Accused killer's DNA samples 'unlikely' to be contaminated, court told

The head of a UK laboratory that found Bradley Robert Edwards' DNA in the fingernail scrapings of the third Claremont serial killings victim has testified at his trial.

The former Telstra worker is fighting accusations he murdered Sarah Spiers, 18, Jane Rimmer, 23, and Ciara Glennon, 27, in 1996 and 1997.

The Supreme Court of WA heard detailed testimony on Monday from DNA expert Jonathan Whitaker, who helped develop the sensitive low copy number profiling technique for the now-defunct Forensic Science Service.

Dr Whitaker has been a key witness in other high-profile cases relying on low-copy DNA, such as that of Bradley John Murdoch, who was convicted for the murder of British tourist Peter Falconio.

The laboratory tested swabs from Ms Glennon's nails in 2008, combining two small samples known as AJM40 and AJM42 to maximise any DNA that was present.

They found Edwards' trace DNA but it wasn't until 2016 that investigators matched it to DNA left behind at two other crimes he has confessed to: the rape of a teenage girl in 1995 and an attack on an 18-year-old woman asleep in her home in 1988.

The 50-year-old's defence team has been raising doubt about the integrity of the evidence, suggesting the fingernail samples may have been contaminated in the laboratory by swabs taken from the rape victim.

Dr Whitaker said that though forensic scientists did everything they could to prevent and detect contamination, it still happened .

"There will never be instances where you can eliminate contamination – it's simply not possible to do that," he testified.

He said, however that it was unlikely contamination had occurred here.

"The probability of that happening was well below very low."

The trial has heard about several instances of laboratory errors, particularly at Perth-based PathWest, but there has been no evidence that the crucial AJM40 and AJM42 samples were tainted.

– Reported with AAP.