A blood test may be able to identify signs of Alzheimer's disease decades before symptoms appear, with higher levels of biomarkers such as tau proteins in the brain signalling worse cognitive performance and an accelerated decline, a study published in The Lancet journal has suggested.

The neurodegenerative condition steadily affects memory and thought processes, eventually interfering with daily functioning. The disease begins with the accumulation of amyloid-beta (Aβ) protein plaques and phosphorylated tau protein tangles in the brain.

Amyloid and tau proteins are important for normal functioning, but can prove toxic upon accumulation and cause neuron death.

Researchers, including those from the University of California San Francisco, US, said measuring the levels of amyloid-beta and tau proteins in blood has become a way to track changes in the proteins, and can be used for diagnosing Alzheimer's disease.

The study measured levels of the biomarkers 'Aβ42', 'Aβ40', and p-tau217 in the blood of 1,350 dementia-free adults in the US with an average age of 61 years.

Standard high cut-off levels of Aβ and tau associated with AD were identified in six per cent (86 out of 1,350) of the study participants.

High levels of the biomarkers were associated with a worse midlife cognitive performance on cognitive processing speed and executive function, which enables planning, focusing attention, and adapting to new challenges.

The researchers also found higher rates of accelerated decline on verbal memory and processing speed tests taken five years apart among those with high levels of the biomarkers.

The findings demonstrate the potential of detecting early-stage AD among middle-aged adults using blood tests, they said.

An early identification of Alzheimer's disease would provide an opportunity for lowering modifiable risk factors such as physical inactivity, smoking, poor sleep and untreated hearing loss, as well the provision of medication, potentially delaying the start of cognitive decline and development of symptoms, the team said.

The study is one among three papers published together in a special issue of The Lancet journal focused on neurology.

In a Linked Comment article, authors from Karolinska Institutet in Sweden who were not involved with the study said that among younger people with no cognitive impairments, the Alzheimer's disease-associated protein blood tests can lead to a higher rate of false positive results.

Therefore, additional diagnostic criteria should always be used alongside the blood tests, they said.

The authors also said that the blood biomarkers are not suitable for large-scale, untargeted screening for Alzheimer's disease pathology in cognitively unimpaired populations or in the community.