<s>[INST] context: PSA velocity Several refinements have been proposed to increase the sensitivity of the serum PSA for younger men more likely to die of prostate-cancer, while reducing the frequency of diagnosing cancers of low biologic potential in elderly men more likely to die of other causes. These modifications include age-specific reference ranges, using a lower "upper" limit of normal for younger males and higher "upper" limit for older individuals. Prostate specific antigen density (PSAD) is calculated by dividing the serum PSA level by the estimated prostate weight calculated from transrectal ultrasonography (TRUS). It was proposed to correct for the influence of benign prostatic hyperplasia (BPH) on the measured level of PSA. Values <0.10 are consistent with BPH, while values >0.15 suggest the presence of cancer. PSAD levels also increase with age. PSA velocity is derived from calculations of the rate of change in PSA before the diagnosis of cancer was established. Increases of>0.75 ng/mL per year are suggestive of cancer. For a 50-year-old male, an increase from 2.5 to 3.9 ng/mL in a I-year period would warrant fuher testing, even though the level is still in the "normal" range. Free and complexed PSA measurements are used to determine which men require a biopsy when the PSA level is in the range of 4 to 10 ng/mL. In cancer, the level of free PSA is lower. Using a 25% threshold of free PSA for patients with levels in the range of 4 to 10 ng/mL, specificity was improved by 20% while maintaining a sensitivity of95%. Fuher refinements to increase the specificity of distinguishing benign and malignant conditions involve the determination of the ratios of free to total, complexed to total, and free to complexed PSA. Using normal ranges for free/total PSA of>O .15, for complexed total PSA of <0.70, and for free/complexed PSA of>0.25 improved specificity in one study by 20%. These modifications are designed to reduce the frequency of biopsies in men without cancer. [/INST] question: Which of the following refinements of serum PSA is used to determine the need for TRUS guided biopsy of prostate, even though the level of serum PSA is still in normal range? | options: A: PSAD (prostate-specific antigen density) B: PSA velocity C: Free/Total PSA D: Complexed/Total PSA | answer: B </s>