Almost a footnote in their original study, however, was the performance of underlying clinical judgment.⁶ In these modern practice settings, the rate of CT was far from the feared 30 to 50 percent witnessed in the derivation studies. Clinicians obtained advanced imaging a mere 8 percent of the time, and in spite of this low rate, their performance was not inferior to the most sensitive decision instrument. In these 20,000 children, there were 160 clinically important brain injuries, and clinicians missed only two of them. Each injury missed by clinicians was diagnosed several days later due to persistent symptoms, and each patient recovered without intervention. In contrast, PECARN TBI missed one injury, and both CHALICE and CATCH each missed at least 10.

So while PECARN TBI has the best sensitivity of the decision instruments, we ought to ask ourselves, why bother utilizing one at all? Clinical judgment in these settings performed statistically identically and at a fraction of the imaging rate. These studies offer some value because the risk factors identified here are almost certainly being incorporated into the judgment of clinicians involved. However, rather than fully relinquishing diagnostic decisions to these instruments, we probably best serve our patients by retaining our independence.

The harm from overreliance on these decision instruments is not just from potential excess imaging but from pervasive misapplication. For example, the Canadian Head CT Rule is intended for alert patients with loss of consciousness or disorientation following trauma. These patients have a non-zero, but also not high, incidence of serious intracranial injury. Bizarrely, a recent serious academic exercise evaluated its use in those with minimal head injury, reflecting indication creep beyond its intended use.⁷

These decision instruments already deliver poor positive predictive values in low-risk populations, and although they certainly can be applied in very-low-risk populations, doing so will increase the frequency of false positives. This pitfall especially afflicts PECARN TBI, which includes a whole host of trivial head injuries in its exclusion criteria. This instrument is almost routinely, and inappropriately, applied to the exceedingly low-risk population, a rabbit hole leading inevitably to unnecessary imaging.

Rather than using these instruments as a crutch, think of them as a guide. In each decision instrument, those elements associated with high risk for intracranial trauma are reasonable indications for advanced imaging. Those children who are obviously low risk on clinical evaluation will clearly meet the PECARN TBI very-low-risk criteria as well, and it is reasonable to document these criteria as justification for added certainty.