Validation of Whole Slide Imaging for Primary Diagnosis in Surgical Pathology

Context.-High-resolution scanning technology provides an opportunity for pathologists to make diagnoses directly from whole slide images (WSIs), but few studies have attempted to validate the diagnoses so obtained. Objective.-To compare WSI versus microscope slide diagnoses of previously interpreted cases after a 1-year delayed re-review (wash-out'') period. Design.-An a priori power study estimated that 450 cases might be needed to demonstrate noninferiority, based on a null hypothesis: The true difference in major discrepancies between WSI and microscope slide review is greater than 4%.'' Slides of consecutive cases interpreted by 2 pathologists 1 year prior were retrieved from files, and alternate cases were scanned at original magnification of 320. Each pathologist reviewed his or her cases using either a microscope or imaging application. Independent pathologists identified and classified discrepancies; an independent statistician calculated major and minor discrepancy rates for both WSI and microscope slide review of the previously interpreted cases. Results.-The 607 cases reviewed reflected the subspecialty interests of the 2 pathologists. Study limitations include the lack of cytopathology, hematopathology, or lymphoid cases; the case mix was not enriched with difficult cases; and both pathologists had interpreted several hundred WSI cases before the study to minimize the learning curve. The major and minor discrepancy rates for WSI were 1.65% and 2.31%, whereas rates for microscope slide reviews were 0.99% and 4.93%. Conclusions.- Based on our assumptions and study design, diagnostic review by WSI was not inferior to microscope slide review (P<.001).