Direct-in-Tube Rapid Papanicolaou Staining (PAPR) for Serous Effusions: Cytomorphological and Diagnostic Performance Evaluation

Background. Serous effusion cytology is essential for evaluating inflammatory and neoplastic processes. Conventional Papanicolaou staining, although considered the reference standard in cytopathology, requires multiple processing steps and substantial reagent consumption.

Objective. To validate a rapid modified Papanicolaou staining method applied directly in tube (PapR) for serous effusions and compare its diagnostic and operational performance with the conventional technique.

Materials and Methods. Eighty serous effusion samples (28 pleural, 26 peritoneal, 26 pericardial) were processed in parallel using conventional Papanicolaou staining (PAP) and PapR. Cytological interpretation followed TIS 2020 guidelines. Concordance analysis, Cohen’s Kappa coefficient, and diagnostic performance parameters were calculated. Slide adequacy was assessed using a structured Degree of Suitability for Interpretation (DSI) score.

Results. Global diagnostic concordance was 97.5% (κ = 0.95, p < 0.001) for the five-category TIS classification. When AUS and SFM categories were grouped as cytologically positive for atypia, PapR demonstrated 100% sensitivity and 98.1% specificity, with a false-positive rate of 1.25% and no false-negative cases identified. Two cases were discordant between methods: one case initially classified as NFM on conventional PAP was reclassified as SFM on PapR (single false-positive result), while the second discordance represented a minor categorical variation that did not modify the positive/negative diagnostic grouping. Cytomorphological features were preserved, and approximately 71% of cellular sediment remained available after staining. When analyzed using binary grouping (negative vs positive for atypia), concordance increased to 98.75% (79/80 cases).

Conclusion. PapR represents a rapid, reliable, and resource-efficient alternative to conventional Papanicolaou staining for serous effusions, demonstrating excellent diagnostic performance while optimizing laboratory workflow and preserving biological material.