PSI is pounds per square inch and is rarely related to blood tests.
On the other hand, "PSA" stands for prostate specific antigen.
Prostate specific antigen is a protein produced by normal prostate cells. It is sometimes used to track the response to therapy in men with prostate cancer. The standard PSA reference range is 0 to 4 ng/mL. PSA levels increase with age which is related to prostate volume. A man younger than 50 should have a PSA level less than 2.5 ng/mL. A man older than 70 could have a PSA of 6.5 ng/mL and still be considered within the reference range.
A PSA level of 4 ng/mL is used as the threshold at which a prostate biopsy is recommended although 1/3 of the cancers occur in men with PSA levels less than 4 ng/mL.
The PSA level tends to rise in men with benign prostatic hyperplasia and is a marker for prostate volume. PSA levels are usually elevated in men with acute bacterial prostatitis. The most valuable measurement of PSA is its change over time rather than the actual serum level.