Hair follicle drug testing has been challenged by Italian researchers in a 2001 paper which claims there is "compelling evidence" that small amounts of drugs can be found in hair from external contamination.

The University of Catania's Institute of Legal Medicine team of Guido Romano, Nunziata Barbera and Isabella Lombardo question the reliability of hair testing even with sophisticated washing and say that it should not be used for stand-alone testing and results.

In the journal Forensic Science International, (Vol. 123, pages 119 -129) they posit: "While a negative result excludes both chronic use and 'contact' with drugs, a positive result cannot and must not be interpreted as a sure sign of drug addiction, but should be further confirmed by urine analysis."

The Italian scientists argue that while the use of alternatives to urine offer "advantages", testers will have to develop methods of distinguishing "with certainty, active users from false positives due to external contamination".

External decontamination, establishing a threshold below which "positive" results are discarded and other methods do not distinguish "active consumption".

Yesterday, Dr. James Brockenbrough said: "The question ultimately remains of how my hair came to test positive for cocaine. This clearly happened without my knowledge. It is well known, from my subsequent review of the literature, that cocaine can be absorbed passively from the environment by people who do not use it as a drug.

"It has been clearly and convincingly described in children of drug addicts and police officers ? who will have positive hair tests even they do not actively take the drug.

"When have I been around cocaine users? The simple answer is that I do not know," Dr. Brockenbrough said. "Patients in the hospital sometimes freely admit to cocaine use, and now doubt others conceal it. It is impossible to tell.

"It is impossible to rule out environmental exposure unless there has been clear evidence established that it is not available from contamination from the environment. If everyone's hair was tested or each doctor's hair was tested, who knows what the results might be?

"There are scientists who believe that hair testing is not a stand alone test ? that the results need to be corroborated by another test."