The Diagnostic Accuracy of the Paris System for Reporting Upper Urinary Tract Cytology: The Atypical Urothelial Cell Conundrum

Background: The diagnosis of upper urinary tract urothelial carcinoma (UTUC) in cytological specimens is challenging, particularly the designation of atypical urothelial cells (AUC). The application of the Paris System for Reporting Urinary Cytology (TPS) has improved the performance of lower tract urothelial carcinoma specimens but has shown variable results in upper tract specimens, which are frequently instrumented. Methods: This retrospective study analysed upper tract selective cytology samples from January to December 2023. Samples were classified under TPS 2.0 categories. Histological specimens were used where available as the gold standard to calculate statistical metrics including sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). Results: Out of 122 samples, 12.2% were considered non-diagnostic or insufficient, with 13.1% designated as Non-high-grade urothelial carcinoma (N-HGUC), 53.3% as atypical urothelial cells (AUC) and 21.3% as positive/suspicious for HGUC. Histopathological correlation was available for 48.7% of cases. The risk of malignancy was: NHGUC (0%), AUC (47%) and HGUC (77.7%). The highest PPV was for HGUC (78%), with a diagnostic accuracy of 81.3% and specificity of 88%. In contrast, AUC had a PPV of 47% in instrumented and non-instrumented samples, rising to 58% in combination with the HGUC category. Conclusion: TPS 2.0 is an effective tool with excellent diagnostic accuracy for HGUC and in excluding malignancies in the N-HGUC category, but in our hands, the high rates of the AUC category, together with the low PPV, remain a major challenge and an obstacle to the correct stratification of patients with UTUC.