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Blood Clots (cont.)

How are blood clots diagnosed?

The first step in making the diagnosis of a blood clot is obtaining a history from the patient for family. Patients rarely are able to make their own diagnosis of a blood clot so the health-care professional will need to ask questions about what might be happening. If a blood clot or thrombus is a consideration, the history may expand to access the patient's risk factors or situations that might have put the patient at risk for forming a clot.

  • Venous blood clots (venous thrombi) often develop slowly with gradual swelling, pain, and discoloration of the involved area, and the signs and symptoms will progress over hours.
  • An arterial thrombus occurs as an acute event. Tissues need oxygen immediately, and the loss of blood supply as a result of an arterial clot creates a situation in which symptoms begin immediately.

Physical examination will help add information to increase the suspicion for a blood clot.

  • Vital signs are an important first step of the physical exam. Blood pressure, heart rate, respiratory rate and oxygen saturation (what percent of red blood cells have oxygen attached to them) may provide a clue as to whether the patient is stable or in potential danger.
  • Venous thrombi may cause swelling of an extremity. It may be red, warm, and tender and sometimes the physical exam may make it difficult to distinguish a venous thrombus from cellulitis or an infection of the extremity. If there is concern about a pulmonary embolus, the examiner may listen to the heart and lungs looking for abnormal sounds caused by an area of inflamed lung tissue, or unusual heart sounds.
  • Arterial thrombus is much more dramatic. If an arm or leg is involved, the tissue may be white because of the lack of blood supply. As well, it may be cool to touch, and there may be loss of sensation and movement. The patient may be writhing in pain, but may not be able to move affected limb. Arterial thrombus is also the cause of heart attack, stroke, and ischemic bowel.

Testing for venous blood clots

Testing will depend on the suspected location of the blood clot.

  • Ultrasound: Venous blood clots may be detected in a variety of ways, although ultrasound is now most commonly used. Occasionally, the patient's size and shape may make it difficult for the ultrasound technician and radiologist to evaluate the deep veins of an arm or leg.
  • Venography: Venography may be another alternative to look for a clot. 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. This test is no longer commonly used, but may be available in certain select circumstances.
  • Blood tests: Sometimes, a blood test is used to screen for blood clots. D-dimer is a breakdown product of a blood clot, and levels of this substance may be measured in the bloodstream. Blood clots are not stagnant; the body tries to dissolve them at the same time as a new clot is being added. D-dimer is not specific for a blood clot in a given area and cannot distinguish the difference between a "good" (necessary) blood clot (one that forms after surgery or due to bruising from a fall) from one that is causing problems. It is used as a screening test in patients with low probability of having a blood clot. In this case, if the result is negative, there is no need to search further for the diagnosis of blood clots.
  • Additional tests: 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 may be done to look for other conditions that can cause chest pain and shortness of breath, the same sings and symptoms of an embolus. The electrocardiogram (EKG) may show abnormalities suggestive of a pulmonary embolus and may also show other reasons for chest pain.

Computerized tomography (CT) is often the test of choice when suspicion of pulmonary embolus is high. Contrast is injected intravenously, and the radiologist can determine whether a clot is present.

On occasion, ventilation perfusion (V/Q) scans are 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. This test is less accurate than a CT scan and is subjective to variances in interpretation.

Should a pulmonary embolus be diagnosed, the amount of clot and potential severity of the disease may require further testing. Echocardiogram is an ultrasound of the heart that may be used to decide how much strain the blood clot in the lung is causing on the heart.

Testing for arterial blood clots

Arterial thrombosis is an emergency, since tissue cannot survive very long without blood supply before there is irreversible damage. In an arm or leg, often a surgeon is immediately consulted. Arteriography may be considered. Arteriography is a technique in which dye is injected into the artery in question to look for blockage. 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 necessary to open the vessel and restore blood flow.

For an acute heart attack, the EKG may confirm the diagnosis, although blood tests also may be used to look for enzymes (troponin) that leak into the bloodstream from irritated heart muscle. With an acute heart attack, the diagnostic and therapeutic procedure of choice is a heart catheterization. A catheter is placed into the coronary artery, the blockage identified and a stent placed to restore blood supply to heart muscle.

Other variations of head CT may be considered to look at blood flow in the brain (perfusion CT) or the arteries themselves (CT angiogram).

For an acute stroke (cerebrovascular accident, CVA), 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. If the stroke symptoms resolve, meaning that a transient ischemic attack (TIA) has occurred, tests may include carotid ultrasound to look for blockage in the major arteries of the neck and echocardiography to look for blood clots in the heart that may embolize to the brain.

Medically Reviewed by a Doctor on 8/30/2017

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