Diagnosing DVT and PE
If your physician suspects that you have deep-vein thrombosis (DVT) or pulmonary embolism (PE), he/she will conduct a complete physical examination and will ask you questions about your symptoms and medical history. There are a variety of tests that your physician can use if he/she suspects that you have DVT or PE. Some of these tests require injection of a dye (contrast agent) to be injected into a vein, in order for your blood flow to be visualized. These dyes can sometimes cause allergic reactions, so if you have ever experienced an allergic reaction to an injected dye before, you should alert your physician.
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Diagnosing Deep-vein thrombosis
Doppler ultrasonography (B-mode sonography)
Doppler ultrasonography (also known as B-mode sonography) is a noninvasive and painless method of diagnosing DVT. It is easy to perform and provides reliable results. Doppler ultrasonography uses sound waves to measure the flow rate of your blood. Your physician will place an ultrasound probe on your skin over a deep vein in your leg and will apply gentle pressure to compress the vein and halt the blood flow. When the compressed vein is released the blood flow will greatly increase. The changes in ultrasound that occur will provide information about the ease of passage of blood through the vein and hence will indicate whether blood clots are present.
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Duplex scanning
Duplex scanning combines two forms of ultrasonography (Doppler ultrasonography and real-time B-mode ultrasonography) and provides more reliable results than either of these methods alone. A probe is placed on your skin over a deep vein in your leg and the operator will gently press down on (compress) the vein. If your vein is compressible, this indicates that there is no blood clot. If your vein resists compression, the inability to compress the vein indicates a blood clot. Real-time ultrasonography can help to visualize the blood clot through the use of a computer, which creates a two-dimensional image of your veins and any blood clots on a computer screen. For the diagnosis of clots in the calf of the leg and, in some conditions, Duplex scanning may give a false 'positive' result which would need to be confirmed by other tests. However, this is a painless and convenient test, which is readily available and is not expensive.
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Venography (phlebography)
Your physician may decide to x-ray your veins to see how your blood is flowing. To do this, a dye will be injected into a vein on the top of your foot. The dye will help the physician to visualize your blood flow. If there is a blood clot in one of your veins, this will prevent blood and dye from flowing in the vein and will show up on X ray as a dye-free area within the vein. Although this is an accurate and reliable test for deep-vein thrombosis diagnosis, it is expensive and can be painful. This procedure is usually readily available.
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Impedance plethysmography
Impedance plethysmography requires you to lie on a bed with your legs raised at an angle and your ankles higher than your knees. A pneumatic cuff will be placed around your thigh (this leg will be bent) and the cuff will be inflated. This prevents the blood in your veins from returning to your heart. The cuff will then be deflated. If you have a blood clot in your cuffed leg, inflation and deflation of the cuff will make little difference to the blood flow in your calf. If there is no blood clot in your cuffed leg, inflation of the cuff will cause your calf to fill with blood and it will empty quickly on deflation. This is a simple and painless test, but it is not very good at detecting clots in your calf, is not very reliable and is used less frequently nowadays.
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D-dimer testing
D-dimer testing is a blood test, detecting the levels of D-dimer, a substance that is found after an important constituent of blood clots called fibrin has been produced and broken down. These tests provide results in a relatively short amount of time and they are used to rule out deep-vein thrombosis or pulmonary embolism, or confirm the results of other tests such as duplex scanning.
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Diagnosing pulmonary embolism
Pulmonary angiography
Pulmonary angiography is regarded as a good method of diagnosising PE because of its accuracy. It is a technique that involves passing a long catheter through a vein in your groin, up through the veins, through the right side of your heart and into the blood vessels that supply the lungs. These blood vessels are then injected with dye and an X ray is taken. If there is a blood clot, it will show up on the X ray as a dye-free area within the blood vessel. Digital subtraction pulmonary angiography uses a computer to provide a clear, digital image of the blood vessels that supply the lungs. Pulmonary angiography is an expensive and difficult procedure that needs to be conducted by experts. As a consequence it may not be be readily available in all hospitals.
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Ventilation/perfusion lung scanning
Ventilation/perfusion (V/Q) lung scanning is a common diagnostic test for PE. This test assesses the flow of air within your lungs and the flow of blood to your lungs. You will be asked to inhale a gas containing a special marker, and the scanner will detect the distribution of the gas in your lungs. A blood clot in one of the blood vessels supplying your lungs will not affect the airflow in your lungs and will not affect the distribution of the gas. You will also be injected with a small amount of safe, radioactive material into a vein in your arm, so that your blood flow can be visualized under the scanner. If there is a blood clot in one of the blood vessels that supplies your lungs, this will show up on the scan as an area within the vessel that is free of the injected material. If the results of your ventilation/perfusion scan are normal, your physician will exclude the possibility of PE.
If your results are normal for the ventilation scan (i.e. you have normal airflow), and abnormal for the perfusion scan (i.e. your blood flow is abnormal), you may have PE. Please note, however, that the results of these tests are not conclusive in approximately half of patients. Diseases such as pneumonia can make this test difficult to interpret, so it may be necessary to perform another diagnostic test to confirm whether you have PE.
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Spiral computed tomography scanning
Spiral computed tomography (spiral CT) scanning is a painless X-ray technique that results in a cross-sectional picture of your blood vessels and any thrombi. It is a very rapid and accurate technique. A dye will be injected into a vein in your arm, so that your blood flow can be visualized under the spiral CT scanner. If there is a blood clot in one of the blood vessels supplying your lungs, this will show up on the scan as a dye-free area within the artery. The limitations of this technique include the fact that blood clots in the smaller blood vessels in the lung are frequently not detected.