Around 18,000 people living with dementia in Wales do not have a diagnosis, leaving families struggling to cope without support and treatment, Senedd members warned.
Mabon ap Gwynfor said Wales has the lowest rate of diagnosis in the UK at 56%, warning that dementia is one of the biggest causes of death in the country.
Plaid Cymru's shadow health secretary paid tribute to Terry Griffiths, the former snooker world champion from Llanelli, who died on Monday following a long battle with dementia.
Mr ap Gwynfor raised an estimate from Alzheimer's Society Cymru of a 37% increase in the number of people living with dementia by 2040 due to an ageing population.
He told the Senedd that dementia costs the Welsh economy £2.3bn a year, which is expected to double to £4.6bn by 2050 if services do not improve.
'Biggest killer'
Leading a debate on a Plaid Cymru motion calling for diagnosis targets, Mr ap Gwynfor said: "Around 18,000 people are left to cope without the support and treatment they need."
"That's 18,000 individuals and 18,000 families struggling in silence, without knowing what to do or where to turn. The situation is even worse in rural areas, such as Powys."
Gareth Davies warned dementia is Britain's biggest killer, yet spending on diagnosis makes up less than 1.4% of UK health care expenditure, "which seems a staggering oversight".
He said families are shouldering 63% of dementia-related costs, according to Alzheimer's Society, with the average bill standing at £29,000 a year rising to £80,000 or more.
His Conservative colleague Altaf Hussain, a former surgeon, focused on alcohol-related dementia and brain damage, warning it is difficult for GPs to identify.
'Cruellest'
Delyth Jewell said her grandma Doreen, who lived to be 100, had dementia.
She told the Senedd: "The cruellest thing about dementia is how bewildering it is for the person going through it. My parents would visit my grandma. They'd call her multiple times each day. But she'd forget that they'd been."
"She'd think that they'd forgotten her. I remember her calling my parents' house one day, and I'd answered the phone, and she'd asked me, 'Why is it that I can't do all the things I used to do?' and she wished that she could walk and walk."
"She'd get frustrated, she'd be lonely and she couldn't understand why it was happening."
Ms Jewell, who represents South Wales East, said diagnosis rates are stubbornly low at 60% in the Aneurin Bevan health board and "there's still no dementia plan for Gwent".
Heledd Fychan, a fellow Plaid Cymru Senedd member, called for greater support for Welsh speakers living with dementia, warning their needs are too often disregarded.
'Vital'
Mike Hedges, who represents Swansea East, said: "Increasing diagnosis rates across Wales is vital to enable people...to take control of their condition and live independently for longer."
"This supports people to stay out of hospital and in their own homes, relieving pressure on our health and care system."
Responding to the debate on Wednesday, Sarah Murphy told the Senedd an independent evaluation of the Welsh Government's dementia action plan will be published in early 2025.
The mental health minister, whose responsibilities include dementia, said the final report will inform a refreshed plan that will include new targets for diagnosis.
She said: "Ensuring people's lived experience is at the heart of the development of services is very important to me, and I am committed to the development of robust diagnosis data."
While the Plaid Cymru motion was defeated, 27-25, a version amended by the Conservatives and the Welsh Government was agreed unanimously.
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