There was no viable opportunity for exhibits held at Western Australia's state-run pathology laboratory and containing DNA matching accused Claremont serial killer Bradley Robert Edwards to be contaminated, a senior PathWest scientist has told a Perth court.

Key points: Edwards's DNA was found under the fingernails of victim Ciara Glennon

Edwards's DNA was found under the fingernails of victim Ciara Glennon Edwards has pleaded guilty to sexual assault charges of two other women

Edwards has pleaded guilty to sexual assault charges of two other women DNA evidence from the assaults was held at the same lab as the samples from Ms Glennon

Edwards, 51, a former Telstra technician, is standing trial in the WA Supreme Court for the wilful murders of Sarah Spiers, Jane Rimmer and Ciara Glennon in 1996 and 1997.

He has pleaded not guilty.

DNA matching Edwards was found underneath the fingernails of murdered lawyer Ms Glennon, and it is the defence case it could have got there through cross-contamination of samples held at PathWest.

Forensic scientist Scott Egan, who has worked at PathWest for 25 years, told the court on Tuesday four items containing Edwards's DNA had been held at the laboratories at various times prior to the discovery of Ms Glennon's body in April 1997.

A kimono found to have Edwards's DNA on it was examined at the lab over a three-day period in 1988 and then returned to police.

Edwards dropped this kimono after attacking a woman as she slept in her bed on February 14, 1988 in Huntingdale — an assault he confessed to before going on trial for the Claremont murders. ( Supplied: Supreme Court of WA )

The silk garment had been left behind at the Huntingdale home of an 18-year-old woman Edwards attacked in her bed in 1988, a crime he admitted to on the eve of his murder trial.

Intimate samples taken from a 17-year-old girl raped by Edwards in February 1995 and containing his DNA were also held at PathWest, as was a pair of hospital pants worn by the teenager after her brutal ordeal at Karrakatta Cemetery and also found to contain Edwards's DNA.

Mr Egan said by the time Ms Glennon's fingernail samples were received in the lab on April 4, 1997, only the samples from the rape were still at PathWest.

Sarah Spiers, Ciara Glennon and Jane Rimmer went missing from Claremont in the 1990s. ( Fairfax Media )

Asked by State prosecutor Carmel Barbagallo SC whether there was any "real as opposed to fanciful opportunity" for the rape samples to contaminate the fingernail samples, Mr Egan said there was not.

"I can't see any viable mechanism for that contamination to occur," he said.

He said the rape samples were held in lidded tubes, sealed inside boxes which also had lids on them, and kept in the PathWest freezer, while Ms Glennon's fingernail samples were held in yellow-topped sterile containers, in a separate lidded box, in a different part of the freezer.

Material collected by police from the burial site of murdered lawyer Ciara Glennon was stored in yellow-topped specimen jars. ( ABC News )

The samples were examined by PathWest scientists "months if not years" apart in time, Mr Egan said.

Mr Egan, who was the lead reporting scientist for the Claremont investigation, reiterated that PathWest took every precaution to avoid contamination, and investigated every instance where contamination had been shown to have occurred.

However, he conceded there were 28 occasions when mistakes had been made in the 25 years that PathWest had dealt with samples relating to the case, ranging from minor typographical errors to the contamination of samples.

This was out of approximately 17,000 separate interactions staff had had with exhibits relating to the case.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 3 minutes 59 seconds 3 m 59 s A timeline of the Claremont serial killings

The error rate of 0.16 per cent compared favourably with the 0.3 to 0.4 per cent error rate of a laboratory in the Netherlands that performed similar work, Mr Egan said.

Edwards's DNA was only found on the crucial fingernail samples, known as AJM 40 and 42, after they were sent to the UK for more sophisticated testing in 2008 and combined.

The yellow-topped container containing AJM 40 was never opened or tested at PathWest before it was sent to the UK, while AJM 42 was only opened twice — once in April 1997 for DNA testing, and again in March 2004 so that its contents could be repackaged and sent to New Zealand for further testing.

PathWest found no DNA on these exhibits or any other samples that came from Ms Glennon's body, including her clothes and underwear — neither Ms Glennon's DNA nor anyone else's.

Mr Egan said the reason DNA was only detected on the fingernail samples and not anywhere else was probably because they would have provided some "protection" for the genetic material.

Ms Glennon's clothing on the other hand, would have been exposed to "light, moisture and time" that all promoted bacterial activity that likely destroyed the DNA, he said.

Edwards's defence lawyer Paul Yovich SC will begin cross-examining Mr Egan on Wednesday.