Background: Sputum cytology via clinical dithiothreitol processing is a well-accepted, reproducible method to assess airway eosinophilia (PMID:10673188). However, this multi-step protocol (~1 hour) requires trained personnel and dedicated space, limiting its routine use in clinical practice.

Aims: We evaluated the feasibility and validity of using formalin-fixed, paraffin-embedded (FFPE) sputum plugs as a simplified, efficient (~10 min.) method to assess airway eosinophilia.

Methods: After clinical processing, excess sputum plugs (~100mg) from 50 patients with airway disease were fixed in 10% formalin overnight, and paraffin embedded. FFPE blocks were sectioned, stained with hematoxylin and eosin, and counted by four observers including two sputum technologists. Mean eosinophil counts were expressed as a percentage of total cells counted and compared with clinical differentials.

Results: 48 out of 50 (96%) FFPE sputum plugs were adequate for cell differentials (>400 cells). Eosinophil counts between the two methods were similar (p=0.15, paired t-test, ns), significantly correlated (?=0.91, Spearman, P<0.0001) and in agreement (mean/SD on Bland Altman -1.10   7.31). The FFPE method detected airway eosinophilia (AUC ROC 0.95, p<0.001) with 91% sensitivity and 82% specificity at 2.3% cutoff. Inter-observer counts showed consistency and agreement (intraclass correlation, ICC, 0.98). All eosinophilic FFPE samples (stratified by >2.3%) showed positive anti-eosinophil peroxidase immunostaining.

Conclusion: FFPE sputum plugs as a ?biopsy tissue block? are feasible for detecting airway eosinophilia, and non-inferior to clinical protocol. Further validation in a healthy cohort is underway.