Blood Clots

  • Medical Author:
    Benjamin Wedro, MD, FACEP, FAAEM

    Dr. Ben Wedro practices emergency medicine at Gundersen Clinic, a regional trauma center in La Crosse, Wisconsin. His background includes undergraduate and medical studies at the University of Alberta, a Family Practice internship at Queen's University in Kingston, Ontario and residency training in Emergency Medicine at the University of Oklahoma Health Sciences Center.

  • Medical Editor: Melissa Conrad Stöppler, MD
    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

Quick GuideDVT in Pictures: Symptoms of Deep Vein Thrombosis, Beyond Leg Pain and More

DVT in Pictures: Symptoms of Deep Vein Thrombosis, Beyond Leg Pain and More

What tests are used to diagnose blood clots?

Testing for venous blood clots

  • Ultrasound: Venous blood clots may be detected in a variety of ways, though ultrasound is most commonly used. Occasionally, the patient's size and shape limit the ability of the person performing the ultrasound to adequately see the veins.
  • Venography: Venography is an alternative test to look for a clot. In this test, a radiologist injects contrast dye into a small vein in the hand or foot and using fluoroscopy (video X-ray), watches the dye fill the veins in the extremity as it travels back to the heart. The area of clot or obstruction can thus be visualized.
  • D-Dimer: Sometimes, a blood test is used to screen for blood clots. D-Dimer is a breakdown product of a blood clot, and its levels in the bloodstream may be measured. Blood clots are not stagnant; the body tries to dissolve a clot at the same time as new clot is being formed and trying to increase in size. D-Dimer is not specific for a blood clot in a given area and cannot distinguish a "good" or needed blood clot (for example, a clot that forms after surgery or due to bruising from a fall), from one that is causing medical problems. It is used as a screening test in low risk patients with the expectation that a negative result will conclude that there is no need to look further for blood clots. The healthcare professional usually counsels the patient that a positive blood test will likely require additional tests being considered. D-Dimer also maybe elevated in women who are pregnant and patients with cancer.
  • CT scan: Computerized tomography (CT scan) is often the test of choice when suspicion of pulmonary embolus is high. Contrast material is injected intravenously, and the radiologist can determine whether a clot is present in the pulmonary vessels. The contrast material injected into the body can be irritating to the kidney(s) and should not be used in patients who have impaired kidney function. In older patients, screening blood tests (serum creatinine) to check kidney function may be required before a dye study is considered. The radiologist may also be able to advise whether the heart is straining to pump because of the amount of clot that might be present in the lung. The more clot there is, the harder the heart has to work to pump.
  • Ventilation perfusion (V/Q) scan: On occasion, a ventilation perfusion (V/Q) scan is performed to look for pulmonary emboli. This test uses labeled chemicals to identify inhaled air into the lungs and match it with blood flow in the arteries. If a mismatch occurs, meaning that there is lung tissue that has good air entry but no blood flow, it may be indicative of a pulmonary embolus. It is less accurate and more subjective than a CT scan, and requires the skill and experience of a radiologist to interpret. Two radiologist may interpret a VQ scan differently and come to different conclusions. The VQ scan is often performed when a CT scan is contraindicated, for example, with a major dye allergy or in a patient with kidney compromise.

Should a blood clot embolize to the lung, this may be a medical emergency. There are a variety of tests to look for pulmonary emboli. A plain chest X-ray will not show blood clots, but the X-ray may be done to look for other conditions that can cause chest pain and shortness of breath, which are the most common symptoms of a pulmonary embolus. An electrocardiogram (EKG) may show abnormalities suggestive of a pulmonary embolus and also may reveal other causes of chest pain.

Testing for arterial blood clots

Arterial thrombosis is an emergency, since tissue cannot survive long without blood supply delivering oxygen and nutrients and shortly causing irreversible damage. When this occurs in an arm or leg, often a surgeon is consulted on an emergency basis. Arteriography may be considered, a test in which contrast material is injected into the artery in question to look for blockage on imaging studies. Sometimes, if there is a large artery that is occluded, this test is done in the operating room with the presumption that a surgical procedure will be needed to open the vessel and restore blood flow. An interventional radiologist may also be part of the treatment team to also try to remove the clot directly or dissolve the clot with medications.

Heart attack: the EKG may establish the diagnosis, although blood tests may be used to look for enzymes (troponin) that leak into the bloodstream from irritated heart muscle. In an acute heart attack, the diagnostic and therapeutic procedure of choice is a heart catheterization. A catheter is threaded into the coronary artery to the level of the blockage, the clot and plaque is squashed in the artery wall (angioplasty) and a stent is placed to keep the artery open. If the hospital does not have a cardiologist or heart catheterization facility available, clot busting drugs (TPA,TNK) may be used to try to restore blood supply to the affected part of the heart. A heart catheterization may then be performed later.

Acute stroke: the test of choice is a computerized tomography (CT) scan of the head to look for bleeding or tumor as the cause of stroke symptoms instead of a blood clot blocking an artery.

Other studies: Other CT studies of the head may be helpful in guiding treatment of a stroke and include CT perfusion to look at brain blood flow and CT angiography (CTA) to map the anatomy of the arteries and look for any acute clot that might need to be treated. An MRI also  may be considered but the test is not always readily available and also takes significantly more time to perform and interpret. If the symptoms of a stroke resolve quickly, the diagnosis becomes a transient ischemic attack (TIA, mini-stroke), and further tests may include carotid ultrasound to look for blockages in the major arteries of the neck and echocardiography to look for blood clots in the heart that may embolize to the brain and cause permanent stroke damage in the future.

Reviewed on 10/28/2016

Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012;141(2_suppl)

Dzsheka MS, et al. Stroke and bleeding risk in atrial fibrillation. Clin Cardiol.2014, Oct, 37(10)

Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.

Kearon, C., et al. "Antithrombotic Therapy for VTE Disease: Chest Guideline and Expert Panel Report." Chest. 2016;149(2):315-352. IMAGES:

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