PSA Cutpoints

psa1

PSA Cutpoints

Vickers and Lilja suggest that cutpoints in general are not a good idea and use PSA as an example (1). To summarize their point of view,

·         The rationale for cutpoints is often unknown (or irrational as with the use of normal ranges)

·         Cutpoints are invariant to patient preferences 

·         Risk prediction should be used in place of cutpoints

·         Patient preferences could be taken into account with risk prediction (but not with cutpoints)

My response to these views is as follows:

·         The value of cutpoints depends on the properties of the marker. PSA is not a very good marker. Men with a negative DRE who are biopsied when their PSA is between 4 µg/L and 10 µg/L do not have prostate cancer (according to the biopsy) 75% of the time whereas, men with a negative DRE and PSA results between 2 µg/L and 4 µg/L have prostate cancer 15% of the time.  So even though there may be no rationale for a PSA cutoff, its properties are known.

·         Whereas lab tests play an increasing role in medical care, they are not the only source of data. Primary care physicians assess history (example: 1st degree relatives with prostate cancer), physical exam (example: DRE) and lab tests (example: PSA) to arrive at an overall assessment of the need to refer the patient to a urologist. The assessment is a risk prediction but is based on more than just the PSA test. Of course, there are other sources of data beyond the ones mentioned as examples.

I don’t quite understand what the authors’ mean when they say “cutpoints cannot include multiple pieces of information.” OK, a 4 µg/L may be a cutoff for PSA regardless of whatever other information is available, but that does not mean a physician would not use the other information. For example, if a man had a history of prostate cancer in his family, a suspicious DRE, and a PSA of 3.5, this would prompt many physicians to recommend a referral to a urologist in spite of the fact that the PSA value is below 4.

·         For patient preferences to be taken into account and to be meaningful, the patient has to be informed (to have a preference). It is not clear how this would occur. If a patient had a PSA value of 1.5 µg/L, a negative DRE, no symptoms, and no 1st degree relatives with prostate cancer, it is hard to imagine a primary care physician starting a process to inform the patient about the probabilities of prostate cancer (to account for a certain class of patients). One must realize that this type of process of informing the patient would apply to every medical encounter and is hardly commensurate with the limited time that a patient spends seeing a primary care physician. Thus, the point at which the physician initiates the process of informing the patient may already be at a higher risk than the patient prefers.

·         Ideally, the patient should ask for a written report of every primary care physician visit, including the results of the physical exam and history, all lab tests, and all other procedures such as an EKG. The patient would then have to acquire sufficient knowledge to understand the implications of this information. This would be a monumental task although it could be eased by hiring other medical professionals although this would be costly.

·         The biases of the primary care physician play a role in informing the patient. The practice of primary care physicians is affected by insurers. There may be an incentive to minimize the referral of patients to a urologist or to refer patients to a urologist who biopsy frequency is lower than his peers (and less costly)(2). The biases of a urologist, to recommend those procedures performed by that urologist and to discount others, must also be considered.

References

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