Empirical evidence that disease prevalence may affect the performance of diagnostic tests with an implicit threshold: a cross-sectional study
- Correspondence to Dr Brian H Willis;
- Received 9 December 2011
- Accepted 20 December 2011
- Published 3 February 2012
Objective To investigate the effects that prevalence has on the diagnostic performance of junior doctors in interpreting x-rays.
Design Two-armed cross-sectional design using systematic sampling.
Setting Emergency department in the UK.
Participants From a sample of 2593 patients (1434 men and 1159 women) taken from an unselected attending cohort between January and April 2002, 967 x-rays were analysed. The sex distribution was 558 men and 409 women, and the mean age of those receiving an x-ray was 34.6.
Interventions The interpretation of x-rays by junior doctors after their triage into high- and low-prevalence populations by radiographers.
Main outcome measures Sensitivity, specificity, likelihood ratios, diagnostic odds ratios and receiver operator characteristic curve.
Results There were statistically significant differences in the performance characteristics of junior doctors when interpreting high-probability and low-probability x-rays. For the high- and low-probability populations, respectively, the sensitivities were 95.8% (95% CI 91.1% to 98.1%) and 78.3% (95% CI 65.7% to 87.2%) and the specificities were 56.0% (95% CI 41.9% to 69.2%) and 92.3% (95% CI 90.0% to 94.2%). Hierarchical logistic regression showed that the sensitivity did depend on the type of x-ray being interpreted but the diagnostic odds ratios did not vary significantly with prevalence, suggesting that doctors were changing their implicit threshold between the two populations along a common receiver operator characteristic curve.
Conclusions This study provides evidence on how the prevalence may affect the performance of diagnostic tests with an implicit threshold and potentially includes the clinical history and examination. This has implications both for clinicians applying research findings to their practice and the design of future studies.
To cite: Willis BH. Empirical evidence that disease prevalence may affect the performance of diagnostic tests with an implicit threshold: a cross-sectional study. BMJ Open 2012;2:e000746. doi:10.1136/bmjopen-2011-000746
Contributors BHW conceived the study, interpreted the data and wrote this manuscript.
Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors. The author was in receipt of a Medical Research Council fellowship during the conduct of this study.
Competing interests The author has completed the Unified Competing Interest form at http://www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declares that (1) BHW had support from a Medical Research Council fellowship during the conduct of this work, (2) has no relationship with any companies that might have an interest in the submitted work in the previous 3 years, (3) spouse, partner or children have no financial relationships that may be relevant to the submitted work and (4) have no non-financial interests that may be relevant to the submitted work.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement No additional data are available.
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