PSA + Free PSA

• unbound, free form of PSA (free-PSA, molecular mass about 33 kD)
• bound, complex form of PSA with 1-antichymotrypsin and α2-macroglobulin (bound-PSA, molecular mass about 100 kD).

The two forms represent the main part of the total PSA (total-PSA).

Total-PSA = free-PSA + bound-PSA.

Measurement of f-PSA has a diagnostic value in parallel with determination of total PSA (t-PSA). The determination of f-PSA and t-PSA makes it possible to draw up the ratio between the two: Free PSA/Total PSA and calculate the percentage of free PSA (% f-PSA). The percentage of free PSA (% f-PSA) has a greater diagnostic value than the total PSA test alone, especially in cases of normal or borderline changed PSA and enlarged prostate dimensions on rectal douching. In men with %-f-PSA values below 25, biopsy is recommended. In fact, prostate carcinomas usually release forms of PSA that are bound to proteins in the blood. This leads to an increase in bound PSA in the blood and hence a decrease in f-PSA (the PSA forms released by prostate cancer are different from the PSA forms released by benign prostatic hyperplasias). The examination of f-PSA and the calculation of % f-PSA greatly increases the possibilities of early diagnosis and treatment of prostate carcinoma.