An inquiry has heard thousands of Queensland DNA crime scene samples went untested following a controversial lab decision. 

The Commission of Inquiry into Forensic DNA Testing in Queensland has been reviewing the effects of a decision not to test certain samples at a state-run DNA lab. 

The lab tests samples for use in criminal investigations. In 2018, allegedly at the request of police, the lab stopped processing any samples that fell below a certain threshold - labelled “DNA Insufficient for Processing” or “DIFP”.

This decision has been at the centre of the inquiry because of concerns that many potentially valid samples did not undergo further testing, leading to questions over the outcomes of legal cases that relied on them. 

On the final day of public hearings, counsel assisting the inquiry Susan Hedge questioned expert witness South Australian Professor Linzi Wilson-Wilde.

She told the inquiry that around 4,000 samples were not tested each year because of the decision.

“Looking at the total samples received … being between 20,000 and 25,000 samples, those numbers which are about 20 per cent, equates to about 4,000 samples every year that weren't progressed?” Ms Hedge asked.

“Yes, that's what the data indicates,” Professor Wilson-Wilde replied.

“That data shows the effect of those thresholds over this period of time, in terms of what was not tested?” Ms Hedge asked.

“Correct.”

Of the samples below the threshold that were progressed for testing, the inquiry was told that the success rate in getting a DNA profile was always above 50 per cent - and in 2022, as high as 97 per cent.

The inquiry should deliver its final report by December 13.