Comparison of PSA test results from Quest and Labcorp

Quest and Labcorp use different instrumentation/standards for PSA testing. A 2005 web article by Dr. William J. Catalona has stated that these different instrumentation/standards can yield test results that vary by as much as 23%. The WHO standard, which is used by Quest, yielded the lower PSA scores in the Catalona article (https://drcatalona.com/quest/psa-tests-are-not-all-the-same/). This article has prompted me to try to find out for myself.

Yesterday, I had the opportunity to have back-to-back blood draws for PSA tests at both labs (literally next door to each other in the same facility). The Quest test was for normal PSA (lower detection limit of 0.1), and the Labcorp test was an ultra-sensitive test (I would have gone for a normal PSA test, but that’s what my doctor ordered).

Here are the results:

Labcorp: 0.094

Quest: 0.11

Both very low, and close, but Quest was 17% higher if you want to compare.

In my case, these low values, and the difference between them, are significant because I am ten years post radical prostatectomy (2015) and until June 2025, my PSA was undetectable on Labcorp tests.

In June, my PSA on a Quest test came back 0.11 (same as today’s results). No change in three months—good news! But, my PSA is above the limit of detection—not good news.

Interesting to note that my ultra-sensitive test at Labcorp was below 0.1, the lower threshold of detection for a normal PSA test.

The 0.11 measurement in June set in motion a PET SCAN and a pelvic MRI, both of which showed “concern for recurrence” in the anastomosis. So, strong imaging evidence for local recurrence, but no imaging evidence for metastatic disease. DREs also detected a small nodule. The evidence for local recurrence seems pretty clear even without a biopsy, and with PSA < 0.2.

I am planning to start EBRT within the next week or two.

Interesting to note that had I used Labcorp back in June for a normal PSA test, that the result may have been “below detection” and I would have been sent home until a follow up in another year. Instead, I am now a two-time cancer survivor. I’m just glad I caught it early and that I am treating it early. A small difference in PSA test results yielded a rather dramatic shift in diagnosis.

It is also interesting to note that even though my anastomosis lit up with an SUV of 13 on the PET scan, that conventional wisdom in the medical world says that PCa doesn’t show up on PET scans until PSA is around 0.5 (again, I’m 0.1). I don’t know how to reconcile that other than to say, conventional wisdom isn’t always right. I suppose there is some very slight chance that the palpable nodule is benign tissue, but highly active metabolically. However, I’m not going with that unlikely possibility, nor are my oncologist and urologist.

So, there you have it. Make what you will of this info. In my case, different tests from different labs yielded slightly different PSA test results that were highly significant.