Nasal cytology in the rhinology-allergy clinic: From rhinitis to chronic rhinosinusitis with nasal polyps (CRSwNP).

Nasal cytology (NC) has evolved into a standardized and clinically relevant diagnostic tool in rhinology, capable of characterizing the cellular patterns of allergic and nonallergic rhinitis, infectious rhinitis, and chronic rhinosinusitis with nasal polyps (CRSwNP). Through minimally invasive sampling and simple staining techniques, NC identifies distinct endotypes-such as nonallergic rhinitis with neutrophilic, nonallergic rhinitis with eosinophilis, nonallergic rhinitis with mast cell, and nonallergic rhinitis with eosinophils and mast cells-overcoming the limitations of conventional classifications and enabling targeted therapies. It also plays a decisive role in diagnosing overlapping rhinitis, a frequent cause of treatment failure. In CRSwNP, NC has contributed to defining the eosinophil-mast cell endotype and to developing the Clinical-Cytological Grading (CCG) and the Prognostic Index of Relapse (PIR), both correlating with disease severity and recurrence risk, while also aiding in the selection of candidates for biologic therapies. Its complementarity with histopathology has highlighted the importance of mast cells-often overlooked with standard hematoxylin-eosin staining-in the most severe disease phenotypes. Technological innovations, including artificial intelligence-assisted cell recognition, digital microscopy, phase-contrast analysis, and the nasal fern test, are enhancing diagnostic accuracy and reproducibility. Teaching remains essential, with hands-on training at the microscope under the guidance of expert tutors integrated into residency programs in otolaryngology, allergology, and pediatrics to ensure skill transfer and the incorporation of NC into clinical practice. Beyond morphology, NC bridges microscopic evidence with macroscopic clinical decisions, establishing itself as a cornerstone of precision medicine for upper airway diseases and as an indispensable tool in the rhinology and allergy clinic.