In conclusion, our study suggests that PSA density (PSAD) has better diagnostic performance than tPSA or f/t PSA for detecting prostate cancer, especially in men with PSA levels in the 4–10 ng/mL “gray zone.” Moreover, combining PSAD with MLR improves the diagnostic accuracy and may serve as a useful, low-cost adjunct to identify patients at a higher risk of PCa. The gene discussed is KLK3; the disease is prostate cancer.