Is it best to have a low number or a high.
The principle behind PSA density is that malignant cells usually put out more PSA than healthy cells, which is why a rising PSA is a warning indicator. However, healthy prostates vary quite a lot since the prostate grows some with age and even more so if there is any BPH.
Since a larger prostate will put out twice as much PSA than one half its volume, PSA density is used to to calculate the PSA per cc of prostate tissue. You divide the PSA value (in ng/mL) by the volume of the prostate gland in cc or mL. (Prostate volume is estimated by either an MRI or by ultrasound.)
So yes, lower is better.