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Medical Author:

Benjamin Wedro, MD, FACEP, FAAEM

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.

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Medical Editor:

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.

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In this Article

• Blood clot facts
• What are blood clots?
• What causes blood clots?
• What does a blood clot look like?
• What are the risk factors for blood clots?
• What types and conditions are caused by blood clots?
• What are the symptoms of blood clots?
• How are blood clots diagnosed?
• What is the treatment for blood clots?
• What are the complications of blood clots?
• How can blood clots be prevented?
• Patient Comments: Blood Clots - Causes
• Patient Comments: Blood Clots - Symptoms
• Patient Comments: Blood Clots - In Leg
• Patient Comments: Blood Clots - Describe Your Experience
• Blood Clots Index

Venous clots do not allow blood to return to the heart and symptoms occur because of this damming effect. Most often occurring in the legs or the arms, symptoms include:

• swelling,
  
  
• warmth,
  
  
• redness, and
  
  
• pain.

Arterial clots do not allow blood get to the affected area. Body tissue that is deprived of blood and oxygen begins to die and becomes ischemic (isch=to restrain + emia = blood)

• Pain is the initial symptom of the ischemia, or oxygen deprivation due to loss of blood supply.
  
  
• Other symptoms depend upon the location of the clot, and often the effect will be a loss of function. Heart attack and stroke are self-explanatory.
  
  
• In an arm or leg, in addition to pain, the limb may appear white, and weakness, loss of sensation, or paralysis may occur.
  
  
• If the blood supply is lost to an area of the bowel, in addition to intense pain, there may be bloody diarrhea.

The initial step in making the diagnosis of a blood clot is obtaining a patient history. The blood clot itself does not cause a problem. It's the location of the blood clot and its effect on blood flow that causes symptoms and signs. 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.

Venous blood clots often develop slowly with a gradual onset of swelling, pain, and discoloration. Symptoms of a venous thrombus will often progress over hours.

Arterial thrombi occur as an acute event. Tissues need oxygen immediately, and the loss of blood supply creates a situation in which symptoms begin immediately.

There may be symptoms that precede the acute artery blockage, that may be warning signs of the potential future complete occlusion of the blood vessel.

• Patients with an acute heart attack (myocardial infarction) may experience angina in the days and weeks prior to the heart attack.
• Patients with peripheral artery disease may have pain with walking (claudication), and a TIA (transient ischemia attack, mini-stroke) may precede a stroke.

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. 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 with hopes that the result will be negative and show that there is no need to look further for blood clots.

The D-dimer blood test is usually ordered with the expectation that it will be negative. It is a useful test in patients who have a low probability of having a blood clot, and the health care practitioner usually counsels the patient that a positive blood test will likely require additional tests being ordered.

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. 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.

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.