Whole Slide Images (WSIs), also known as digital or virtual slides, are digitized images generated from traditional glass slides using specialized hardware and software. WSIs have been widely used as an alternative to glass slides across various clinical diagnostic and research fields, but they raise questions about validation and consistency, in a field where accuracy matters above all else.
Some drug developers and sponsors have begun to explore the use of WSIs for primary diagnosis in toxicological pathology under Good Laboratory Practice (GLP) conditions, which makes effective validation even more crucial.
By following these tips, researchers can ensure compliance with regulatory expectations and, equally important, feel confident in WSIs’ interpretations.
Know the Correct Validation Guidance Documents
Validation documents are crucial to researchers and sponsors because they provide the framework required to prove a WSI system is fit for purpose. They define expectations, support regulatory confidence, help shape validation protocol, create consistency across studies and organizations, and protect scientific credibility.
For validation guidance, reference documents include “Validating Whole Slide Imaging Systems for Diagnostic Purposes in Pathology (College of American Pathologists (CAP)), Validation of Digital Pathology in the Regulated Nonclinical Environment (Digital Pathology Association (DPA)), and Best Practice Recommendations for Implementing Digital Pathology (Royal College of Pathologists).
Relevant regulatory frameworks should be considered, such as Title 21 Part 11 of the CFR (Electronic Records and Electronic Signatures), EU GMP Annex 11 (Computerized Systems), and Japan’s Pharmaceutical and Food Safety Bureau Notification 0401022. For nonclinical validation of WSIs, ICH guidance such as Q9 (Quality Risk Management) and Q10 (Pharmaceutical Quality System) can provide useful context. The Drug Information Association (DIA) provides guidance on validation workflows and phase definitions.
These documents form a comprehensive basis for developing risk-based, regulatory-compliant validation strategies for digital pathology systems.
Acknowledge the Purpose of Validation
Understanding the purpose of validation ensures it does not become a box-ticking exercise. The DPA identifies the core purposes of validation as follows:
- To ensure the system is designed, deployed, and tested for its intended use.
- To assure pathologists that the digital images faithfully represent the glass slides, including color rendition, magnification, resolution, etc.
- To ensure data integrity.
- To provide documented evidence for regulatory review, demonstrating that the system has been validated for its intended use.
Establish the Personnel Required for Validating WSIs
WSI validation sits between pathology, technology, quality systems, and regulatory compliance, so it’s important to know which personnel are required and where responsibility ultimately falls. Personnel involved in WSI validation typically include the system owner, pathologists (end users), IT staff, and quality assurance.
The system owner defines the intended use of the WSI system and coordinates responsibilities between the different groups. IT staff provides the technical foundation, including installation, configuration, network access, user permissions, cybersecurity, storage, backup, system performance, software updates, and integration with other systems. Quality assurance ensures that the validation process is properly documented and compliant with internal procedures, GLP expectations, and regulatory requirements.
The pathologist is the end user, and although IT provides the technical support and implementation expertise, they are ultimately responsible for confirming that the system meets clinical requirements and intended use.
Prioritize Effective Concordance Comparison
Concordance comparison—checking whether two methodologies produce the same results— is used to show that WSI can support pathological interpretation equivalent to traditional light microscopy, even though regulators do not consider scanned WSIs to be exact copies of glass slides. Regulators emphasize that scanned WSIs must preserve the relevant information from the original glass slides, including tissue structure, color, and any artifacts present.
Concordance assessment must confirm that tissues, organs, cellular elements, basic pathological processes, and relevant histopathological changes can be identified in digital images.
The best sample size for this assessment remains debated. Clinical guidance suggests anywhere from 20 to 60 cases, but toxicological pathology may require a tailored approach based on the study objectives, lesion types, and intended use. Comparison methods include having the same pathologist review cases by both light microscopy and WSI after a washout period, or having different pathologists review the same cases using different modalities.
Reported concordance rates between microscopy and WSI can vary widely, but the DPA considers 95% a feasible rate. Discordances are generally classified as minor or major. Minor discordances usually reflect subjective interpretation, terminology, grading, or training differences. Major discordances, such as a disagreement about whether a lesion is present or which organ is affected, can indicate limitations in the WSI system.
A Final Word
WSI offers many potential advantages. A digital slide can be viewed remotely, archived, and retrieved more easily. This allows for simpler collaboration between team members, including when personnel are located across the globe, and a more efficient organization. Digitization also enables the use of machine learning to analyze slides.
WSI is becoming increasingly important in toxicological pathology, especially as organizations explore its use under GLP conditions. But its adoption requires more than impeccable scanner performance or digital convenience. Because digital slides are not considered exact copies of their glass counterparts, validation is critical to preserving the information pathologists need to make reliable interpretations.
