Theory and data

From the Herald (from the Daily Telegraph)

A revolutionary blood test, which acts like a smoke detector to spot cancer up to 10 years before symptoms appear, could be available within five years.

It looks like this is genuinely impressive research, and deserves its spot at the British Science Festival, but it’s harder to assess the realism of the claims. What do we actually know now? Well, less than we should, because the claim is based on a press release and interviews about unpublished research. However, earlier research by the same group is available, with a bit of detective work.

In a conference abstract published in February, they report what they were trying to do: measure mutations in a specific gene in red blood cells. As the Herald story says:

Scientists at Swansea University have discovered that mutations occur in red blood cells way before any signs of cancer are evident.

But it’s more than that. Mutations in red blood cells occur before cancer even exists — another reason this test is potentially useful is for studying low levels of mutations that would have a very low chance of leading to cancer, so that the risk of realistic doses of potential carcinogens can be assessed. Since the test picks up mutations in the absence of cancer, there’s justification for worrying about false positives.

In the February abstract they had used the test on 121 people, and were claiming five-times-higher mutation rates in people with cancer than healthy people. Now they have 300 people and are claiming ten-times-higher rates — one possible explanation is that they’ve made the test more selective somehow and so are picking up fewer uninteresting mutations.  In any case, progress. The earlier data didn’t look as if it could support a useful test; the new data might be able to.

We still don’t know about the false-positive rate — with 300 people tested, it’s too early to say.  The false-positive rate is important for another reason, though.  The Independent has another story, quoting the lead researcher

Professor Jenkins said they needed to find evidence that it would work for other cancers, but added it would be hard to imagine that it would not.

“It would be really difficult to think why it would only affect oesophageal cancer,” he said.

As he says, it’s hard to think why oesophageal cancer would be unique — though you might expect some cancers to be different. For example, in cervical cancer, the mutations are caused by a virus that only infects certain cell types, so it might not cause mutations that show up in red blood cells.  But if we assume many cancers show the same pattern of red blood cell mutations, assessing the usefulness of the test gets more difficult. Suppose a positive result means you’re going to get some type of cancer over the next ten years, but it could be almost any type. What would the next step be?

There’s another important point in the first sentence of the Herald story. It contains two numbers. One is bigger than the other.  As far as I can tell, this test is done on freshly-collected blood, and hasn’t been done on large numbers of healthy people yet. If the test is available within five years, it will, at best only come with reliable information for five years after testing.