For men over age 50, the best means to screen for prostate cancer require a combination of tests: the prostate specific antigen blood test and a digital rectum scan, according to Dr. Michael Cookson of the Department of Urological Surgery at Vanderbilt University Medical School. The numbers from a PSA test can return a false positive for cancer, which often occurs in men with benignly enlarged prostate glands, notes the University of Pennsylvania's Abramson Cancer Center. Relating the results of a PSA test to the size of a man's prostate can help doctors to determine whether the higher level of PSA results from a benign enlargement or from cancer. To find this number, an accurate volume of the prostate gland must be made using a transrectal ultrasound, which can make finding the PSA density expensive, according to John's Hopkins. Dr. Michael Cookson notes that the results of a PSA density might show a false positive or negative for cancer, as the PSA test results vary daily and the accuracy of TRUS for finding prostate volume does not produce accurate results.

Look at the PSA test results to determine whether the patient needs a transrectal ultrasound to find the size used in calculating PSA density. Opt for a TRUS if the PSA test returns results of 2.5 ng/ml or higher. The University of Pennsylvania notes that some researchers recommend a biopsy or additional testing, while others continue tests at levels of 4.0 ng/ml to 10 ng/ml for the PSA test.

Conduct a transrectal ultrasound to find the prostate volume by measuring the length, width and height of the prostate. Multiply these numbers together and multiply the result by 0.52 to find the prostate volume.

Divide the PSA test number by the size of the prostate gland found during the transrectal ultrasound to find the PSA density. For instance, if a patient has a PSA test of 6.0 ng/ml and a prostate volume of 40 ml, divide 6.0 by 40.

6.0/40 = 0.15.

Evaluate the results of the PSA density for numbers of 0.15 or higher, lessening the likelihood that the elevated PSA test came from a benign growth, according to Cookson. Consider either a “watch and wait” approach or biopsy for further evaluation of the patient's health.