A test which is used to help assess patients with dementia is a worthwhile exercise, scientists have confirmed.
Early diagnosis benefits dementia patients
It was feared the commonly-used reading test was not a good measure of basic intelligence, which is useful as mild dementia symptoms may not be obvious.
But Alzheimer's Research Trust experts have shown the test can indicate intelligence accurately.
This means doctors can rely on it as a basis to diagnose how advanced a patient's dementia is, the Trust says.
It can be difficult for doctors to diagnose dementia because of a lack of availability of objective measures of previous mental ability.
Such measures are particularly important in the mild to moderate stages of dementia when the symptoms are not always obvious.
An accurate diagnosis allows doctors to rule out other illnesses with similar symptoms, such as depression.
The patient can then be prescribed drugs which can temporarily improve the quality of life of the patient and their carer.
The test studied was the National Adult Reading Test (NART), which asks patients to read irregularly pronounced words such as ache and thyme.
It is widely used by doctors, but some experts had expressed doubt over whether patients' performance in the test was itself affected by dementia.
The researchers looked at NART scores for just over 500 people aged around 80, 45 of whom had been diagnosed with a form of dementia.
As children, all had had taken part in the Scottish Mental Survey of 1932 - in which children aged around 11 were given psychometric intelligence tests.
The researchers compared mental ability in youth and NART scores in old age for patients with mild to moderate dementia and for healthy people.
It was found there was no difference in the range of NART scores between those with mild to moderate dementia, and those who were healthy.
They also found that people's scores were in line with how they fared in their childhood tests, suggesting the NART test accurately indicated people's intelligence.
The Scottish team say the test appears to be less useful for diagnosis of advanced dementia, as ability to pronounce difficult words is affected by the disease at that stage.
The team hope to carry out further research to discover more about which brain areas are involved in pronouncing irregular words.
Dr Brian McGurn, the first Alzheimer's Research Trust Clinical Fellow, told BBC News Online assessing the extent of someone's illness was difficult based on traditional cognitive tests alone - because their scores would be coloured by their previous mental ability:
"A retired professor would score highly, even if he had advanced dementia," he said.
Dr McGurn added: "We are all delighted that this work could lead to improved early detection of Alzheimer's and other dementias so that new and future treatments can be given as soon as possible."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "Early accurate diagnosis of Alzheimer's is vital in the fight against this devastating disease.
"These exciting results have significant implications for clinical practice and bring us a step closer to finding an answer to dementia.
"However, there we have a long way to go and a lot more research is needed if we are to find an effective treatment or prevention for dementia."