A forensic expert in Amanda Knox's appeal against her conviction for murdering her British roommate in Italy said Saturday that no DNA was found on the blade of the presumed murder weapon.

The original trial for the 2007 murder in the medieval Italian university town of Perugia had heard that traces of victim Meredith Kercher's DNA were found on the blade of the kitchen knife and that Knox's DNA had been identified on the handle.

"There was starch but certainly no DNA on the blade," Carla Vecchiotti, who has been reviewing DNA evidence in the case, told the appeal hearing in Perugia.

The knife, which was found in the house of Knox's then boyfriend and fellow convicted murderer Raffaele Sollecito, played an important part in the conviction.

Experts on Monday had also cast doubt on tests on a bra clasp originally found to have traces of Sollecito's DNA, saying the evidence was likely contaminated.

The head of Italy's forensic police wrote a letter that was read out in court defending the work carried out by forensic agents in the original trial, saying the technology was world-class and the officers were highly experienced.

"Never before" has there been such criticism of his agency, he said.

Patrizia Stefanoni, the police forensic scientist who originally found the DNA on the kitchen knife and the torn bra clasp, has said she will fight the appeal trial experts' accusation that basic errors were made on her watch.

Knox, from the US city of Seattle, also signed a statement during interrogation following the murder saying she was in the house while Congolese bar owner Patrick Lumumba killed Kercher.

Lumumba, Knox's former employer, has since been cleared of all charges and Knox alleges that the statement was extracted from her by police under duress.

Kercher, an exchange student from Leeds University, was found half-naked in a pool of blood in the cottage she shared with Knox. Her throat had been slit.

Prosecutors have said they believe Knox, Sollecito and a third person, Rudy Guede, killed Kercher in a drug-fuelled sexual assault, but the blond-haired Knox, now 24, says she was at Sollecito's house at the time of the murder.

Knox and Sollecito were sentenced to 26 years and 25 years each in 2009.

Guede was also convicted. While he has admitted to having sex with Kercher shortly before the killing, he says he was out of the room when she was attacked.