Why do different cancer centers use different PSA undetectable limits?

A large number of places consider < .1 To be undetectable. That’s what Mayo goes by so does Kaiser.

They're different tests.

The regular PSA test goes down to 0.1 (or sometimes 0.2). The ultrasensitive PSA test goes down to 0.01 or lower.

If you're expected to have some PSA — e.g. routine screening or monitoring — then there's no extra benefit from the uPSA test. However, if you have metastatic castrate-sensitive prostate cancer, to goal is to hold your PSA at zero, and the uPSA test gives earlier warning if it's starting to move.

There are rare cases of prostate cancer progressing when PSA is < 1.0, and very rare cases < 0.1, but I have read of none (yet) of cancer progressing when PSA < 0.01. That's helpful for oncologists trying to decide whether a new spot of lucency on a scan is cause for concern.

When they indicate that your PSA is “undetectable” by using the mathematical “< “ (less than) symbol, that’s less about what your PSA actually is, and more about the limitations of the testing lab’s equipment.

PSA testing equipment have different levels of sensitivity (as well as different calibration methods).

For example:
> The PSA testing equipment at the cancer center that I go to can detect PSA down to 0.008 ng/mL. During the time that I was on Eligard, my PSA went below the sensitivity of their lab equipment to detect it, so they reported it as “< 0.008.” I have no idea what my actual PSA was, simply that it was “undetectable with their lab equipment.”

> A week or so later, I went to one of our local CompuNet laboratories (the one that my family doctor uses) for a self-ordered PSA test. The PSA testing equipment at CompuNet can detect PSA only down to 0.02 ng/mL. During that same time period that I was on Eligard, my PSA was below the sensitivity of their lab equipment to detect it, so they reported it as “< 0.02.” Again, I have no idea what my actual PSA was, simply that it was “undetectable with their lab equipment.”

And that made sense to me. If my PSA was undetectable at the cancer center (that can detect PSA down to 0.008 ng/mL), it was certainly to be undetectable at CompuNet (that can detect PSA only down to 0.02 ng/mL).

If your PSA is undetectable, it will always be preceded with the mathematical “< “ (less than) symbol, meaning that their lab equipment doesn’t have the sensitivity to test lower than that.

Even though they may say “we consider your PSA to be undetectable,” look for the “< “ symbol.

(Even now at 4-3/4 years post-proton radiation treatments, the cancer center that I go to still reports my PSA to 3 decimal places, my last one being 0.374 ng/mL just two weeks ago.)

Looking for this elusive number once, I asked my surgeon once what he used as a definition of undetectable, he said "that which can not be detected." My initial response was "nobody likes a wiseguy surgeon."

Then he explained. We're using a lab that has equipment that can detect down to .003, if they can't detect anything they say it was < .003 therefore undetectable by that lab.

I asked customer service at my local clinic why every doctor, department or nurse doesn't check blood pressure the same way. I have yet to get a response.

I just had a rather nice conversation with Grok about this, but it gets a bit lengthy. If you tell Grok that it's for patients post-radiation and treated with ADT over a period of months and years it finally focuses on the issues. The most sensitive ultra-readings will have perhaps 20% variability below 0.01% so it's best used over several tests to confirm the values. Radiation tends to leave bits of prostate still active, too, and these will cause variability. Once you get above the normal test or ultra test lowest levels precision gets better, towards just 5% variability.

And there are some aspects of the disease that are more or less sensitive to small variations.

I know with home tests for things like blood pressure and blood glucose the variability tends to be higher, for typical blood panel labs a few percent plus or minus is seldom critical. Few people seem to handle these statistical matters easily, and that includes me, but I at least try to be aware of them. If you remember the testing company Theranos that got in so much trouble, they weren't doing it right at all.

So what numbers different labs choose can depend on their equipment, the kinds of cases involved, and always some judgment by the lab's management as to what to report and what is significant.

The cost of transparency is sometimes confusion. If they just wrote "PSA: 0" then we'd all be happy, but we wouldn't know how sensitive the lab's test was. I appreciate their disclosing that, despite the extra cognitive overhead

FWIW, my oncologist calls < 0.01 "effectively zero," which is still nice to hear. 🙂

Different labs use different equipment, which are sensitive to extremely low levels of detection.
At less than .05 (MSK), .03 (Hopkins) or .02 (Quest Diagnostics), we are counting how many angels can dance on the head of a pin.
And as noted above, what is important is whether you are undetectable with any of the uPSA testing and if the PSA reading moves.
And (almost) all of us are understandably very sensitive (often overly sensitive) to numbers.
It's natural and human nature.
For the New Year, I wish everyone low numbers and good results.

To me the low numbers .0 something is not as important
as when psa has been down and starts raising to .2 plus.
Which might mean something is coming back.

Not to hijack this thread, but if, with my first PSA test after RALP the results are undetectable (0.03 more accurately), does that mean I'm effectively cured? Am I in remission?
How might I describe my state of health to people?