An ultrasound of the leg is often ordered when your doctor suspects a deep vein thrombus. This often presents with leg swelling, redness and pain. The danger is that you can develop a pulmonary embolism or a thrombus that travels to the lungs. This can be life threatening in some cases. Therefore prompt detection of this condition is necessary. More chronic consequences can include pain in the leg that lasts after treatment.
It is also necessary to identify any potential underlying conditions such as certain genetic conditions that make your blood prone to clotting. In some cases, patients have an underlying cancer which also can promote clotting of the blood. Other risk factors include recent surgery, trauma, immobilization, pregnancy, and estrogen replacement.
An ultrasound will be ordered to look at the deep veins of the legs. A thrombus in the deeper lying veins poses a greater risk of embolism or clot to the lungs. The more superficial veins, or those lying closer to the skin can also develop clots. The ultrasound will be done by a specially trained technologist. She will usually start at the groin and work her way down to the calf. She will look to see how good the blood flow is, and compress your leg all the way down.
If a vein can compress, then this means that there is no thrombus inside. A vein which does not compress often means it is clotted. The technologist will also see how the blood flow is and whether any clot is visible. She will also see how extensive the clot is and whether it blocks the vein and blood flow completely.
A radiologist will interpret the study in many cases. He will call the ordering doctor if the test is positive for clot. This test is done most commonly from the emergency room in my experience. The emergency doctor will treat the clot with blood thinners. You will follow up with your primary care doctor or a specialist called a hematologist for further care and follow up.
In some cases where the clot is extensive, a specially trained interventional radiologist may try to break up the clot directly with medicines placed directly into the vein. If you have any symptoms of blood clot to the lungs such as shortness of breath or pain, a chest CT may be ordered to look for clot in the arteries of the lungs. This is sometimes seen as well when you have a clot in your leg.