Please realise that, just because a detail is omitted in a news report, it does not mean that the team who did the research didn't think of it, or tried to hide it. This is just the usual shoddy reporting by the BBC.

Recently the woman who could smell Parkinson's Disease got one test wrong but when they rechecked they found that she was right, it was just undiagnosed in that person.

Read a little more about it --> Researchers identified 10 molecules that Parkinson's patients emit and could be used for early detection. Pretty cool. http://www.telegraph.co.uk/health-fitness/body/meet-woman-ca...

Is this so? I thought the original researchers would be able to follow-up if they had accounted for it in their consent forms. In other words, they surely have identifying patient information, and even though an outside team couldn't screen the same cohort, that original team could find those 7 patients and ask again in 2 years.

Is that about right taking into account pre-screening? We can identify a population that has a 5% chance of having one of these cancers?

At 70 people have a much higher risk of cancer than 7. Long term smokers have higher risks than non smokers. Combine multiple risk factors and find groups with 5+% risk of these kinds of cancers is not that difficult.

With such an excellent specificity this sounds promising as a cheap screening tool, although the sensitivity is too low in some cancer types to be used as a means to rule out cancer.

Also, I assume it would be mostly done on higher risk patients (certain work conditions, certain age, etc..). Only once in couple of years on younger population for example. Or if it's really cheap everyone could get it and then there could be more focused blood screening done next so you don't have to get full body cat scans.

But first of all you need to account for the negative effects of a false positive, and they are much worse than that. They include anything from stress and psychological effects, cost and risks of all the additional screening, to the much worse issue that you also need to account for the fact that for some cancers in some instances it will be hard enough to determine if the growth is malignant in the sense that some proportion of cancerous growth will never pose a threat to the patient, but once diagnosed there tends to be a strong pressure to treat. As a result too frequent screening will save some, but will also result in a large amount of unnecessary chemo, radiation therapy and/or surgeries - all of which come with risks in addition to the pain and discomfort.

Then you need to account for frequency of testing. A 1% false positive rate per test looks distinctly worse if the test is repeated every year for a 10-20 year timespan, for example.

Then you'll also want to account for how much it improves outcomes. If the cancers it detects are ones that would generally be detected in time anyway, and/or have low mortality, and where treatment outcomes are good, the incremental improvement would not necessarily be big enough to justify the negatives.

For breast cancer, for example, there has in recent years been a push to reduce large scale routine mammogram screening because it's not clear if it does more good than harm for many groups of patients (in large part because it leads to overtreatment). This article goes into some of the issues related to that in some detail (and presents both proponents and detractors of large scale screening):

http://www.healthbeatblog.com/2009/04/mammography-screening-...

If I remember correctly, the argument went like this: early screening may detect some forms of prostate cancer which, given the estimated natural lifespan of the patient, would have failed to kill/discomfort him before he died of other causes. But once it's been detected, the recommendation will be to treat the cancer; and treatment has a chance to cause discomfort (such as impotence) right now. So in effect, early detection of some forms of prostate cancer may negatively impact the quality of life of patients who would have otherwise lived a normal life without noticing the cancer.

I might be remembering some details incorrectly.

"Given the considerable time that has passed since PSA screening began, most of this excess incidence must represent overdiagnosis."

true positive rate (TPR, a.k.a. hit rate, recall, specificity)

TPR = TP / P = TP / (TP + FN)

specificity (SPC) or true negative rate

SPC = TN / N = TN / (TN + FP)

"Sensitive" has a clearer intuition: For example when you want to detect a weak signal, then if your antenna is more sensitive, you can better pick up the signal, thus you are better at telling whether the signal is really there (you have a better true positive rate).

I cannot think of a better name for TNR, though, other than simply sticking with TNR and TPR.

Still, this would be great, probably combined with (nearly) equally simple follow-up tests.

C being cancer, H healthy, P positive test P(C|P) = [P(P|C) * P(C)] / [P(P|C) * P(C) + P(P|H) * P(H)] = [ 0.7 * 0.05] / [ 0.7 * 0.05 + 0.01 * 0.95] = 0.035 / (0.035 + 0.0095) = 0.7865

That's pretty good. If you have a positive test result, in 4 out of 5 cases it actually is cancer. It gets a bit worse if you account for the fact that this test only works for a subset of the cancers. But this would be useful as a pre-screening.

You are using % of population diagnosed with cancer. The correct value would be the % of population with undiagnosed cancer.

In the end you'll need a properly conducted study with random undiagnosed people to get reliable numbers though.

You also need to keep in mind that that it is very possible the false positive rate will be per test, in which case with regular screening 1% quickly turns to 10%...

Cancer treatment is highly lucrative, though.

It should be noted that at the moment, the researchers are still working on determining which of the 8 cancers is present in the case of a positive result.

Screening tests are about cheaply directing further testing.

Early screening will occasionally save some. The question is whether it will save enough to make up for that harm if we increase the frequency of testing.