Rapid Validation of Whole-Slide Imaging for Primary Histopathology Diagnosis

A Roadmap for the SARS-CoV-2 Pandemic Era

Megan I. Samuelson, MD; Stephanie J. Chen, MD; Sarag A. Boukhar, MBChB; Eric M. Schnieders; Mackenzie L. Walhof; Andrew M. Bellizzi, MD; Robert A. Robinson, MD, PhD; Anand Rajan KD, MBBS


Am J Clin Pathol. 2021;155(5):638-648. 

In This Article

Abstract and Introduction


Objectives: The ongoing global severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic necessitates adaptations in the practice of surgical pathology at scale. Primary diagnosis by whole-slide imaging (WSI) is a key component that would aid departments in providing uninterrupted histopathology diagnosis and maintaining revenue streams from disruption. We sought to perform rapid validation of the use of WSI in primary diagnosis meeting recommendations of the College of American Pathologists guidelines.

Methods: Glass slides from clinically reported cases from 5 participating pathologists with a preset washout period were digitally scanned and reviewed in settings identical to typical reporting. Cases were classified as concordant or with minor or major disagreement with the original diagnosis. Randomized subsampling was performed, and mean concordance rates were calculated.

Results: In total, 171 cases were included and distributed equally among participants. For the group as a whole, the mean concordance rate in sampled cases (n = 90) was 83.6% counting all discrepancies and 94.6% counting only major disagreements. The mean pathologist concordance rate in sampled cases (n = 18) ranged from 90.49% to 97%.

Conclusions: We describe a novel double-blinded method for rapid validation of WSI for primary diagnosis. Our findings highlight the occurrence of a range of diagnostic reproducibility when deploying digital methods.


Digital pathology and whole-slide imaging (WSI) are versatile tools that fulfill many roles in pathology teaching, clinical conferencing, slide archival, and research. Limited but focused routine implementations in telepathology and frozen-section interpretation have been carried at many institutions in the United States. Unlike radiology, which has incorporated digital imaging into routine practice for decades, pathology has moved much more slowly toward digitization. In 2017, the US Food and Drug Administration (FDA) approved the first WSI system for primary diagnosis in surgical pathology.[1] This occurred after a long process primarily because digital microscopy with WSI scanners was classified as a closed end-to-end class III device by the FDA and had to follow a premarket approval pathway requiring a clinical trial.[2] Subsequent devices, however, are considered class II, with a different FDA approval pathway.[3]

Until recently, primary digital diagnosis and consultation in histopathology remained predominantly a value-added service[4] rather than a new value proposition. Prior regulatory barriers—both perceived and real—have significantly and negatively affected the digital performance of clinical services by surgical pathology departments and practices, especially when carried out remotely.[5,6] The FDA-approved Phillips and Leica implementations offered standardized pathology revenue-generation workflows incorporating digital methods but in the form of end-to-end closed systems. This in turn affects acquisition and deployment of the components of a digital-focused infrastructure, which carries significant up-front monetary and personnel costs. Digital pathology has been mired in the resulting cul-de-sac for close to a decade[6,7] in the United States.

The global severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic[8] has adversely affected functioning of pathology departments in various ways.[9] It has become apparent that hospital and health systems' services are better provided by remaining open and functional, to the maximum extent possible, even under pandemic conditions.[10] The Clinical Laboratories Improvement Amendments (CLIA) regulations require cytology and histopathology to be practiced in laboratory locations licensed for the purpose. This necessitates exposure of essential components of the clinical workforce—laboratory staff engaged in routine histopathology work, consulting and attending anatomic pathologists, residents, and fellows—to increased risk on a daily basis for an as-yet unforeseen length of time. Recognizing the potential for digital pathology to mitigate this risk, on March 26, 2020, the US government announced "enforcement discretion" of the CLIA regulations, amounting to a temporary relaxation of the rules prohibiting digital remote sign-out of cases.[11] On April 24, 2020, the FDA joined by declaring that it "does not intend to object to" to previous FDA-cleared restrictions on digital pathology devices and their marketing for remote use.[12] With large-scale noninferiority trials showing clear equivalency in diagnostic performance between glass and digital methods,[2,13] the College of American Pathologists (CAP) (March 26, 2020) issued a "COVID-19 Remote Sign-Out Guidance" that stated pathologists "may use a non-FDA approved system as long as it has been properly validated" for primary diagnosis. These brisk changes have greatly spurred interest in deployment of digital pathology for consultation and primary diagnosis as a medium- and long-term pandemic mitigative measure, even if not for the benefits that digital pathology brings Table 1.[4] As part of digital mitigative measures to be put into place, validation of the use of WSI for primary and consultative diagnosis would need to be completed by laboratories.

Validation of WSI for primary diagnosis ensures clinical precision and accuracy in unique, individual settings and provides a way for laboratories to demonstrate equivalence (or noninferiority) with the use of glass slides in diagnosis. In 2013, after extensive data review, validation guidelines applicable to WSI systems for diagnostic purposes were published by CAP.[15] The CAP guidelines offer 12 recommendations gathered from the review of 23 published studies. The recommendations chart a pathway for laboratories to perform a comprehensive end-to-end assessment of diagnosis using digital means while providing flexibility in the specific mechanisms of implementation.

Implementation of digital pathology is a multistep and continuous process[16] involving histotechnologists, imaging technologists, and information technology expertise, of which a validation study would be only the first step. The Department of Pathology at the University of Iowa Hospitals and Clinics is a midsized academic anatomic pathology program (36 faculty, 20 residents, 7 fellows) that had previously committed to a process of stepwise integration of digital capabilities into quantitative image analysis, tumor boards and teaching conference presentations, social media dissemination, case archiving, outside slide retention, and allied applications. The department acquired a P1000 whole-slide scanner in mid-2018 that was in use for teaching and research. No case or image management software was provided with the instrument or in use. As part of this effort, pathologists acquired experience in the basic functions of the WSI slide viewer program. We sought to perform validation of digital pathology for primary diagnosis in this context. We had 2 goals: (1) to perform rapid, robust validation of the scanner and associated digital infrastructure (computer hardware, software, file handling protocols) and (2) to move through the development life cycle[17] of digital diagnosis so that recommendations, problems, and barriers could be identified and iteratively addressed. We evolved and implemented specific methods that sought to fulfill recommendations in the CAP guidelines in an evidence-based manner. We present data that show successful validation utilizing well over 150 cases and involving 5 pathologists.