The prostatic specific antigen (PSA) is a blood-test biomarker originally FDA-approved for use as a measure of prostate cancer activity NOT prostate cancer detection.
PSA-based screening for prostate cancer detection is highly UNRELIABLE.
The PSA is not prostate-specific and often fluctuates. The bigger the prostate, the greater the PSA. The so-called normal levels of 0-4 ng/ml are arbitrary and many situations can raise the PSA in the absence of cancer while other situations can lower the PSA without guaranteeing protective value.
PSA-based screening may be somewhat more informative by measuring several PSA formats before considering an mp-MRI and or, a prostate biopsy. The following PSA tests when considered together may improve screening reliability; the total PSA, PSA derivatives (free and percent free), PSA kinetics (velocity and doubling time), PSA density, age specific values, PCa3 test and evaluating the PSA after a 3 month course of finasteride (proscar). Sometimes this requires monitoring over some time before an MRI/ biopsy is considered.

PSAs and Prostate Cancer: Mayhem and Gore
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The Imperfect PSA, the Fraudulent Robotic Prostatectomy and Medical Ethics. The US Preventive Services Task Force Weighs In
Made Sick from Prostate Cancer Awareness and Screening
PSA and Prostate Cancer Screening? Maybe. Carefully and Selectively
Uro Health Blog