Deep Vein Thrombosis: Tests & Diagnosis

Instructor: Amanda Robb

Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.

In this lesson we'll review what deep vein thrombosis is and how it affects the body. Then, we'll explain the five main types of tests, D-dimer tests, ultrasound, venography, CT and MRI scans and how they are used to diagnose deep vein thrombosis.

What Is Deep Vein Thrombosis?

You're walking to work, but you come across a blocked-off sidewalk. All foot traffic is diverted onto a small, unpaved pathway where bikes and pedestrians and locked in a gridlock. You're nearly late to work because of this backed-up traffic.

Something similar can happen in your body. In a condition called deep vein thrombosis, blood clots form in the deep veins of the body, usually the legs. Blood backs up and can damage both the clogged veins and veins nearby. Sometimes, the damage can become permanent, even after the clot is removed. Let's look at the tests needed to diagnose deep vein thrombosis.

Deep vein thrombosis is caused by blood clots in the deep veins
deep vein thrombosis


John has been traveling from Boston to Shanghai for work, and he is often on flights lasting ten hours or more. His doctor warned him about a condition called deep vein thrombosis. Inactivity can cause the blood to clot more easily, so long flights can be a risk factor. After one international flight, he noticed some swelling, redness and pain in his leg. Thinking it could be a blood clot, he went to the doctor.

First, the doctor did a physical exam. He asked John about his symptoms and examined his leg and noticed the redness and swelling. John's doctor decided to do some additional tests involving blood work and scans.

D-dimer Test

The doctor explained that when there is a blood clot in the body, your body will try to dissolve it by making natural clot busting factors. When the blood clot is broken down it releases a small piece of protein called a D-dimer. A D-dimer test detects the presence of this molecule and can indicate that a blood clot is present, but can't help locate where. If the test is positive, the doctor will have to do some scans to find where the clot is located. D-dimers are a pretty good indicator of deep vein thrombosis, but more information is needed about where the clot is to make a final diagnosis.

A patient gets blood drawn for a D-dimer test
blood drawn


John's test was positive for D-dimers, so the next step is to locate the clot with an ultrasound, the same type of imaging used to look at developing fetuses during pregnancy. Ultrasound is often the first step because it doesn't require exposing the patient to radiation.

In an ultrasound, a wand, or transducer, emits sound waves that hit solid masses inside the body and bounce back to the machine, which creates a picture. If this image reveals the location of the clot, doctors can accurately diagnose. If the clot is not visible, additional tests may be needed.

Exam room and equipment for ultrasound detection of deep vein thrombosis

MRI Scan

One additional test, an MRI scan, gives a better picture of the veins than an ultrasound and can be used to visualize clots through a cast, which is helpful for patients who may experience clots due to surgery. In an MRI, a tube like machine emits radio waves, long waves that cause hydrogen ions inside cells to line up. When the radio waves stop, the hydrogen ions return to their original location. When they do this, they emit energy that the MRI machine can detect. The hydrogen ions give off different signals depending on how the blood is flowing. So, if the blood behind a clot is backed up, the MRI will be able to detect it.

MRI machines like this can visualize veins more clearly than an ultrasound
MRI machine


Venography uses X-rays to visualize the veins and identify clots. Normally, the X-rays travel through soft tissues, like muscle and blood vessels, but doesn't go through bone.

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