Evaluating the Performance of the Paris System in Neobladder Cytology: The Role of Cellularity and Volume Adequacy Criteria

Background Urinary diversion (UD) including neobladder (NB) samples play a crucial role in diagnosing recurrent urothelial carcinoma (UC) following cystectomy. However, urine cytology (UCy) interpretation in this setting is challenging due to degenerative changes and a necroinflammatory background. The diagnostic performance of UCy using The Paris System (TPS) has not been well established for NB specimens. Moreover, given the distinct characteristics of NB samples compared to other urine specimens, the application of specific adequacy criteria may be necessary. Methods This retrospective study analyzed 442 NB urine specimens from 243 patients who underwent cystectomy with UD. One‐year surgical pathology or cytology follow up was collected. All specimens were prepared using the cytospin technique, stained with Papanicolaou stain, and assessed using TPS criteria. The presence of preserved benign urothelial cells and urine volume were analyzed to determine specimen adequacy. Youden's index and likelihood ratio testing were applied to identify an optimal volume cutoff. Results By applying TPS, positive cytologic diagnoses (atypical or higher) were identified in 3.39% of NB specimens. Among 139 cases with histological or cytological follow‐up, UC was confirmed in 10.8%, with 86% being high‐grade UC (HGUC). The sensitivity and specificity of TPS for detecting HGUC were 61.5% and 96.8%, respectively. Notably, 60.5% of NB specimens in this cohort lacked preserved urothelial cells. Redefining adequacy criteria to require the presence of at least one preserved urothelial cell markedly increased sensitivity (100%) with only modest changes in specificity and overall diagnostic accuracy. A specimen volume ≥ 45 mL emerged as the optimal cutoff for identifying atypical urothelial cells or higher TPS categories, although statistical significance remained limited ( p = 0.12). Applying this threshold led to a modest improvement in the sensitivity of UD cytology (66.67%). Conclusions The diagnostic performance of TPS for NB samples was comparable to the performance of pre‐TPS cytology in the UD setting, as reported by other studies. Incorporating adequacy criteria based on the presence of at least one preserved benign urothelial cell substantially improves the sensitivity of UCy in NB samples. Additionally, a minimum specimen volume of approximately 45 mL may serve as an adequacy indicator for NB cytology, although its impact on diagnostic performance is limited.