Creed J, et al. - The clinical performance of a blood-based test was determined 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 quantified a previously described 3.4 kb mitochondrial DNA (mtDNA) deletion. For men in the PSA ‘grey zone,’ the 3.4 kb deletion in plasma appeared an accurate predictor of CS cancer. Used in advance of biopsy for improved patient selection, this deletion could decrease the number of biopsies required to diagnose CS prostate cancers.