October 10, 2015


Blood Clots

Blood Clots

What tests are used to diagnose blood clots?

Testing for venous blood clots


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.

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 them at the same time as new clot is being formed. D-Dimer is not specific for a blood clot in a given area and cannot distinguish a "good" or needed blood clot, one 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.


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.

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.

Testing for arterial blood clots


For a heart attack (acute myocardial infarction, MI), 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.


Reviewed on 2/27/2015
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