Researchers have found people with the mental illness have significantly higher levels of sulphite in their urine than healthy individuals.

The substance appears to mark an inherent trait rather than being a symptomatic by-product of the disease.

It means in future doctors may be able to spot a predisposition to schizophrenia and provide early treatment before the illness becomes serious.

But doctors acknowledge there may be difficult ethical considerations to screening for schizophrenia.

~Abnormal metabolism

Schizophrenics produce characteristic urine

Dr Theodore Soutzos looked for increased sulphite levels in schizophrenics after earlier studies indicated that the illness might be linked to abnormal metabolism.

Sulphite should not be present in the urine of normal, healthy individuals but could be excreted due to a particular defect in body chemistry.

A study of about 40 schizophrenic patients at Guy's and St Thomas's Hospitals, London, showed that 90% had high levels of sulphite. None had no sulphite in their urine.

They were compared with a group of healthy individuals. A few of these had low levels of sulphite but the majority had none at all, and none had high levels.

Eighteen people with clinical depression had levels higher than the healthy individuals but significantly lower than the schizophrenics.

'Quite startling'

Dr Soutzos said: "Schizophrenics stand out massively as having high levels of sulphite. It's really quite startling."

One patient was tested both before and after receiving treatment which removed all his symptoms.

In both cases the level of sulphite was high.

"This shows that it's a trait marker and not a state marker," said Dr Soutzos.

Looking for a system

He said a number of research teams had been looking for a system of early detection of schizophrenia. But it was difficult to overcome the danger of "false positive" results providing a wrong diagnosis.

The sulphite discovery offered the first hope of a clear-cut and practical test.

Dr Soutzos, who was attending the annual meeting of the Royal College of Psychiatrists in Belfast today, added: "It would be amazing to be able to detect which people were likely to become psychotic and treat them at the first sign of the disease. You could potentially save this country billions of pounds."

However, he recognised that screening for schizophrenia could present ethical problems, with the risk of insurance companies and employers discriminating against people who tested positive.

Simple urine test

Dr Soutzos is now working with doctors at the Guy's and St Thomas's medical school on patenting a simple sulphite urine test. For the purposes of the research, samples had to be sent away to specialist laboratory.