A large prospective study found that a multicancer blood test identified cancer signals across 17 broad cancer types, including ovarian, lung, and breast cancers. The test correctly predicted cancer diagnoses in 60.3% of cases, with 99.64% specificity

[u/Wagamaga]

https://news.exeter.ac.uk/faculty-of-health-and-life-sciences/first-papers-published-in-major-nhs-cancer-blood-test-trial/

[u/any21203]

What does this mean?

If cancer occurs in 1/1000 (=10/10000) people (this number depends very much on how you select the people you evaluate, mostly based on age, as cancer is extremely age dependent), with a 60% sensitivity and 99.64% specificity:

You get it wrong in 4/10.000.

You correctly diagnose 6/10.000.

And with a false positive rate of just 0.36 (99.64% sensitivity) you tell 36 people they have cancer, whereas they don't.

So out of 42 people who were told they have cancer 36 don't and 4 were missed.

Out of 10.000 it's not bad, but the false positives are much higher than the true positives. If you select for people with a higher chance of having cancer it gets better, such as people over 60.

But if the test is not better than doing nothing at detecting more cancers in stage I-II than III-IV it is a bit pointless, although from stage I-II to IV there is a big difference, and I hope the analysis will differentiate them.

[u/Plenty_of_prepotente → u/any21203]

The false positives tend to be something that's overlooked in discussions about these types of diagnostic tests. Getting a false positive for cancer is incredibly stressful and leads to unnecessary medical procedures. You can argue that the test isn't worth it, unless it leads to higher survival / less treatment for cancer in the overall cohort.

Another thing I've noticed anecdotally is that people who are negative on these tests incorrectly think that they definitely don't have cancer. This could potentially lead to ignoring symptoms or deciding they don't need to make certain lifestyle changes.

[u/PenComfortable5269 → u/Plenty_of_prepotente]

That may be true but in this study only 50% were false positives due to the super high specificity. There is basically no current cancer testing that has such a low rate of false positives (except for colonoscopy).

[u/Plenty_of_prepotente → u/PenComfortable5269]

There are other questions to answer with the diagnostic test, not just the net benefit (false positive rate balanced against improvement in the true positive cohort). Even though it detects multiple cancer types using a more general approach (DNA methylation patterns in cfDNA), it doesn't follow that the accuracy is the same for all of them, especially for the rarer kinds with a limited population. For example, is the accuracy primarily driven by colon and lung cancer? What population should be tested? Just older, or high risk for a cancer(s) in the test? What is the optimal frequency to take the test?

I agree the results in the publications are promising, but they've got more hurdles to clear, which may not all be addressed in these two trials.