She said: "I just want to know if she's alive or dead."
Speaking to publicise a new child rescue alert system, she added: "We need to know. The worst thing is not knowing."
Kate, 46, said she still longed for the return of her daughter nearly seven years to the day after Madeleine went missing while on holiday with her family in Portugal.
But in an interview with The Sun, she said: "We obviously want Madeleine back number one - but we do want an answer, whatever.
"I have spent days thinking, 'What would you rather? "Not know, or find out something you didn't want to hear?'
"Obviously our ultimate hope is that we find Madeleine and she comes home and we re-establish her into our family and spend the next few years of her life getting it all as good as it can be."
She added: "I'm not under-estimating the blow of hearing bad news that your child had been killed, because obviously you're not going to go, 'OK at least we know'.
"But I've spent hours thinking about that and, each time, I still come up thinking we need to know.
"Regardless, we need to know."
Kate and husband Gerry, 45, hope a Met Police investigation could lead to Madeleine, who would now be 10, being found alive and well.
Kate was speaking three days ahead of the seventh anniversary of Madeleine's disappearance to promote the launch of the Child Rescue Alert system, which can be triggered when a child's life is in danger. Kate is urging people to sign up to receive the alerts.
The system will go live on 25 May.
Kate said: "When a child is abducted, families are devastated and entire communities are torn apart. The agony of not knowing where your child is almost impossible to imagine. The helplessness is at times overwhelming.
"But there is now something we can all do to help. Please sign up to receive alerts - you could save a child's life."
A Child Rescue Alert is a mechanism to centralise possible clues and leads from the public.
An alert can be triggered by any British police force - but only when a case matches three criteria:
• The child must be under 18
• There is a "reasonable belief" that the child is in imminent danger of serious harm
• There is sufficient information for the public to be able to help the police investigation
The revamped system will allow alerts to be issued via text, email, social media and digital billboards.
Alerts have been in place across England and Wales since 2005 and members of the public can already sign up to receive them.
The updated system will come into force on on 25 May to coincide with International Missing Children's Day.
CHILD RESCUE ALERTS
• Alert the public to an abduction or other high-risk child disappearance
• Based on the 17-year-old American AMBER alert system
• First introduced in the UK by Sussex Police in 2002
• Managed by the Child Exploitation and Online Protection Centre (Ceop)
• A decision to issue a CRA is made by a senior investigating officer on the lead police force