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Comment & Response |

A Closer Inspection of the Number Needed to Biopsy

Michael A. Marchetti, MD1; Stephen W. Dusza, DrPH1; Allan C. Halpern, MD1
[+] Author Affiliations
1Dermatology Service, Department of Medicine, Memorial Sloan Kettering Cancer Center, New York, New York
JAMA Dermatol. 2016;152(8):952-953. doi:10.1001/jamadermatol.2016.0936.
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To the Editor To reduce the costs of treating skin cancer, Shahwan and Kimball1 suggest restricting the performance of biopsies for skin cancer to clinicians with favorable number-needed-to-biopsy (NNB) ratios. However, when viewed in isolation, the NNB is a limited surrogate for diagnostic accuracy and a problematic metric for assessing cost-effectiveness.

The NNB is essentially the inverse of positive predictive value, which reflects a combination of diagnostic accuracy, the applied threshold for diagnostic sensitivity, and disease prevalence. The diagnostic accuracy of a test (or clinician), is defined by sensitivity and specificity. The inherent trade-off between sensitivity and specificity dictates that 2 clinicians with identical diagnostic accuracy but with different thresholds of sensitivity for their examinations will have discordant NNBs, as graphically illustrated by receiver operating characteristic curves. Similarly, clinicians with identical diagnostic accuracy and sensitivity thresholds examining patient populations that differ in disease prevalence will have discordant NNBs, as increased disease prevalence leads to higher positive predictive value. Hence, considering NNB without attention to the sensitivity applied to the detection of skin cancer and the prevalence of skin cancer could prove misleading.


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August 1, 2016
Kathryn T. Shahwan, MD; Alexa B. Kimball, MD, MPH
1Clinical Unit for Research Trials and Outcomes in Skin (CURTIS), Department of Dermatology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
JAMA Dermatol. 2016;152(8):953. doi:10.1001/jamadermatol.2016.0937.
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