It is broadly recognised that the PSA test is not an ideal screening test for prostate cancer.  Some new research enables us to be more specific about exactly how useful it is, or isn't.

A team of Swedish researchers studied 1,500 men who had been given a PSA screening test prior to developing any prostate cancer symptoms. They then looked at the proportion of men developing prostate cancer afterwards. 

They found that the PSA performed poorly when used as a screening instrument to predict subsequent prostate disease.  This was the case for PSA "cut-off" (threshold) values of 3, 4 or 5 ng/ml.

Making sense of this paper depends on understanding a bit about how diagnostic tests are evaluated.

The first thing to realise is that few tests are 100% accurate all the time.  PSA is one such test.  You may have a raised PSA and not have prostate cancer;  conversely, you may have a low PSA yet still go on to develop prostate cancer. 

The key question is "what is the likelihood that a man with a given PSA test result will go on to develop prostate cancer"?

Getting to grips with the numbers

The study presented its results as "likelihood ratios".  A likelihood ratio tells you how much more likely the disease is given a particular test result. For example, if a PSA test result of 4ng/ml or higher had a likelihood ratio of 2 it would mean that men with this PSA score would be twice as likely to develop prostate cancer as men with a lower score.

To put things in perspective, a test is usually considered to "rule out" a disease if a negative result has a likelihood ratio of 0.1 or less. It is considered to be good at ruling in the disease if its likelihood ratio is 10 or more. 

In this study, the authors found that the likelihood ratios for ruling in or ruling out prostate cancer lay in between this range, meaning that the PSA test on its own is not definitive enough to act as a screening instrument.

• Read more about likelihood ratios and diagnostic tests on the Agency for Health Care Research and Quality (AHRQ) website.

Links

• British Medical Journal: Holstrom et al, Prostate specific antigen for early detection of prostate cancer: longitudinal study BMJ 2009, 339:b3537. [Full text]
• Comment from The Prostate Cancer Charity
• More about prostate cancer screening on UK Prostate Link
• More about the PSA test on UK Prostate Link