Hundreds of older people may be living with undiagnosed dementia in Mansfield and Ashfield, figures suggest.
NHS Digital data shows that 1,081 people aged 65 or over in Mansfield, and 1,235 people in Ashfield, had a recorded dementia diagnosis in September.
But estimates in the same data, based on the local population, suggest the real number could be 1,373 in Mansfield, meaning around 292 pensioners may have dementia without it being recorded by their doctor.
The alarming figure is similar in Ashfield, with estimates suggesting the figure could be 1,482, meaning around 247 Ashfield pensioners have also gone undiagnosed.
The NHS figures were collected in response to former prime minister David Cameron’s Challenge on Dementia 2020, which included a target for at least two-thirds of people with dementia to be diagnosed.
The numbers show that about 79 per cent of expected dementia sufferers in Mansfield, and 83 per cent in Ashfield, were diagnosed in September - well above the target.
It was also about the same as the previous September.
But The Alzheimer’s Society says that, while diagnosis rates for the condition have improved in recent years, the level of detection varies drastically across England – with the disease now thought to be the country's biggest killer.
Sally Copley, Alzheimer’s Society’s director, said: "The number of people with dementia is set to double over the next two decades, and as data shows, it’s still the UK’s biggest killer.
"It has never been more urgent to ensure a proper system of social care is in place."
Dementia is a term used to describe symptoms such as loss of memory, behaviour changes and problems in reasoning.
The most common cause of dementia is Alzheimer's disease, but it can result from brain damage caused by a stroke or neurological conditions such as Parkinson's.
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said early diagnosis of dementia was not always in the best interests of the patient.
But she said increased diagnosis in recent years showed GPs' recognition of how important it is to get patients the treatment they need.
She added: "Increases in diagnosis must be matched by capacity in the system – we need to see more specialist services in the community geared up to helping diagnose and support patients with dementia, such as memory clinics, but access is patchy across the country."
A Department of Health and Social Care spokeswoman said: "More people are being diagnosed with dementia than ever before, and we are committed to improving this further with better access to care and support, increasing public awareness and putting millions of pounds of funding into dementia research."