There was a study done by an Italian group on the CA19-9 assay and one of the recommendations in the paper is using the same lab for consistency in reporting results. This is why I always go to the same medical center to have the test done. And as I noted in the previous reply, the one time I was directed to have it done by a National Reference Laboratory due to Covid concerns, the value reported out was double- 10 instead of 5. So to try and reduce the amount of variables in testing, I use the same lab. The same holds true for when I have imaging done. I continue to do to the hospital where I was treated rather than a more convenient neighborhood imaging center. Going to the medical center maintains consistency in the instrumentation and the radiologists doing the interpretation of the image. There is a familiarity between my case history and the same radiologists reviewing the images.
https://academic.oup.com/ajcp/article/138/2/281/1761200?login=false