The tests were ordered as part of a third trial in the case in which Knox and her former boyfriend, Italian Raffaele Sollecito, are accused of killing British student Meredith Kercher in 2007 while the two women lived together in Perugia, Britain's The Daily Telegraph reported.
Investigators believe the kitchen knife, which was found in Collecito's apartment, was used by Knox to kill Kercher, as forensic experts initially said they found traces of DNA on the blade of the knife that belonged to Kercher, and Knox's DNA on the handle.
The original evidence was used to convict Knox and Sollecito in 2009, but was later thrown out during their appeal.
The latest results, which were revealed in court Thursday, "pushes even further away the possibility that this was the murder weapon," said Luciano Ghirga, Knox's lawyer.
The new tests found only tiny trace amounts of Kercher's DNA on the blade of the knife, which the defense has argued only proves that the knife was once in the apartment Knox and Kercher shared and could have gotten on the knife at any time.
"The forensic experts appointed by the Perugia appeals court, who at the time of their work didn't have instrumentation available to carry out a thorough examination on such a low quantity of DNA, carried out their work well," Ghirga said.
"This test excludes categorically that the knife was the murder weapon," said Giulia Bongiorno, Sollecito's lawyer.
Knox has opted to not be present at the current trial, which was ordered earlier this year by the Supreme Court in Rome. Sollecito, who has so far been absent from the trial, is scheduled to appear in court Wednesday.