LONDON (Reuters) - Chris Froome's positive test for excessive levels of the asthma drug Salbutamol at this year's Vuelta a Espana was unusual but dehydration or metabolism could have played a part, an anti-doping expert said on Wednesday.
Tom Bassindale, a forensic scientist who works at Britain's Sheffield Hallam University, said further tests under controlled conditions could clear Britain's four-time Tour de France winner if it showed other factors at play.
"He gets the chance to prove that either his metabolism is particularly different from everyone else’s or that dose has a bigger excretion into his urine compared with the standard," he told Reuters.
Froome, who has not been suspended, said he had followed medical advice and taken an increased dosage of Salbutamol -- within the limits -- after his asthma worsened.
Cycling's governing body, the UCI, said on Wednesday the Briton's urine sample contained twice the allowed limit of Salbutamol, a legal drug, when he was tested at the Vuelta in September.
"It’s not that normal," Bassindale said of the amount of Salbutamol reported.
"Unless there’s a lot being done on the quiet, and these tests have been finding that lots of people can produce values higher than the threshold, then I think it is quite uncommon."
Team Sky rider Froome, 32, risks missing next year's Tour de France, and losing his Vuelta crown, without a satisfactory explanation for the failed test during the Spanish race.
Bassindale said it would be hard to recreate the same conditions as on the Vuelta.
"The lab tests don’t stipulate that he will be exercising, just that it’s a lab test," he said.
"Trying to recreate it exactly is going to be very difficult because he’d have been dehydrated at the end of that day as well as cumulative effects such as two weeks of cycling and so on. But that’s the next step, to do that study."
Bassindale said he was not optimistic about the outcome of any investigation.
"The most recent similar case was I think a year ban for a similar level. That would be probably my expectation of a starting point, unless his control test shows significantly different results than expected," he said.
Italian rider Alessandro Petacchi was given a 12-month suspension and stripped of his five stage victories in the 2007 Giro d'Italia for excessive Salbutamol use.
Team Sky said analysis of Froome's urine sample showed a concentration of 2,000 nanograms per millilitre (ng/ml) of Salbutamol compared with the World Anti-Doping Agency threshold of 1,000ng/ml.
Team Sky also highlighted evidence of "significant and unpredictable variations" in the way Salbutamol is metabolised and excreted.
"A wide range of factors can affect the concentrations, including the interaction of Salbutamol with food or other medications, dehydration and the timing of Salbutamol usage before the test," said the team.
The recommended maximum use of a standard inhaler is eight puffs over a 12 hour period. Bassindale said a similar reading to Froome's would normally require well above that number.
Some medical studies have suggested inhaled Salbutamol, rather than in tablet or liquid form, offers little in the way of performance gain, however.
The drug makes it easier for an asthmatic to breathe by relaxing the muscles of the airways into the lungs, reducing wheezing.
"I think the consensus is generally that it's not that much of a performance enhancer. For asthmatics it can return people who are suffering an asthmatic attack to their normal function," said Bassindale.
"But it’s debatable whether it can increase performance above that function."
(Reporting by Alan Baldwin, editing by Ken Ferris)