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brain scan

Alzheimer's and brain scans: not needed yet

Carl Heneghan
Last edited 7th March 2011

New system uses brain scans to spot early Alzheimer's says the BBC and so does the Daily Mail. Therefore, it must be important.

"We have some of the best scientists and facilities in the world and today's announcement will help ensure we continue to be at the cutting edge of life sciences."

An advanced computer programme compares a patient's brain scan with a database of 1,200 existing images of brains already affected by the disease. Scientists say early tests show the new technique is 85% accurate and can deliver results in just 24 hours.

In relation to diagnosis the word accurate keeps cropping up, and when it does we normally reveal the test is useless. If you want a precursor then see why Autism can’t be diagnosed with brain scans in the guardian last year. It's seems to be a modern feature of news stories that there seems to be little checking of the original data.

This is what Kings College have to say:
The 'Automated MRI' software automatically compares or benchmarks someone’s brain scan image against 1200 others, each showing varying stages of Alzheimer’s disease. This collection of images is thought to be the largest of its kind in the world.

The scan has been developed by scientists at the IoP, together with colleagues from the Karolinska Hospital in Stockholm.
The system is being 'field tested' over the next 12 months with patients attending SLaM memory services in Croydon, Lambeth and Southwark. The ‘field test’ will also provide a supply of research grade images, which has important implications for the development of the next generation of drugs for dementia and individualised treatments.

My assumptions for now, given after searching 35 news articles without a single reference to any study, is that 85% accurate refers to the proportion of people with established disease who have a positive brain scan. This figure, if it is the correct interpretation, bears no resemblance to the real question that needs to be asked? If I have a positive brain scan do I have Alzheimer’s?

I'm not sure how these news stories help us, but for now, I'm glad I can still spot nonsense when I see it. When I can't, it looks like I'll need the Brain scan.

autism and brain scan test: the real predictive value

Carl Heneghan
Last edited 11th August 2010

A brain scan that detects autism in adults could mean much more straightforward diagnosis of the condition, scientists say. Reported the BBC, Sky the Guardian and many more.

I had great difficulty getting hold of this paper, it wasn’t published online at the time of the press release. I managed to get a copy via Ben Goldacre at Bad Science and Evidence Matters who sent me the full text. Given this problem in getting the paper, it is highly likely no one who released the story has actually read the paper.

The news all report the headline ‘The researchers detected autism with over 90% accuracy, the Journal of Neuroscience reports.’

Sounds impressive, but this is one of the most obvious mistakes to make in interpreting a diagnostic test result. Never mind this is not the correct study type.

What has happened is the sensitivity has been taken for the positive predictive value, which is what you want to know: if I have a positive test do I have the disease?

Sensitivity: The proportion of people with disease who have a positive test.
Positive predictive value (+PV): The proportion of people with a positive test who have disease.

So, for a prevalence of 1% the actual positive predictive value is 4.5%. That is about 5 in every 100 with a positive test would have autism. Even at a prevalence of 2%, only 8.5% would be correctly identified.

Suddenly, not that great a test. This has to be one of the worst examples of misinterpreting diagnostic test results in the media I’ve ever seen.

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