The court heard there were three possible explanations.
There was a two-week gap between lab testing on a murder victim's fingernail sample and a swab from a rape victim, the Claremont serial killings trial has heard, as the defence seeks to establish possible contamination.
Bradley Robert Edwards, 51, admits raping a 17-year-old girl at Karrakatta Cemetery in 1995 and attacking an 18-year-old woman in Huntingdale in 1988 but denies murdering Sarah Spiers, 18, and Jane Rimmer, 23, in 1996, and Ciara Glennon, 27, in 1997.
The epic WA Supreme Court trial last week heard when Ms Glennon's left middle fingernail was initially tested, only her DNA came up - not Edwards'.
But in a combined sample with Ms Glennon's torn left thumbnail in 2008, Edwards' DNA was detected by UK scientists, although the ex-Telstra technician was not identified until 2016.
Yesterday, the court heard there were three possible explanations: the clipping never contained Edwards' DNA; it was always there but not detected by the technology used at the time; or it was present but not in the extracts initially analysed.
There were several tests conducted on various samples and sub-samples in WA over the years.
Pathwest scientist Anna-Marie Ashley testified the rape exhibits were not tested on the same day as Ms Glennon's nail samples.
In fact, Ms Glennon's samples were tested two weeks earlier in May 1997 and that was the closest they ever got to each other, Ms Ashley said.
The defence claims the rape swabs contaminated Ms Glennon's fingernail exhibits via secondary transfer in the lab.
Asked under cross-examination whether it was ever communicated to her that police were under pressure for results in the case, Ms Ashley replied: "I can't recall that being communicated to me."
Ms Ashley said improvements were later made to reduce the chance of contamination in labs, including masks, hair nets and double gloves.
Defence counsel Paul Yovich also asked Ms Ashley if she recalled anyone working on phone lines in the lab but she said no.
During re-examination, prosecutor Carmel Barbagallo asked Ms Ashley if she was aware of any contamination in relation to the two cases and again, she replied no.
WA Police Commissioner Chris Dawson sat in court yesterday morning and spoke with Ms Glennon's father, Denis, and detectives.
"The purpose of me visiting today is just to encourage the prosecution and the police teams who are working very, very hard in a very difficult and challenging trial," Mr Dawson said outside court.
Source: 9News https://www.9news.com.au/national/claremont-killings-trial-two-week-gap-between-testing-dna/8f50cb26-c100-4353-a278-94f8e020dd86