If these patients can be identified at an early stage, it may be possible to give them drugs that will reduce the risk of death or serious illness.
The development of DVT, or blood clots in the legs, is common in patients who have had a stroke. In most cases the clots dissolve in a few days.
And in a minority of cases pieces of clot may dislodge and pass to the lungs, causing a potentially life-threatening condition known as pulmonary embolism (PE).
PE is responsible for up to 25% of early deaths among people who have had a stroke.
Clots can be treated effectively once they are diagnosed.
However, because the clot in the leg often causes no other symptoms, PE can be totally unexpected.
Early diagnosis
A team at Guy's and St Thomas' Hospital, led by Dr James Kelly, is working on a technique to identify patients who may be at risk early enough to give them treatment.
They believe it may be possible to identify such patients using a simple blood test which measures levels of a substance released from clots known as a D-dimer.
The test has proved helpful in the diagnosis of clots in other groups of patients.
However, it is currently unclear whether it would be useful in helping diagnose clots after stroke because stroke itself is thought to increase D-dimer levels.
Test accuracy
Dr Kelly's team, which has received a grant of £121,000 from the Stroke Association, will be using a new and powerful technique called magnetic resonance direct thrombus imaging to identify clots in stroke patients so that the accuracy of the test can be determined.
The technique will also provide valuable information on how common DVT and PE are after stroke, and how they behave once they occur.
For example, some types of clot may simply dissolve after a few days and not need treatment, whereas others may grow larger and pose a significant threat to the patient's life.
Stroke care
If the project is successful, checking D-dimer levels at intervals after a stroke to identify patients who may be developing DVT could become a standard part of stroke care.
Dr Kelly said: "Many DVTs are minor and would not require treatment, particularly as blood thinning drugs slightly increase the risk of bleeding into the damaged area of brain immediately after a stroke.
"However, patients with more extensive DVT, who are at much higher risk of pulmonary embolism, would be likely to benefit from such treatment.
"Early detection and treatment of DVT may prevent deaths and improve the outlook in patients who survive stroke."