Risk assessment and evidence-based medicine

The article by Roychowdhury & Adshead starts to place violence risk assessment in the context of medical care. ¹ Although this is welcome, their partial defence of risk assessment in general, and of structured professional judgement in particular, is based on some significant distortions.

The first distortion is the gross overestimation of the power of risk assessment to discriminate between low-risk and high-risk people. The authors present a contingency table that they imagine shows the ‘potential’ outcomes of a violence risk assessment (Table 2). Using their tabulated data, a diagnostic odds ratio for risk assessment can be calculated to be 81, indicating that the risk of violence in the high-risk group (50%) is hugely higher than in the low-risk group (1.2%). These figures are totally unrealistic. In fact, the diagnostic odds ratio of violence risk assessment in replication studies was recently estimated by meta-analysis ² to be 3. Roychowdhury & Adshead overestimate the discriminating power of risk assessment by 27 times. Moreover, even an unrealistically powerful risk assessment with diagnostic odds of 16 is of little or no value because of failure to detect potential violence in the low-risk group and the large proportion of false positives in the high-risk group. ³

The second distortion relates to the underestimation of the precision of medical tests. In fact, the authors seem to have had difficulty finding any medical test with diagnostic odds that they could compare to a violence risk assessment. Instead they chose to compare two medical treatments. They argue that the high number-needed-to-treat as a result of a violence risk assessment is acceptable in psychiatry because in cardiology the number of bypass grafts needed to prevent one fatal outcome has been calculated to be 53. ³ However, the meta-analysis they derived this figure from compared coronary bypass surgery to angioplasty - both of which are highly efficacious treatments for angina. ³ In reality, medical tests that are used to diagnose conditions with serious implications for the patient are very accurate - biopsy is an excellent indicator of cancer and an angiogram a good indicator of coronary heart disease.

Despite these limitations, I support the authors' general idea of viewing risk assessment as a medical procedure. I would go further: surely violence risk assessment should be judged by the standards of evidence-based medicine. The real questions then become: (1) are there any rational interventions that can be justified in terms of cost and benefit that might reduce violence among high-risk patients (many of whom will not be violent) and yet should not be offered to low-risk patients (who commit as many or even the majority of acts of violence); and (2) is there evidence that shifting treatment resources from low-risk to high-risk people can, in any way, reduce overall levels of harm?

The answer to both these questions is no. ^4,5 There is no doubt that medical diagnostic tests serve as a good basis for medical treatment and that medical and surgical treatment can save lives. It is simply disingenuous to suggest that the same can be said of violence risk assessment.