Daniel Yorath, who died from sudden death syndrome

The test works by picking up delays in the transmission of electric impulses in the heart that keep the heart functioning.

This is linked to ventricular fibrillation, which is a potentially fatal disturbance in the heart rhythm which causes the heart to quiver or "fibrillate" in a disordered way.

Once patients at risk of SAD have been detected, they can be fitted with an implantable cardioverter defibrillator (ICD).

This restarts the heart if the electrical impulses become erratic.

Genetic risk

The researchers at Papworth Hospital, near Cambridge focussed on patients who have a condition called hypertrophic cardiomyopathy (HCM), one of the most common genetic risk factors for SAD.

HCM is a thickening of the heart muscle which leads to an irregular heartbeat.

It affects people in their late teens or early 20s, which could affect about one in 500 people in the UK.

The condition can cause breathlessness and chest pains, but many sufferers will not know they are affected until they drop dead.

Those at risk can come to medical attention because a relative dies of SAD, through medical checks or through a family history.

But at the moment, doctors around the world vary on who should receive an ICD. It is hoped this test will provide a way of deciding who should receive a defibrillator.

Dr Richard Saumarez, the lead researcher, told BBC News Online: "The research will hopefully ensure patients who need an IDF will get them and equally as important, that patients who are lower risk can be reassured.

"If you have someone who has a family history of SAD, there's the dilemma of whether they need an ICD."

Sir Charles George, medical director of the BHF, said: "HCM is a condition that may never show any symptoms but could result in sudden death when the heart goes into cardiac arrest.

"As the researchers have already shown, a test can be developed to identify those people that are more prone to sudden death, and in these cases an ICD could be fitted that might save their lives."

Gabby Yorath, whose brother died when he was 15, said: "It is important that we can understand more about this disease that can take a young life so suddenly.

"If we can identify who may be prone to the condition, they can be fitted with a defibrillator that might help save their lives."