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About thrombosis
Venous thrombosis
Diagnosis of deep vein thrombosis
If a DVT is suspected, two different sets of tests
are used to make a diagnosis.
The first is a simple blood test, called a D-dimer test. This measures the activity of the clot breakdown system in the blood. When a clot occurs, this system starts automatically. Therefore when the test is negative, it is unlikely that a clot is present.
However the clot breakdown system can also be activated by other conditions, such as a heart attack, stroke or chest infection.
If the D-dimer test is positive, a further test is carried out to confirm that a DVT is present. This involves detecting the clot through imaging, either by using ultrasound or by performing a venogram.
Ultrasound scan
A Doppler ultrasound scan can detect a clot with reasonable
accuracy, especially if it is behind the knee or in the thigh. However it
is not reliable for detecting a thrombosis in the calf.
Venogram
A venogram is currently the 'gold standard' for routine diagnosis
of DVT. Radio-opaque dye is injected into a vein in the foot and travels up
through the leg veins. The dye is then visualised using X-ray imaging. If
there is a thrombosis the dye will not flow through the vein and will appear
as a gap in the pattern of dye.
Magnetic Resonance Imaging
Safer and more accurate methods are constantly being devised. One recently
developed technique, Magnetic Resonance Direct Thrombus Imaging (MRDTI),
provides a very accurate picture of the clot.
MRDTI uses magnetic resonance imaging to detect a venous thrombosis, without the need to use dye or X-rays.
Diagnosis of pulmonary embolism
If a pulmonary embolism is suspected, diagnosis involves the same combination of tests as for a deep vein thrombosis.
A D-dimer test is given and if negative, it is unlikely that a pulmonary embolism is present.
If the D-dimer test is positive, a further test is carried out to image the pulmonary embolism. This is usually either a ventilation/perfusion scan or a CT pulmonary angiogram.
Ventilation/perfusion scan
A ventilation/perfusion scan uses small amounts of
radioactive materials to image the lungs. If a pulmonary embolism is present,
this will show up as a missing area on the perfusion scan.
The ventilation scan measures how much of the lung is being aerated. If there is a pulmonary embolism, there will be an area of lung that is aerated but has no blood flowing through it.
CT angiogram
In a CT angiogram, dye is injected into the bloodstream. The
chest is imaged using Computerised Tomography (CT). If a pulmonary embolism
is present, this will show up as an area where there is no dye.
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