A D-dimer test is a blood test that can be used to help rule out the presence of a serious blood clot.
When you get a cut, your body takes a bunch of steps to make your blood clump up. It’s a normal part of healing -- without it, you’d keep bleeding and have a much more serious problem to deal with.
Once the bleeding stops, you don’t need the clot anymore. So your body takes a series of steps in the other direction and breaks the clot down.
At the end of all that, you have some leftover substances floating around in your blood -- like how you’d have wood dust all over after a building project.
One of those leftovers is called D-dimer. It’s part of a protein. Normally, with a little time, it goes away. But you can get high levels of D-dimer in your blood if you have a major clot like with deep vein thrombosis (DVT).
With DVT, you have a clot deep in one of your veins, usually in your legs, and it can lead to serious problems.
Your doctor may use this test, which checks the level of D-dimer in your blood, to figure out whether you might have a blood clot. You may also hear this test called:
- Fragment D-dimer test
- Fibrin degradation fragment test
When Do I Need This Test?
Some tests help you know for sure that a certain disease or condition is causing your symptoms. Other tests are more useful to rule out a certain condition as the cause. The D-dimer test can be used both ways, based on what your doctor is looking for.
To rule out DVT and other conditions: The D-dimer test is most useful when your doctor thinks something else is causing your symptoms and wants to quickly rule out these causes:
- DVT, which may give you swelling, pain, or redness in your leg
- Pulmonary embolism, or PE, a blood clot that has traveled to your lungs that may lead to trouble breathing, fast heartbeat, pain in your chest, and coughing
In this case, the test is only helpful if you’re not too likely to have blood clots. A positive D-dimer test doesn’t mean you have a blood clot. Other tests will be needed to check for that. If your odds of having a clot are higher, you’ll need different tests. You have higher odds of a clot with:
- Antiphospholipid syndrome, a disease in your immune system
- Clotting diseases that you’re born with
- Major surgery, such as a knee replacement
- Major injury, such as a broken leg
- Long periods of sitting or lying down, such as a long plane ride or hospital stay
- Pregnancy or if you recently had a baby
- Some cancers
To test for Disseminated Intravascular Coagulation: D-dimer can also be used to help test for what’s called disseminated intravascular coagulation (DIC), in which blood clots form in small blood vessels throughout your body while also causing bleeding. It can be life-threatening.
It’s also used to check on the treatment for DIC. If D-dimer levels drop, it’s a sign that the treatment is working.
What Happens During the Test?
You don’t need to do anything special to get ready for a D-dimer test. Your doctor uses a thin needle to take a small amount of blood. You’ll feel a pinch or stinging when the needle goes in. You may get some soreness or a bruise where the blood is taken, but that’s usually it.
Typically, you get results quickly. This test is often used in emergency rooms.
What Do the Results Mean?
Different labs may do the test differently, so keep in mind that what’s normal can vary. Your doctor can help you understand more clearly what your results mean.
If your result is "negative," you most likely don’t have a problem with blood clots, such as DVT.
If your result is "high," it means you’ll need more testing to see whether you have a blood clot. This test can’t confirm that you have DVT or PE. It can only help rule them out.
You can also get a high result for reasons other than a clot, such as:
- Infection
- Liver disease
- Some cancers