Would you have a blood test for dementia?

Would I? The announcement yesterday that Alzheimer's can be diagnosed with 90% certainty three years before symptoms develop didn't feel to me like a good news medical story.

My first reaction was to be filled with dread.

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This malady will claim one in three of us. However, except in rare cases, it shouldn't strike before we are 70. Even then, why meet trouble half way especially as there is nothing (yet) that we can do to prevent its onset?

Surely, I thought, it is better to travel hopefully than to seek the medical equivalent of the judge's black cap a minute sooner than necessary?

For most of us the prospect of dementia is crueller than other terminal illnesses. It threatens us with the loss of our identity while we are still living. It threatens our families with a duty of care to someone who may be changed almost beyond recognition; someone who eventually may not recognise even their spouse or their children.

If it is our fate, there is nothing we can do about it but practise acceptance. Acceptance is one thing. Seeking out a diagnosis before symptoms show is something else.

And yet Alzheimer charities welcomed the news. So I tried to put aside my knee-jerk first reaction and set about reading more. Would it change my mind?

In Scotland only 64% of dementia sufferers have a diagnosis. That figure is much higher than parts of England where it is as low as one-third in some places. The charities want to see the figures high because diagnosis is a gateway to support, information and possible treatments.

In Scotland each sufferer receives post-diagnostic support, co-ordinated by a link worker, for a minimum of a year. Each health board now has a dementia nurse consultant. Scottish Health Secretary Alex Neil said at the time of the announcement in 2012: "This is a world first commitment and will lead to significant improvements in the ability of people to live well with dementia for as long as possible."

And therein lies the rub: for as long as possible. At the moment there is no cure.

As things stand, there is eight times more money spent on cancer research than on dementia. This is partly because dementia is vastly under-reported as a cause of death. A study in America followed 2566 people over eight years, giving them an annual dementia test. It then confirmed with an autopsy how many died from Alzheimer's. Their findings were five-to-six times higher than those recorded in the general population.

As well as poor diagnosis, a tendency to record conditions such as pneumonia as the cause of death masks the true extent of the challenge dementia poses.

Globally, the figure for dementia of all sorts stands at 44 million sufferers. By 2050, that is predicted to rise to 135 million, of whom three-quarters will be poor or in the middle-income bracket. More than half will have Alzheimer's.

That adds up to a colossal economic headache for governments. It is this economic imperative - rather than sentiment - that is fuelling the interest of the G8 nations.

Scotland is estimated to have 61,000 people suffering various forms of dementia, of whom around 33,500 have Alzheimer's. We are an ageing population so those numbers too will inexorably rise.

Even a short delaying of symptoms offers important benefits in sparing suffering and in terms of cost of care.

That is one reason to seek the earliest possible diagnosis; one reason to welcome a predictive test. Another is that drug treatment could be trialled at a much earlier stage in the disease.

Whether you or I decide to have the test at the moment is, fortunately, academic. More research work will need to be done before it becomes generally available. The breakthrough study by Georgetown University in Washington was small. Researchers followed 525 healthy men and women aged over 70 for five years. They analysed blood samples from 53 who developed dementia against 53 who remained cognitively normal. Biomarkers in the blood predicted with 90% accuracy those who developed Alzheimer's disease.

A larger clinical trial will be needed. But, if the findings are replicated with larger numbers, a blood test for all is just a matter of time. It has clear advantages from the point of clinical research. The earlier the disease can be detected, the better we will understand its causes and the more chance there will be of finding better treatments; perhaps, even a means of preventing its onset, or a cure.

On the other hand, when the test becomes available and before the discovery of a cure, many thousands might be sentenced to three extra years of fearful suffering. Certainly, they will have time to make financial decisions, organize a will, communicate with their loved ones and decide on their future care.

They will have time to see and do what is important to them before the nerve cells in their temporal lobes start to die.

Or maybe they won't. Might they be so overwhelmed by the prospect facing them that they are unable to enjoy the healthy life they have left? I wonder.

Once we are able to take the test, will it remain voluntary? The health costs of the diagnosis are high so will insurance companies make the test a prerequisite of taking out a policy?

Another worry: it has a 10% inaccuracy rate. What if we were to get a false prediction? Would those of us of an imaginative bent develop imaginary symptoms? Might some become suicidal? There is a sense here that we might be offered more knowledge than we can bear.

The drugs currently available work best if given early. They can reduce symptoms but do not slow the disease's progression. However, the Americans plan a further study in which people at high risk of developing the disease will be offered experimental treatments which, it is hoped, will delay or prevent the disease.

We can hope and pray for a breakthrough before the blood test becomes available. But if that does not happen so neatly, on reflection I think I would take the test despite my fears. I would take it because it would seem self-centred to a fault to decline it while knowing that the wider the knowledge base, the greater the research material, the better the possibility of treatment, of prevention. The more of us who have the blood test when it is available, the quicker that will come about.

Yes, I know I might have to spend three years confronting a fate I would shrink from. But along the way I'll have the small but comforting consolation that, for once, I've done the right thing. If I'm unfortunate enough to be told my test is positive I will keep it to myself. It's not a burden I would wish my family to share or to see any of them make decisions about their lives in preparation for my decline. I will instead cling to the hope that I am one of the 10% who was wrongly diagnosed.