> In the case of pancreatic cancer, it is much more aggressive and you need to catch it early.

It's not clear that the cancers that you would find early with a more sensitive test are those more aggressive cancers.

The pancreatic cancers we find with our current detection (generally after becoming symptomatic) are typically quite aggressive. But are they all the cancers? Likewise, if the cancer is aggressive, it can grow quite a bit between screening intervals and not be found all that early.

(Part of why we think that "finding cancer early" is such a benefit is that because the smaller/earlier cancers we find are less aggressive than the cancers that we first find when they're huge and spread. There is definitely an effect from earlier detection but our estimate of it has been confused by this effect.)

As we've increased cancer screening, we've found that survival rates have gone up, as have survival times after detection... but unfortunately we've often also found that the screening doesn't always reduce the number of people dying of that cancer at a certain age. Instead, you find more cancers, and you find them earlier so more people live to 5 years, even if you've changed nothing. Cancer treatment has gotten better, but most of the benefits we have expected from better cancer screening have not materialized.

Finding pancreatic cancer early sounds good. And it may be able to reduce mortality from pancreatic cancer, but it's not a sure thing.

My grandfather (a doctor) always used to say this. There’s also an aggressive fast growing kind of prostate cancer, but treatment basically does nothing for survival rates (or at least that was the case decades ago when he was practicing.)

So his advice was, don’t look, don’t treat. Either you have the slow one and treatment is harmful, or you have the fast one and you’re going to die soon anyway.

Your grandfather's take has become increasingly accepted for prostate cancer. There is more of a watch and see attitude to make sure that the patient doesn't have a rare case of aggressive growth.

As you mentioned, the outcomes aren't significantly different, regardless of how you treat it.

From the article linked above:

> Researchers followed more than 1,600 men with localized prostate cancer who, from 1999 to 2009, received what they called active monitoring, a prostatectomy or radiation with hormone therapy.

Over an exceptionally long follow-up averaging 15 years, fewer than 3 percent of the men, whose average age at diagnosis was 62, had died of prostate cancer. The differences between the three treatment groups were not statistically significant.

The irony is he died of prostate cancer. He ignored his own advice and treated it. It did not change the outcome or buy him much time, if any.

You can just run test multiple times to eradicate this possibility or you can confirm it with another method.

> You can just run test multiple times to eradicate this possibility

The measurements are not independent and the quality of the measurement is not improved by this.

> you can confirm it with another method.

Yes. And usually the other method is invasive and expensive and bears some risk.

And then you get results like the blood test saying "very likely cancer" and the biopsy saying "uh, probably not?" that you need to decide what to do with.

I'm confused. If this blood test gave a false positive that wasn't due to an anomaly in the blood itself, then why can't we assume that the likelihood of getting a false positive twice is lower than getting it once?

> I'm confused. If this blood test gave a false positive that wasn't due to an anomaly in the blood itself,

That's the errant part. A small amount of the false positives will be because of lab issues. The rest will be because this patient is different in some way, but not all of them are cancer. Medicine doesn't have very many perfectly specific tests.

So you have a patient who has some weird enzymes around because they're genetically different, for example, and they always pop positive on this particular test. Or has an unusual diet that causes some other non-tested-for-enzyme level to be high enough to set off this sensor. Or whatever.

In this case, the specificity is 98%, so this false positive rate is about 2%.