PSA testing and inverse bell-shaped curves

Human behavior fascinates me. And physician human behavior fascinates me even more. What motivates us to treat specific diagnoses in specific patients the way we do? How do we decide what medicines to prescribe? What kinds of tests do we order? One of my former mentors used to tell me that physician performance, actions, decisions, outcomes, etc., all fall on a bell-shaped curve. No matter what one is measuring, there are a small number of low performers, a growing bulge of middle performers, and a number of high performers which gradually tapers down. Here is an image of a typical bell-shaped curve:


                                        Bell-shaped curve

But I recently read a fascinating study in JAMA which showed an interesting phenomenon – physician practice can demonstrate an inverse bell-shaped curve. The study looked at PSA ordering. While there is some controversy over PSA testing for younger men, no national professional society recommends PSA screening for men aged 75 or older. There is no evidence of benefit, and significant evidence of possible harm. As with most realms of human behavior or performance, one would logically predict that physician ordering of PSAs in older men would fall on a bell-shaped curve. But what the researchers found, in looking at 1,963 primary care physicians in Texas for the year 2010, is that physician ordering of PSAs fell on an inverse bell-shaped curve. What does an inverse bell-shaped curve look like? The following image is an example:


                                 Inverse bell-shaped curve

Overall, 29% of men aged 75 or older in Texas received screening PSAs in 2010. While this mean average figure seems fairly high, what was most striking about the ordering pattern is that 24% of all PCPs were ordering PSAs far more frequently than the average, and 16% were ordering far less. Ordering practices fell on an inverse bell-shaped curve.

What does this tell us? I am sure there are some snarky readers of this blog who will surmise it has something to do with Texas. Maybe medical practice in Texas is warped because of the weather there, or it is somehow connected to what happened at the Alamo in 1836. But I suspect we may find a similar pattern in other states as well.

It is hard to know what this data means, but one possible factor is whether it reflects how physicians as a group, a very large group, come to adopt best practices – gradually. Perhaps there is a significant group of doctors who have not yet appreciated the data on PSA screening and have not yet incorporated it into their practice. Perhaps another large group are “early adopters” who have already implemented PSA screening recommendations. And perhaps there is a middle group somewhere in between.

Who knows, maybe the inverse bell-shaped curve phenomenon applies to other new practices and knowledge. They may just take time to diffuse throughout the medical community.

Richard Fleming, MD

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