The initial step in making the diagnosis of a blood clot is obtaining a patient history. If a blood clot or thrombus is a consideration, the history may expand to explore risk factors or situations that might put the patient at risk for forming a clot.
Physical examination can assist in providing additional information that may increase the suspicion for a blood clot.
- Venous thrombi may cause swelling of an extremity. It may be red, warm, and tender; sometimes the appearance is difficult to distinguish from cellulitis or an infection of the extremity. If there is concern about a pulmonary embolus, the clinician may examine the lungs, listening for abnormal sounds caused by an area of inflamed lung tissue.
- Arterial thrombus symptoms are much more dramatic. If a leg or arm 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.
Arterial thrombus is also the cause of heart attack (myocardial infarction) and stroke (cerebrovascular accident) and their associated symptoms.
Testing for venous blood clots
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 for ultrasound to provide a definitive answer.
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. It is used as a screening test with hopes that the result will be negative and show that there is no need to look further for blood clots.
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 it may be done to look for other conditions that can cause chest pain and shortness of breath, which are the 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.
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.
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.
Testing for arterial blood clots
Arterial thrombosis is an emergency, since tissue cannot survive long without blood supply before there is 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.
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, myoglobin, CPK) 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.
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 symptoms resolve, the diagnosis is 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.
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