How the D-Dimer Test Measures Blood Clot Formations (2024)

The D-dimer test is a blood test doctors can use to rule out a severe blood clot. A normal result would be less than 500 nanograms per milliliter (ng/mL) and anything above that is considered a positive test. It's useful if your doctor suspects you have a blood clot in your lung or deep within a vein of your leg or pelvis.

These clots can be fatal. Getting prompt treatment increases your chances of surviving and avoiding other medical problems.

This article discusses the D-dimer test, how doctors use it, and its limitations. It also explains the link between COVID-19 and elevated D-dimer levels.

What Is a D-Dimer Test?

D-dimer is a substance involved in the body's healing process. When you get an injury that causes you to bleed, your body uses proteins to clump up your blood. The clot that forms plugs the damaged vessel.

Once the bleeding stops, your body sends out other proteins to slowly break down the clot. Afterward, you end up with fragments of D-dimer in your blood.

These protein fragments usually dissolve over time. However, if a clot doesn't break up or another one forms, you'll have high levels of D-dimer in your blood.

The D-Dimer test is a blood test that can measure the amount of these fragments in your blood.

What Is a D-Dimer Test Used For?

A D-Dimer test is used to warn of the possibility of a severe blood clot and diagnose certain conditions before they become life-threatening.

As many as 100,000 deaths occur each year in the U.S. due to deep vein thrombosis, a blood clot deep within a vein, and pulmonary embolism, a blood clot in your lung. The symptoms you may have include:

• Swelling or redness, usually in the lower leg but sometimes in the thigh, pelvis, or arm
• Pain in the leg, thigh, pelvis, or arm
• Difficulty breathing
• Fast heartbeat
• Chest pain
• Sweating a lot

Diagnosing these conditions and getting prompt treatment increases your chances of surviving PE and DVT. It also helps you avoid other medical problems that can hurt your quality of life.

Ruling out DVT and PE is the main reason doctors order D-dimer tests. However, the test can help doctors evaluate and manage other severe conditions that involve blood clots as well. These include:

• Coronary artery disease: People with severe heart disease have higher levels of D-dimer. People who are treated for a heart attack but still have elevated D-dimer levels are at an increased risk of having another heart attack or dying from one.
• Stroke: Higher levels of D-dimer are associated with an increased risk of stroke.
• Disseminated intravascular coagulation (DIC): This is a rare condition in which blood clots form in vessels throughout the body. Elevated D-dimer levels are part of the scoring test for DIC.
• Hyperfibrinolysis: This blood clotting disorder is similar to DIC. The D-dimer test also helps doctors evaluate this disorder.

Why Do I Need the D-Dimer Test?

Diagnosing these conditions can be tricky. One study found that nearly 70% of people seen in outpatient clinics and emergency rooms with symptoms of a DVT didn’t have one.

Doctors used to have to send all blood samples to a central lab for analysis. This caused delays and meant the test couldn't be used for emergencies. So doctors were forced to send patients for expensive imaging tests instead.

The D-dimer test is a simple and relatively quick way to diagnose what could be a rapidly worsening and life-threatening condition. Healthcare providers can order D-dimer tests in emergency situations and have the results back within 24 hours.

What to Expect

Before the Test

There's no special preparation before taking the D-Dimer test. You do not have to fast or do it at a certain time of day. That being said, check with your healthcare provider if there are any special instructions they want you to be aware of.

During the Test

The D-Dimer testing process is simple. A blood sample is taken from the vein, usually in the arm, using a small needle. For most people, having blood taken feels like a pinch or prick when the needle is inserted into your arm. If your veins are difficult to find, it may be more challenging for the phlebotomist to get a sample of your blood and analyze it. Your arm may feel a little sore at the site of the blood draw later in the day or the next day.

After the Test

Most people will feel normal as soon as the test is complete. You may have mild soreness in the arm muscle or slight bruising. Typically, results will be ready in a day or two.

What Do the Results Mean?

Results can vary depending on the test your doctor used and the design. Doctors need to know the ranges of normal and abnormal levels for the test they're using. The reference range can vary from laboratory to laboratory, but in general, a normal D-dimer range is 220 to 500 ng/mL.

If your results are in the lower range, your doctor can safely rule out a blood clot. If your results come back abnormal or high, you'll likely need more tests. The D-dimer test cannot be the sole basis to diagnose DVT or PE.

Patients with a low risk of blood clots and whose D-dimer levels are in the lower to middle range likely don't have a pulmonary embolism. Studies show the D-dimer test is comparable to ultrasonography or CT angiography in ruling out PE.

Nearly all patients with DVT have elevated D-dimer levels.This makes the test beneficial in ruling out the condition for patients with levels in the lower to middle range. The test is also helpful if your symptoms aren't that clear. If your levels are high, your doctor will order more tests.

What Does a High D-Dimer Mean?

High or abnormal results suggest you may have a blood clot, but it does not definitively mean a clot is present. There could be other reasons why your results are high, and your doctor will order further testing to determine the cause.

Also, many people who've had a recent PE will still have elevated D-dimer levels. So the test isn't helpful for them.

In addition to PE, many diseases, treatments, and lifestyle factors can raise your D-dimer levels. Thus, it's essential to thoroughly answer your doctor's questions about your medical history. People with blood clots often have one or more of the same risk factors. They include:

Medical conditions and treatments:

• Heart disease: Patients with unstable angina or who have had a heart attack have higher levels of D-dimer and a higher risk of future blood clots.
• Cancer: Some cancers can increase the risk of a blood clot.
• Cancer treatment: Chemotherapy and certain breast cancer drugs can increase the risk of blood clots.
• Treatment with estrogen: Birth control pills and hormone replacement therapy can increase the risk of DVT and PE.
• Surgery: Patients who have had major surgery, like a hip or knee replacement, have a higher risk of a blood clot. (Drugs are prescribed to prevent this.)
• Infectious diseases: COVID-19 and pneumonia can cause inflammation and trigger blood clots.
• Kidney disease: For reasons that aren't fully understood, kidney disease increases the risk of DVT and PE.
• Liver cirrhosis: People with severe liver disease have a higher risk of clots in the large vein of the liver.
• Pregnancy: D-dimer levels rise two- to four-fold by delivery. Women have an increased risk of DVT or PE for up to three months after delivery.

Other risk factors:

• Age: People over 60 years of age have a higher risk of blood clots.
• Cigarette smoking
• Race: African Americans have higher levels of D-dimer compared to people of European ancestry.
• Gender: Women have higher levels of D-dimer than men.
• Obesity
• Sedentary lifestyle: Not exercising or not moving for an extended period can increase the risk of DVT or PE. An example is a long plane ride or being in the hospital.

Doctors may order other tests to make sure you don't have PE or DVT:

• Other blood tests: To see if you have a bleeding disorder
• Ultrasonography: A test that uses high-frequency sound waves to take pictures of your blood vessels, tissues, and organs
• Ventilation-perfusion lung scan: A test that uses a radioactive substance to help doctors see if air and blood can move through the lungs or if you have a blockage
• Computed tomography angiography: A test where you receive an infusion of a special dye. Doctors use a CT scan to take high-definition pictures from different angles. The dye lights up the blood vessels and tissues they need to check for blood clots.

Summary

Doctors may order a D-dimer test if they suspect you might have a dangerous blood clot. The test helps doctors rule out two conditions that can be fatal: deep vein thrombosis, a blood clot in a vein, and pulmonary embolism, a blood clot in the lung. The test can also help diagnose other conditions that cause blood clots such as stroke and coronary artery disease.