The defence pressed to retest crucial forensic evidence from the crime scene with a handful of items re-examined by Thailand's Central Institute of Forensic Science (CIFS) including a garden hoe, the suspected murder weapon.
"The DNA on the weapon did not match with the two suspects," Porntip Rojanasunan, director-general of the CIFS, said after testifying at Koh Samui Provincial Court on Friday.
Porntip also told the court that there was no DNA found on other items tested by the forensics institute including a shoe and some plastic bags.
In July a witness at the trial had testified to removing and washing down the garden hoe after coming across it shortly after the tourists' bodies were found on the island.