What Is Heparin Monitoring?

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  • Written By: Mary McMahon
  • Edited By: Nancy Fann-Im
  • Last Modified Date: 28 December 2019
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Heparin monitoring is ongoing evaluation of a patient on heparin therapy to make sure the medication works properly and the patient's risks are kept to a minimum. A doctor may recommend this anticoagulant medication for a patient at risk of clots or a patient with clots that need to be resolved to prevent thrombosis. While on heparin, patients are at increased risk for serious bleeds, because their blood does not clot as quickly. If they reach an unsafe dosage on the medication, this could create significant risks.

When a doctor prescribes heparin, she will prescribe a loading dose to get the patient's clotting rate down, and then periodic boluses to maintain a therapeutic level of the medication. Within six hours of the loading dose, and every six hours afterward, blood needs to be drawn for a heparin test. This heparin monitoring allows the doctor to determine when and how to adjust the dose. Once the patient is on a stable dose, a single daily test is enough. Heparin monitoring every six hours will resume if the doctor needs to change the dose.


There are several different tests a hospital laboratory can perform for heparin monitoring. One is the partial thromboplastin time (PTT) test. This measures how long it takes for the patient's blood to clot and provides quick feedback for the doctor. The lab will give the doctor a reference range of values based on other patients so he can determine if a patient's blood clots normally, too quickly, or too slowly. The goal is to reach a consistent clotting time and maintain it.

Another option is the anti-Xa assay, which directly measures the amount of heparin in the patient's blood. A doctor may request this test if a patient appears resistant to heparin or if there is a problem with the PTT test, like a delayed result. This can be useful for heparin monitoring when the doctor wants an absolute value on how much heparin is in the patient's circulation.

As a patient's condition improves, the doctor may start to taper the heparin dose back down. This will require more heparin monitoring to catch any clotting problems early. If the patient's blood still won't clot normally, the doctor can adjust the dosage again until she is more stable. Patients may need to step down slowly, especially if they have significant underlying health problems. Sometimes they stay in the hospital during this stage so care providers are immediately available if complications develop.


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