulmonary embolism (PE) and even in the arteries of the fingers and toes, leading to gangrene. Patients with HIT can also have unusual symptoms such as severe skin rashes or low blood pressure. HIT is a reaction that can occur with any type of heparin preparation, including IV heparin and heparin shots.

Diagnosis of HIT
The diagnosis of HIT is suspected when a patient receiving heparin therapy has a drop in the platelet count (called thrombocytopenia), which is measured on a complete blood count test (CBC). This heparin reaction usually occurs five to 14 days after heparin therapy has been started. A patient can also be diagnosed with HIT when a new blood clot develops while receiving heparin. When a doctor suspects the possibility of HIT, other causes of a low platelet count need to be ruled out, such as other medication, bleeding resulting in blood transfusion, and infection. Although HIT is mainly diagnosed based on the judgment of a doctor, there are multiple blood tests that can be useful to help confirm the diagnosis. While it may take a few days for the results of these tests, the doctor may start treatment for HIT with another blood thinner (see below).

Treatment of HIT
Once HIT is suspected, heparin of any type must be stopped immediately. Unfortunately, this is not sufficient to improve the patient's condition, and so a blood thinner other than heparin needs to be started to prevent the blood clotting that may occur during HIT due to the abnormally activated platelets. Multiple blood-thinning medications are available for the treatment of HIT (the most common agents are known as direct thrombin inhibitors). Once heparin is stopped and the alternative blood thinner is administered, the doctor will follow the platelet count until it begins to return to normal. This is expected to occur over a few days. Once an improvement in platelet count is noted, warfarin (Coumadin®), a blood thinner which comes in a pill form, is usually started. The patient with HIT is treated with warfarin for at least three months and possibly longer, depending on other factors and the reason why the blood thinners were started in the first place.

Prevention of HIT
It is important that a patient who has had HIT avoid any future heparin or low-molecular-weight heparin exposure and tell his or her health-care providers about this reaction. Heparin should be entered into the medical chart as a medication allergy, even though it is not a typical allergic reaction. It is particularly important to record prior HIT in the medical record because the abnormal blood test results that are used to diagnose HIT can be completely normal within weeks. Patients with a history of HIT should also consider carrying a wallet card or wearing a medical-alert bracelet that lists heparin as an "allergy."

There are a few rare cases in which patients with a history of HIT may be safely re-exposed to heparin for a very short period of time. An example of this is during cardiac surgery, where heparin is the preferred blood thinner due the availability of an antidote (a medication that can be given and will reverse the blood thinning caused by heparin).

Finally, one must know that HIT is not common, and one should not avoid heparin for fear of getting this reaction. Heparin has many advantages as a blood thinner, including the fact that it is a well-established medication with which healthcare providers have had a lot of experience. That being said, health-care providers need to routinely monitor patients while receiving heparin therapy to look for any sign of HIT.

About the Author: Carmel Celestin, MD, is a vascular medicine specialist at Cleveland Clinic Foundation, Cleveland, OH, and is involved in the diagnosis and treatment of a wide variety of vascular diseases.