PathWest veteran Martin Blooms testified on Tuesday at the WA Supreme Court trial of former Telstra technician Bradley Robert Edwards, 51, who denies murdering Sarah Spiers, 18, Jane Rimmer, 23, and Ciara Glennon, 27, in 1996 and 1997.
Mr Blooms tested swabs taken from a 17-year-old girl Edwards admits abducting and twice raping at Karrakatta Cemetery in 1995, and said lab staff were conscious of cross-contamination and were very careful not to encourage it.
Sex assaults were given high priority and dealt with swiftly, with exhibits testing likely beginning within less than one hour of arriving, he said.
Moist samples were dried out in an oven over several hours, grouped together in cases so there were no mix ups.
They were contained in tubes kept ajar by about 1cm to allow air flow and there were “no issues with that technique at all”, he said.
“Things don’t just fly about.”
Prosecutor Carmel Barbagallo made a similar comment in her opening address.
“DNA within a particular sample doesn’t just fly around a laboratory,” she said. “It cannot pass through a barrier such as plastic and it cannot selectively contaminate an exhibit with only part of the sample from which it is contained.”
Prosecutors say if DNA Edwards left on the rape victim was mixed up with DNA taken from underneath Ms Glennon’s fingernails in the lab, it would be expected the rape victim’s DNA would also be present in Ms Glennon’s samples, but it was not.
Under cross examination, Mr Blooms insisted he and his colleagues were “always very aware of the potential for contamination”, even in the mid-90s when forensic science was in its infancy.
Asked if precautions taken were in part driven by what was affordable and practical to do, he said: “I would defend any assertion that we shortcut cleaning procedures, for example.”
Mr Blooms also told the court tests found no semen on Ms Rimmer, but negative results were expected given her body was recovered from bushland 55 days after she vanished.
“With the state of decomposition, these results don’t indicate anything one way or the other.” He said the same about Ms Glennon, whose body was found in bushland 19 days after she was last seen alive.