How Blood Clots Are Diagnosed

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The blood clotting mechanism is vitally important to life. When a blood vessel is damaged by injury, the normal clotting mechanism ensures that blood loss will be limited. Furthermore, the blood clot that forms at the site of injury provides the body’s first step toward healing.

However, if a blood clot forms when it should not, serious problems can result, because the clot can cause significant damage to the organ supplied (or drained) by the blocked vessel.

This is why diagnosing a blood clot—which is possible with lab and imaging tests—is so important.

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When to Diagnose a Blood Clot

It is often important to diagnose the presence and location of a thrombus or an embolus, two types of blood clots, because either kind has great potential to produce tissue damage. Making the correct diagnosis is critical to instituting the most effective therapy.

Common medical conditions that are very often due to either thrombus or embolus include the following.

  • Stroke is most often caused either by thrombosis of one of the arteries that supply the brain, or an embolus that travels to the brain (most often from the heart).
  • Heart attack is usually due to the rupture of an atherosclerotic plaque, which causes a thrombus to form within a coronary artery.
  • Deep vein thrombosis (DVT) is a clot that forms in one of the major veins of the leg, thigh, or pelvis.
  • Pulmonary embolus is a blood clot that travels to the lungs, usually from a deep vein thrombosis.
  • Other conditions include thrombosis of the major vein that drains the liver (portal vein thrombosis), thrombosis of the vein that drains a kidney (renal vein thrombosis), and embolization of a clot to an arm or leg.  

With any of these conditions, before effective treatment can be administered, it is important to show beyond a reasonable doubt that a blood clot is actually causing the problem.

Blood Clots Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

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Lab Tests

Lab tests for diagnosing a blood clot can test for abnormal activation of the blood clotting system and offer clues that will help your physician narrow down your diagnosis.

D-Dimer Blood Test

The D-dimer blood test detects whether there has recently been an abnormal level of clotting activity somewhere within the bloodstream. This test is the most useful in helping doctors pin down their suspicion that either DVT or pulmonary embolus has occurred.

Cardiac Biomarkers

Cardiac biomarkers are used to diagnose a heart attack. These blood tests do not strictly diagnose blood clots; rather, they detect whether heart muscle damage has occurred—which is almost always caused by a ruptured coronary artery plaque, along with subsequent thrombosis formation.

Imaging Tests

Your doctor will order the imaging tests he or she deems necessary for making a proper diagnosis. Ask your doctor what the test will reveal, and make sure to bring up any concerns you have about the procedure.

Compression Ultrasound

The compression ultrasound test is a noninvasive test that can be performed at the bedside that is often very useful in diagnosing a DVT. 

V/Q Scan

A ventilation perfusion scan (V/Q scan) is a test using a radioactive dye to examine blood flow to the lungs in order to detect whether a pulmonary blood vessel has been blocked by a pulmonary embolus.

CT Scan

The CT scan is a computerized X-ray technique that can show an impressive amount of anatomic detail. The CT scan is particularly useful in confirming that a stroke has been caused by an embolus or a thrombus, and is often the first test used in diagnosing a stroke. The CT scan can also be quite helpful in confirming a pulmonary embolus.

MRI Scan

As with CT scans, MRI scans can be used to detect clots within blood vessels. These tests are logistically more difficult to perform than CT scans, so when time is of the essence, CT scans are more often employed.

Angiography or Venography

These are catheterization techniques in which a dye is injected into a blood vessel where a clot is suspected and X-rays are taken to detect the clot.

Pulmonary angiography can be used to diagnose a pulmonary embolus, whereas venography is used to diagnose DVT. Thanks to the availability of CT scans and MRI scans, these invasive tests are needed for diagnostic purposes much less frequently than they were in the past to diagnose a thrombus or embolus.


Echocardiograms are often used in patients who have had embolisms affecting an artery—especially in people who have had an embolic stroke. To get into a cerebral artery, in almost every case an embolism will have to either originate within the heart or travel through the heart.

The echocardiogram can detect a thrombus that has formed in the heart (typically in the left atrium in a person with atrial fibrillation, or in the left ventricle in a person with severe dilated cardiomyopathy). The echocardiogram can also detect cardiac problems that can allow an embolus to traverse the heart, such as a patent foramen ovale.

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