Prostate-specific antigen (PSA) is a serine protease of the kallikrein family. In the serum, PSA exists in two forms: free PSA and PSA complexed with alpha-1-antichymotrypsin (ACT). In benign prostatic hyperplasia (BPH), the ratio of free:total PSA is:

• A Lower (<10%) compared to prostate cancer
• B Identical to prostate cancer, making the ratio clinically useless
• C Entirely free PSA with no complexed form
• D Higher (>25%) compared to prostate cancer ✓

Explanation

In BPH, prostate tissue produces relatively more free PSA. In prostate cancer, malignant cells produce more PSA that complexes with alpha-1-antichymotrypsin (ACT) and alpha-2-macroglobulin, reducing the proportion of free PSA. A free:total PSA ratio >25% favours BPH; <10% suggests a higher probability of malignancy. This ratio is clinically used in the 'grey zone' of total PSA (4-10 ng/mL) to decide on biopsy, improving specificity while maintaining sensitivity.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.