Transcrainial Doppler ultrasonography allows doctors to identify electronically the number of blood clots floating free in the bloodstream.

Scientists from King's College of Medicine in London claim the technique provides a more reliable measure of the effectiveness of drugs than was previously available, and could remove the need to test new drugs on animals.

Technique could substitute for animal research

They have used the technique to test the effectiveness of a new drug called GSNO (S-nitrosoglutathione), which releases nitric oxide specifically targeted to dissolve blood clots.

GSNO was compared with the standard treatment of aspirin and heparin in a group of 24 patients who underwent surgery to clean a major blood vessel in the neck.

Patients who undergo this operation are at high risk of blood clots that can block blood flow to the brain and cause a stroke.

Patients treated with GSNO were found to have significantly lower numbers of clots during a three-hour period after the operation.

Identify patients

Dr Hugh Markus, a member of the research team, said: "Before this technique assessing a drug meant either doing animal tests, or taking blood from people and studying it under the microscope - neither was a very good measure of what would actually happen when the drug was used in man."

Dr Markus said the technique could also be used to identify patients who were most at risk of stroke, and who would benefit most from corrective surgery.

New drugs will still have to be thoroughly assessed in large-scale clinical trials, but the new technique will help scientists to decide which products should go to full blown trial.