Monday, 26 June 2017

I do therefore I am or the bias of intervention in medicine

Cristina Roure

This week, thanks to a post of Sergio Minué in the blog "El Gerente de Mediado", I discovered the recent publication of "Ending Medical Reversal" The book refers to situations in which new studies more robust than the pre-existing ones contradict the standards of commonly accepted practices, which have now proved ineffective or even harmful.

Some will recall such vivid examples as the use of protein C activated in sepsis, high dose chemotherapy combined with autotransplant of stem cell in metastatic breast cancer, the aprotinin in cardiac surgery, or hormone replacement therapy in postmenopausal women. All of them were used for years until proven harmful.

I have not been able to read the book yet, but I read a report by the same author; Dr. Vinay Prasad in Mayo Clinical Proceedings exposes his vast research. Over ten years, 363 studies evaluating established practices were published, out of which 146 (40%), between 12 and 19 per year, were revoked (1). The author explains in the following video that such revocations usually occur after the precipitous adoption of new therapies based on incomplete or inadequate studies.

The consequences of aversion to revocation are evident:

a) Patients’ exposure to adverse effects in exchange for a zero benefit
b) Waste of health resources and threat to the system’s sustainability 
c) Patients’ distrust in the system and in the professionals
d) Contamination of the shared clinical decision process.

Therefore, it would be logical to correct quickly and abandon the revoked practices as soon as possible, but this isn’t usually the case, and on average the practice persists for ten years before being abandoned.

Cognitive bias is the tendency to maintain a partial perspective that distorts the perception of what is obvious is called cognitive bias and appears to be prevalent in medical practice.

Intervention Bias  is the unconscious tendency of professionals or the medical community to intervene, either with drugs, diagnostic, tests or with procedures, when non-intervention would be a reasonable alternative or even better. The existence of this intervention bias has been demonstrated in several surveys carried out with professionals who, before two equivalent options, they usually choose the interventionist versus conservative option (2).

The causes that explain these biases are diverse. We are often seduced by elegant and sophisticated rationally impeccable and basic science arguments, with the illusion that they can predict what will happen in practice, and we adhere to it without waiting for the necessary evidence regarding its benefits and the real risks to be generated. Here we can add the confirmation bias, or the unconscious tendency to favour the information that confirms our hypothesis (3). Conflicts of interest, not necessarily economic, don’t help either; these sometimes, intentionally, conceal or delay the publication of the complete results of studies when they are negative (publication bias). Defensive medicine that abuses diagnostic tests while being aware of their futility doesn’t help and neither does the relative incompetence of the medical community for the critical evaluation of evidence or the understanding of risk that explains the tendency of professionals to overestimate the benefits of treatments against risks.

But if faced with the new options we can frequently observe an intervention bias, in contrast, when faced with the revocation of established practices, other biases such as the status quo bias, the anchoring or aversion to loss biases predominate. One example is the persistence of screening programs for some cancers despite accumulated evidence of their ineffectiveness in reducing mortality.

The status quo bias occurs when the individual gives more value to the losses associated with the decision than to the potential gains. I recommend you read Jordi Varela’s post illustrating the bias of the status quo with some examples where, despite having been revoked, the often preventive clinical practice is maintained. This type of bias is reinforced by other biases such as the aversion to loss or anchoring bias or the omission that occurs when faced with two reasonable alternatives; the fear of negative consequences of action is greater than that fear derived from the eventual consequences of the inaction and, therefore, the maintenance of the status quo is strengthened.

We can see that the medical community is enthusiastic about new practices and tends to adopt them quickly, even when they are based on studies with clear limitations, but the same medical community is much more reluctant to abandon them when they are ineffective or harmful, even when faced with clear evidence. Despite the famous “primum non nocere”, if the question is to do or not to do, it’s clear that in medicine, the tendency is toward the first option. 


  1. Prasad V et al. A Decade of Reversal: An Analysis of 146 Contradicted Medical Practices. Mayo Clinical Proceedings 2013;88(8):790-798.
  2. Foy A, Filippone E. The Case for Intervention Bias in the Practice of Medicine.Yale Journal of Biology and Medicine 2013; 86: 271-280.
  3. Schiff G et al. Principles of conservative prescribing. Arch Intern Med 2011; 171(16):1433-1440.

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