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What Is a Bethesda Unit?

By Jillian O Keeffe
Updated Feb 09, 2024
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Doctors sometimes assign names like "Bethesda Unit" to certain measures of biological activity. In this case, Bethesda units are measurements of how efficient an antibody is at inhibiting a blood clotting factor. People who have a medical condition called hemophilia, which derives from an inability to clot blood properly, require medical attention to help control bleeding. Some people with this disease develop antibody molecules that block circulating blood clotting factors and make the disease symptoms worse. One Bethesda unit is a measure of how much of a patient's antibody can block half of the activity of a blood factor in half an hour at 37° Celsius.

Blood clotting factors are a group of molecules that healthy people make and which circulate around in the bloodstream. When the person suffers a cut, or begins bleeding, internally or externally, then the blood clotting factors work together to signal the body to patch up the point where the blood is getting through and stop the bleeding. People who have hemophilia have genetic problems in creating a healthy level of blood factors, and so they experience problems like uncontrolled bleeding.

Modern medicine can treat these patients with synthetic forms of blood clotting factors, and other drugs that help the blood clot normally. In addition, people with hemophilia can have some level of blood clotting factors naturally moving around the body. Some people with the condition, though, can develop antibodies against the factors, that can make the hemophilia more dangerous, even if it is under treatment. An antibody is a molecule that the immune system makes and that can recognize specific substances. Once it has recognized the substance, it tells the immune system to attempt to destroy or otherwise remove it.

The reason that the body develops these antibodies is unknown, but they act to block the action of the clotting factors, thus decreasing the ability of the hemophiliac patient to stop bleeding after injuries. It is important for a doctor to know whether a patient has developed antibodies. He or she also needs to know how strong the effect of the antibodies are on blocking the clotting factors.

Measuring this can help doctors figure out which medications can help improve the patient's symptoms. All measurements require standard units, and the Bethesda unit is the unit to measure this in. The Bethesda unit is defined as the amount of antibody that can block half of the activity of a clotting factor over two hours at 37° Celsius.

Typically, this is measured by mixing the plasma component of blood from a patient, and plasma from a healthy person in a 50:50 ratio. After the two hours are up, the levels of blood clotting factor in the sample is compared to a control sample that only contains healthy plasma, and therefore a normal level of clotting factor without any antibodies. The result is then converted into Bethesda units by placing it onto a graph that contains known comparison values.

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