The American writer has already served three-year-term handed down by the court.
Knox initially identified Congo-born Lumumba over the murder of the 21-year-old British exchange student, a crime for which the American was first convicted with her Italian ex-boyfriend Raffaele Sollecito.
Knox was finally acquitted after almost four years in prison.
Lumumba had nothing to do with the murder.
Knox filed the appeal to definitively annul the slander sentence on the basis of a European Court of Human Rights ruling that her defence rights were violated during the initial investigation.
In October, Italy’s supreme Court of Cassation overturned the conviction and ordered a retrial.
Knox listened beside her husband Chris Robinson and her lawyers as the ruling was read out.
“Amanda feels very bitter,” her lawyer Luca Luparia Donati said in an initial response, adding that she planned to appeal.
Knox, now aged 36, earlier accused Italian police of threats and violence as she sought to clear her name in the last outstanding case against her.
“The police threatened me with 30 years in prison, an officer slapped me three times saying ‘Remember, remember’,” she told the court on Wednesday, saying that police wanted her to blame Lumumba.
“I never wanted to slander Patrick.
“He was my friend, he took care of me and consoled me for the loss of my friend.
“I’m sorry that I couldn’t resist the pressure and that he suffered.
“I humbly ask to declare myself innocent,” she concluded.
She described herself at the time as “frightened and deceived” 20-year-old.
Lumumba was a civil plaintiff in the trial and his lawyer had asked for calumny conviction to be reinstated.
Knox and Sollecito were arrested five days after the murder and convicted by a court of first instance, but this conviction was subsequently overturned.
The appeal sentence was then thrown out by the Court of Cassation, Italy’s supreme court, which ordered a new trial on appeal leading to their re-conviction in 2014.
Knox and Sollecito were eventually acquitted definitively by the supreme court the following year.
Rudy Guede, an Ivorian, was convicted and sentenced to 16 years for the murder. He was released from prison in November 2021 after serving 13 years.
In February, a court ordered him to be placed under special surveillance after he was found guilty of beating his ex-girlfriend.
ANSA
