PURPOSE: To determine the clinical performance of a blood-based test for clinically significant (CS) prostate cancer (PCa) (grade group ≥ 2) intended for use in men with prostate serum antigen levels in the 'grey zone' (PSA < 10 ng/ml). The test quantifies a previously described 3.4 kb mitochondrial DNA (mtDNA) deletion. METHODS: In a first prospective study of an MRI-guided re-biopsy population (n = 126), the 3.4 kb deletion and 18S rRNA gene were amplified from plasma. A diagnostic threshold was selected from the coordinates of the receiver operating characteristic curve and tested in a second population of men who were (n = 92) biopsy naïve when the mtDNA deletion was assayed and for whom those diagnosed with cancer on initial biopsy were treated with radical prostatectomy. RESULTS: The 3.4 kb deletion was a good predictor of CS PCa in the image-guided re-biopsy population [AUC 0.84, (95% CI 0.73-0.95)] and the selected threshold corresponded to a sensitivity of 87% [95% CI, 70-96%], specificity of 68% [95% CI, 47-85%] and negative predictive value (NPV) of 97%. Applying this threshold to the second population showed this deletion to be a strong predictor of CS cancer [AUC 0.98, (95% CI 0.94-1.02)], independent of PSA or age [sensitivity 100% (95% CI, 93-100%), specificity 90% (95%CI 73-98%) and NPV 100%]. CONCLUSION: The 3.4 kb deletion in plasma is an accurate predictor of CS cancer for men in the PSA 'grey zone'. Used in advance of biopsy for improved patient selection, this deletion may reduce the number of biopsies needed to diagnose CS prostate cancers.