Medical ethicists draw some parallels between knowing you are at high risk of Alzheimer’s and Huntingdon’s disease, an incurable neurological disorder caused by a single gene defect which can be diagnosed even before birth. Individuals with Huntingdon’s begin to develop symptoms between the ages of 30 and 50.

“If somebody has the gene for Huntington’s disease, they have a similar or worse prognosis than for Alzheimer’s,” says Dominic Wilkinson, professor of medical ethics at the University of Oxford. “What we know from that is that there are some people who do want to know that information for planning their life. Do they save up for a pension or spend their savings on travelling the world? But it’s important that people are aware of what they’re letting themselves in for before taking a test that might reveal such information. Some might want to live their life without having a Sword of Damocles hanging over them.”

One of the additional complexities of Alzheimer’s is that genetic factors only represent 60-80 per cent of the risk for the disease. This means that no genetic test could ever be 100 per cent accurate. Environmental components such as whether someone remains sufficiently cognitively stimulated into old age, as well as diet, smoking, exercise and even loneliness, represent significant contributing factors towards the disease.

“It’s a complicated picture,” says Sims. “If you’ve got a high genetic risk of the disease, does that mean you’re going to get Alzheimer’s? The answer is no, because if you’ve got environmental factors that protect you, they can alleviate the cumulative effect of the genes. And vice versa: you can be at low risk genetically, but have some really high environmental risk factors in your life.”

At the moment, our understanding of the genetic landscape of the disease is predominantly based on white Caucasians, meaning that such tests could be less accurate in people of different ethnicities. However, scientists like Sims and Tanzi are working on developing gender-specific and ethnicity-specific tests which also incorporate environmental data, for example whether a person is a lifelong smoker or not.

Sims believes that this additional information, combined with the levels of certain protein biomarkers in the cerebrospinal fluid – the colourless liquid in your brain and spinal cord – could one day lead to a test which can predict Alzheimer’s risk with an accuracy of more than 90 per cent.

“If we get to the point where we’re combining those different datasets, we might well be able to be more certain in our accuracy with regards to predicting who’s going to get the disease,” she says.