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  • Letter to the Editor
  • Open Access
  • External validation of a clinical decision rule: we need events in a population in order to rule them out!

    Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine201725:3

    • Received: 10 October 2016
    • Accepted: 18 December 2016
    • Published:


    We respond to the Lorton et al. article on external validation of the PECARN study. With an event rate of only 0.6%, we believe that their study failed to confirm the safety of this rule. Such a low number of events should call for caution when interpreting the results of diagnostic tests.


    • Public Health
    • Confidence Interval
    • Compute Tomography
    • Primary Endpoint
    • Diagnostic Test

    To the editor,

    Lorton et al. [1] recently published a prospective study that aimed to evaluate the diagnostic performances of the PECARN clinical decision rule (CDR). We congratulate the authors for performing an external validation of this rule, which aims to guide and reduce the number of computed tomography (CT) head scans among children with minor head trauma.

    In their study, the authors reported a sensitivity of 100% (95% Confidence Interval [CI] 66.4 to 100.0%), with a wide 95% CI that they acknowledged would limit their conclusions. However, when relying on a CDR to decide whether or not a child should undergo a CT, clinicians need to know the false negative rate they are dealing with. Therefore, the negative predictive value (NPV) is of utmost importance in such studies. Accordingly, Lorton et al suggest that this CDR is safe because its reported NPV is 100% [99–100%]. Indeed, an NPV with the lower bound of the 95%CI higher than 99% seems almost perfect, and could be used to validate the safety of this rule.

    However, we would like to highlight that a major limitation of these results is the very low rate of events (namely intracranial hemorrhage) in their study population. Only 9 children out of 1499 met the primary endpoint, a rate of 0.6%. Therefore, statistically, if the CDR used was a coin flip “heads or tails”, the NPV would still be 99.6% (95%CI of 99.0 to 99.9%).

    Such a low number of events should call for caution in the interpretation of the results of diagnostic tests.






    Availaibility of data and supporting materials

    Not applicable.

    Authors’ Contribution

    ALP and YF conceived this letter. YF drafted the study. ALP provided substantial revisions. Both authors read and approved the final manuscript.

    Competing interests

    The authors declare that they have no competing interests.

    Consent for publication

    Not applicable.

    Ethics approval and consent to participate

    Not applicable.

    Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( applies to the data made available in this article, unless otherwise stated.

    Authors’ Affiliations

    Emergency Department, hôpital Pitié-Salpêtrière, Paris, France
    Paris Sorbonne Université, Univ-Paris06, Paris, France


    1. Lorton F, Poullaouec C, Legallais E, et al. Validation of the PECARN clinical decision rule for children with minor head trauma: a French multicenter prospective study. Scand J Trauma Resusc Emerg Med. 2016;24:98. doi:10.1186/s13049-016-0287-3.View ArticlePubMedPubMed CentralGoogle Scholar


    © The Author(s). 2017