A 66-year-old man on finasteride 5 mg daily for benign prostatic hyperplasia has a screening PSA reported as 2.0 ng/mL. His baseline PSA before starting therapy was 3.8 ng/mL. The correct interpretation is:
- A PSA has fallen appropriately with treatment; no further action needed
- B Finasteride halves measured PSA, so the corrected value is approximately 4 ng/mL and further evaluation is warranted if clinically indicated ✓
- C Finasteride does not alter PSA, so the fall indicates laboratory error
- D Finasteride raises PSA, so this value confirms assay interference
Correct answer: B. Finasteride halves measured PSA, so the corrected value is approximately 4 ng/mL and further evaluation is warranted if clinically indicated
Explanation
Finasteride, a selective 5-alpha reductase type 2 inhibitor, lowers serum PSA by about 50 percent within months of starting therapy. Interpreting values without doubling them can mask prostate cancer, so the measured level must be multiplied by two for comparison against standard thresholds. A drop from 3.8 to 2.0 is exactly what the drug produces and carries no reassurance.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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Written and medically reviewed by the StethoPrep medical team.