Objective: To investigate the diagnostic efficiency of systemic immune response (SII) in prostate cancer (PCa) in patients with PSA < 10 ng/mL undergoing fusion prostate biopsy., Methods: The prospective study included patients who were planned for fusion prostate biopsy and had PSA < 10 ng/mL and a PI-RADS ≥ 3. All the patients underwent 12-core standard transrectal prostate biopsy followed targeted biopsy (combined biopsy). Based on preoperative complete blood count parameters, SII was calculated using the following formula: SII = platelet × neutrophil-to-lymphocyte ratio. Correlations between PI-RADS score, platelet, neutrophil-to-lymphocyte ratio, PSA, PSA density, SII and PCa were determined using ROC curve analysis. Optimal cut-off values were determined using the maximum Youden Index (defined as: sensitivity + specificity - 1)., Results: The study included 508 patients with a mean age of 62.49 ± 6.86 years and a median PSA level of 7.28 (5.69-8.70) ng/mL. The overall clinically significant PCa rate was 39.4%. Although SII had no significant diagnostic value in PCa patients with low ISUP grades (grade 1 and 2) (AUC = 0.487, P = 0.622), it was revealed as a significant marker in PCa patients with an ISUP grade ≥ 3 (AUC = 0.811, P < 0.001). The cut-off value of SII was 533.0. While the combination of SII with PI-RADS score is the most effective marker, neutrophil-to-lymphocyte ratio and platelet were also revealed as effective markers in predicting ISUP grade 3-5 PCa, though not as effective as SII., Conclusion: SII and SII combination with PI-RADS score appear to be a significant diagnostic marker in patients with high-grade PCa (ISUP grade 3-5). These values were found to be higher compared to those of patients with a benign pathology and patients with lower ISUP scores., (Copyright © 2021 AEU. Published by Elsevier España, S.L.U. All rights reserved.)