She said nowadays, of the 50,000 samples that are tested in the NZ lab, "one or two a year" may return a positive result for having been contaminated with DNA from an unrelated source, such as a staff member. 

She said in 2004, the number would likely have been similar, although she could not recall. 

Justice Stephen Hall has asked if it's able to be determined whether the four examples of cross-contamination occurred at Pathwest, when they extracted the samples, or at ESR. 

"[I can't] 100 percent [discount that the contamination occurred in the ESR lab] but ... there was a smaller window of opportunity and the amplification process [we carried out] included a negative control, introduced at that step, and those were clear," Dr Harbison said. 

She said there were more opportunities for the Qiagen extracts to have been contaminated in the Pathwest lab. 

Today, the unrelated DNA could be tracked to the person it came from, however in 2004, this was a manual process, and ESR did not carry out an investigation at the time. 

He has now asked what the minimum threshold amount of DNA was to return a profile using its SGM+ testing and Y chromosome testing. 

Dr Harbison said for SGM+, a full DNA profile could be obtained by samples with 0.1 nanograms of DNA, and a partial profile could be obtained from even less. 

For the Y chromosome testing she said a full profile could typically be obtained from 0.1 nanograms, and sometimes less than that. 

For reference, 100 DNA cells weigh 0.6 of a nanogram. 

Mr Yovich is now asking Dr Haribon when the NZ lab changed its practices to reduce the risk of contamination following the introduction of more sensitive DNA technologies. 

"As we moved through the 1990s that's definitely the case, by the time we were using the SGM+ testing [in 1997]... our work practices haven't changed significantly since then," she said. 

She said in 1997, a sample would have been handled with disposable, single-use blades and tweezers which were cleaned with ethanol between each use. 

Before 2004, she said scientists switched from wearing cotton lab coats to disposable paper lab coats. She also said tweezers and other lab equipment started to be washed in solution, then ethanol, and then UV treated, between each use. 

She also said by 2004, gloves were single-use only to prevent risk of cross-contamination from one item to another through the gloves. 

Mr Edwards was arrested in late December, 2016. 

Ciara's reference profile matched the DNA profile found on Ciara's left index fingernail (AJM41), which had the most DNA present of the four fingernails tested by the lab. 

The DNA testing of this sample revealed 10 loci, all matching Ciara's reference sample. 

One of the 10 loci, that each usually give a result for two alleles, showed a result for an additional third allele. 

"This may be a genuinely mixed profile with DNA from another person in the fingernail sample, it's also possible that this is DNA introduced into the sample during the processing, there is a small chance that sometimes it could be what you would observe with a mutation event, however that is not reflected in the reference loci," Dr Harbison said. 

She confirmed if the third allele came from another person, it was likely a female. 

Ciara's reference profile was also a partial match at her left middle fingernail (AJM42) which had less DNA present, and was more degraded. 

Dr Harbison said all the available locus (6/10) matched Ciara's DNA profile. 

Of the causes of degrading of DNA, Dr Haribson said: 

"Prolonged exposure to the environment is one factor, for example sun or water, if a sample is dry and kept out of direct sunlight it will last for many years, but a sample that's been outside or in a humid environment for example, the DNA will degrade quite quickly.”

The testing only revealed one DNA profile, being Ciara's. 

AJM42, Ciara's left middle fingernail, is one of the state's critical pieces of evidence. It alleges once combined with AJM40 (Ciara's left, torn thumbnail), and tested in 2008 in the UK, it revealed a mixed DNA profile consistent with Ciara and accused man, Bradley Edwards.

It's alleged Mr Edwards' DNA got under Ciara's fingernail when she scratched him in a fight for her life. 

Prosecutors claim the yellow top container which housed Ciara's torn left thumbnail was only opened once - in 2008, and have called the sample "pristine". 

AJM42's yellow top container was opened in 1997 and 2004. After the New Zealand testing, which found the sample only contained Ciara's DNA profile, the sample container was not opened again until 2008 in the UK. 

Mr Edwards' defence team is arguing Ciara's fingernail samples - at some stage while in the Pathwest lab - became contaminated with the accused's DNA profile, which was housed in the lab within exhibits relating to the rape of a 17-year-old girl in Claremont in 1995. 

Mr Edwards' has pleaded not guilty to the murders of Sarah Spiers, Jane Rimmer and Ciara Glennon. 

For a full catalogue of WAtoday's coverage of the trial, click here.