A Better PSA Calculation

By: Dan Sperling, MD

By now, any discussion of the PSA (prostate specific antigen) blood test is practically old hat. The adage that “tests beget tests” is particularly true for PSA, because it is not specific for cancer. Too many other conditions—even a simple act like riding a bicycle—can bump up the amount of this antigen in the bloodstream. Many doctors still consider an elevated or rising PSA as a red flag, and send patients for a biopsy. And then we get into the problem of biopsies…but don’t get me started!

PSA alone is far from perfect, but it’s not completely hopeless as a communicator. If it begins to rise, and is followed periodically over a year, the velocity (speed) with which it is rising can be a true warning sign.

Authors Stacy Loeb, E. Jeffrey Metter et al published a way to assess cancer risk by assigning “risk values” to PSA progression.[i] They developed a simple formula for calculating prostate cancer risk based on PSA velocity using a threshold of <0.4 ng/mL rise per year as being zero risk. (This threshold was based on previous studies done by themselves and others, correlated with pathology evidence from prostatectomy specimens.) They defined two points, PSAV1 (the first PSA to “trigger” a tracking period) and PSAV2, the final PSA at diagnosis, with no blood draws closer than 6 months apart. Their population sample size is impressive: 18,214 men from a screening study.  Their work is summarized in this table:

PSA Velocity “Risk Count”

Risk Count

Definition

0

Both PSAV1 and PSAV2 increase less than 0.4 ng/mL per year

1

Either PSAV1 or PSAV2 increase more than 0.4 ng/mL per year

2

Both PSAV1 and PSAV2 increase more than 0.4 ng/mL per year

 

Statistically, they demonstrated that a risk count of 2 indicated 8 times the risk of prostate cancer, with 5.4 greater risk that the patient was harboring Gleason 8-10 cancer.

My feeling is that we’re rapidly approaching more widely used tests for molecular markers that will make PSA testing obsolete. However, with increasing economic pressures to keep medical costs down, in many demographic locations where low income predominates and community physicians don’t have access to more sophisticated technologies, PSA will continue to be of use. Until everyone catches up with the coming tests—and they are affordable—perhaps the scoring system of Loeb et al can play a meaningful part in “red-flagging” PCa before the risk count reaches 2.



[i] Loeb S, Metter EJ, Kan D, Roehl K, Catalona W. Prostate specific antigen velocity risk count improves the specificity of screening for clinically significant prostate cancer. BJUI. 2012 Feb;109(4):508-514.

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