WHO Reporting System for Lung Cytopathology: Insights Into the Insufficient/Inadequate/Non-Diagnostic, Atypical and Suspicious for Malignancy Categories and How to Use Them

The World Health Organization Reporting System for Lung Cytopathology (WHO System) is an international effort aiming to serve patients worldwide in all medical resource settings and improve patient care globally. It is an evidence-based standardised reporting system applicable to all respiratory cytopathology specimens. The WHO System consists of five diagnostic categories including Insufficient/Inadequate/Non-diagnostic, Benign, Atypical, Suspicious for malignancy and Malignant. Each category has an associated risk of malignancy established from the current literature and recommendations for further management to establish as precise a diagnosis as possible. The key diagnostic cytopathological criteria for each entity are established, a differential diagnosis based on cytopathological features that is globally applicable is discussed, and best practices in appropriate ancillary studies are presented. The Insufficient/Inadequate/Non-diagnostic, Atypical and Suspicious for malignancy are diagnostic categories that are particularly challenging for cytopathologists and clinicians and patients due to the organ's diversity in sampling methods, cell types and the wide variety of neoplasms. Herein, we have reviewed the role of these three categories, discussed their definitions and cytopathological criteria, appropriate applications, risk of malignancy, clinical recommendations for patient's further diagnostic workup and application of ancillary studies. The aim was to increase cytopathologists and clinicians understanding of the three categories and provide a framework for the essential discussions that should follow.