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Urine Cytology and The Paris System
Overview
The Paris System for Reporting Urinary Cytology (TPS) provides a standardized reporting framework that refocuses urine cytology on its greatest strength: the detection of high-grade urothelial carcinoma (HGUC). The system explicitly acknowledges the inherent limitations of urine cytology for detecting low-grade urothelial neoplasms and establishes clear morphologic criteria with an implied risk of malignancy for each diagnostic category.
The Paris System: Diagnostic Categories
| Category | Diagnosis | Risk of Malignancy | Key Criteria |
|---|---|---|---|
| 1 | Nondiagnostic / Unsatisfactory | — | Insufficient cellularity or obscuring elements |
| 2 | NHGUC | <5–10% | Normal urothelial cells, reactive changes |
| 3 | AUC | ~20–30% | Equivocal atypia, insufficient for SHGUC |
| 4 | SHGUC | 50–70% | Features of HGUC but <5–10 cells |
| 5 | HGUC | >90% | N:C >0.7, severe hyperchromasia, angular membranes |
| 6 | LGUN | 60–70% | Papillary clusters, mild atypia, fine chromatin |
| 7 | Other malignancies | Variable | SCC, adenocarcinoma, small cell carcinoma |
1. Nondiagnostic/Unsatisfactory
A specimen is classified as nondiagnostic when there is insufficient cellularity, excessive degeneration, or obscuring elements that preclude evaluation. For voided urine, a minimum cellularity threshold is not strictly defined but the specimen must contain sufficient cells for meaningful interpretation. Instrumented specimens (catheterized, washing, or brushing) are generally more cellular and better preserved. Specimens that are acellular or contain only squamous cells are nondiagnostic.
2. Negative for High-Grade Urothelial Carcinoma (NHGUC)
This category includes normal urothelial cells at all maturation levels: superficial umbrella cells with abundant cytoplasm, intermediate cells, and basal cells (which are smaller with higher nuclear-to-cytoplasmic ratios). Benign changes encompassed under NHGUC include reactive urothelial cells showing nuclear enlargement with smooth membranes and fine chromatin often in clusters, polyomavirus (BK virus) cytopathic effect with enlarged "decoy cells" containing ground-glass intranuclear inclusions that can mimic HGUC, lithiasis-related changes with clusters of mildly atypical reactive cells, and instrumentation artifact showing larger tissue fragments with preserved architecture. The risk of malignancy for NHGUC is less than 5-10%. Critically, NHGUC does not exclude low-grade urothelial neoplasm, as these tumors are poorly detected by cytology.
3. Atypical Urothelial Cells (AUC)
This category applies when cells show features exceeding reactive changes but insufficient for a suspicious or definitive malignant diagnosis. Criteria include nuclear enlargement, hyperchromasia, or irregular nuclear membranes that are equivocal in nature. As a quality metric, this category should represent less than 10% of all cases. The risk of malignancy is approximately 20-30% for HGUC, and management includes cystoscopy follow-up with consideration of ancillary testing.
4. Suspicious for High-Grade Urothelial Carcinoma (SHGUC)
This category is used when cells have features of HGUC but are present in insufficient number (fewer than 5-10 cells) or show suboptimal preservation. The atypical cells demonstrate markedly increased nuclear-to-cytoplasmic ratios, hyperchromatic and irregular nuclei, and coarse chromatin, but the quantity is insufficient for a definitive HGUC diagnosis. The risk of malignancy is 50-70%, and management is cystoscopy with biopsy.
5. High-Grade Urothelial Carcinoma (HGUC)
The cytologic criteria for HGUC include markedly increased nuclear-to-cytoplasmic ratios (greater than 0.7), severe nuclear hyperchromasia, irregular (angulated) nuclear membranes, and coarse clumped chromatin. Cells may appear singly, in clusters, or in papillary tissue fragments. Background necrosis and tumor diathesis are common in invasive tumors. Eccentric nuclei and cytoplasmic homogeneity produce characteristic cercariform cells. A minimum of 5-10 definitively malignant cells is recommended for a confident diagnosis. The risk of malignancy exceeds 90%, and management includes cystoscopy with biopsy and staging workup.
6. Low-Grade Urothelial Neoplasm (LGUN)
Low-grade urothelial neoplasm presents as papillary clusters of urothelial cells with mild nuclear enlargement, fine chromatin, and smooth nuclear membranes. Fibrovascular cores may be visible within papillary fragments. The Paris System acknowledges that cytology has low sensitivity for LGUN (approximately 15-30%), as cystoscopy is the primary detection method for these lesions. The risk of malignancy (mostly low-grade tumors) is approximately 60-70%. This category is separated from HGUC because clinical management differs significantly.
7. Other Malignancies
Non-urothelial malignancies such as squamous cell carcinoma, adenocarcinoma, and small cell carcinoma are reported with specific classification.
Specimen Types and Their Characteristics
Voided Urine
Voided urine is the least invasive specimen type but has the lowest sensitivity, especially for low-grade lesions. Cells are more degenerated, making morphologic assessment more challenging. This specimen type is best suited for monitoring known HGUC. Early morning void is often recommended but paradoxically produces more degenerated cells; some practitioners prefer second-void specimens for better preservation.
Catheterized/Instrumented Specimens
These specimens provide better cellularity and preservation than voided urine. However, instrumentation artifact can produce larger tissue fragments with preserved polarity that may mimic neoplasia. The specimen type should always be noted in the report as it affects interpretation thresholds.
Bladder Washing
Obtained during cystoscopy, bladder washings provide the best cellularity and preservation of all urine specimen types. They offer the highest sensitivity for both high-grade and low-grade lesions, and the AUC rate may be lower because of superior sample quality.
Upper Tract Specimens
Upper tract specimens are obtained during ureteroscopy and serve as lateralizing specimens for upper tract urothelial carcinoma. They are typically small volume with variable cellularity, and interpretation follows similar principles to bladder specimens with the clinical context directed toward the upper urinary tract.
Ancillary Testing
UroVysion FISH
UroVysion is a multiprobe FISH assay detecting gains of chromosomes 3, 7, and 17 along with loss of 9p21 (CDKN2A locus). It is positive in HGUC with approximately 70-80% sensitivity and can detect malignancy in cytologically negative specimens, termed an "anticipatory positive" result. It is most useful in equivocal cases (AUC category) and for surveillance after BCG therapy. A limitation is false positivity with polyomavirus infection and reactive urothelium.
Immunocytochemistry
CK20 shows aberrant strong, diffuse expression in urothelial carcinoma (normally expressed only in umbrella cells). p53 overexpression in a mutant pattern supports HGUC. CD44 expression is lost in HGUC but retained in reactive and benign urothelium. GATA3 serves as a urothelial lineage marker, positive in urothelial carcinoma.
Other Molecular Tests
Additional urine-based tests include Cxbladder (a gene expression assay), BTA (bladder tumor antigen immunoassay), and NMP22 (a nuclear matrix protein assay that is FDA-approved for monitoring). All have variable sensitivity and specificity, and none currently replace cystoscopy as the gold standard for bladder cancer detection and surveillance.
Challenging Scenarios and Pitfalls
Polyomavirus (BK Virus) Infection
BK virus cytopathic effect is the most important mimic of HGUC. Infected cells show ground-glass nuclear inclusions, nuclear enlargement, and hyperchromasia. They are distinguished from HGUC by their smooth nuclear membranes, homogeneous chromatin pattern, and the characteristic intranuclear inclusions. SV40 immunohistochemistry confirms polyomavirus and is negative in HGUC. This infection is particularly common in immunosuppressed patients, especially renal transplant recipients.
Post-BCG/Immunotherapy Changes
BCG treatment for bladder cancer causes granulomatous inflammation and reactive urothelial atypia, including multinucleated giant cells and epithelioid histiocytes. Distinguishing post-treatment reactive changes from recurrent HGUC is challenging, and UroVysion FISH may be helpful in this clinical setting.
Lithiasis-Related Atypia
Ureteral and renal calculi cause reactive changes that can mimic malignancy, presenting as cohesive clusters of urothelial cells with mild nuclear enlargement. Clinical history of stone disease is essential to avoid false-positive diagnoses.
Ileal Conduit Specimens
In patients with ileal conduits, intestinal epithelial cells are the predominant population. Degenerated intestinal cells can mimic malignancy, creating a diagnostic pitfall. The clinical context is monitoring for urothelial carcinoma recurrence in the remnant upper tract.
<image>A medical illustration showing the cytomorphologic criteria for the major diagnostic categories of The Paris System for Reporting Urinary Cytology. Panel A (NHGUC): Normal superficial umbrella cells with abundant cytoplasm and small bland nuclei alongside intermediate urothelial cells. Panel B (AUC): Urothelial cells with nuclear enlargement and mild hyperchromasia but smooth nuclear membranes and equivocal features. Panel C (SHGUC): A small number of cells (fewer than 5) with markedly increased N:C ratio, hyperchromatic irregular nuclei, and coarse chromatin. Panel D (HGUC): Numerous malignant cells with severely increased N:C ratio exceeding 0.7, angular nuclear membranes, coarse clumped chromatin, and eccentric nuclei (cercariform cells), with background necrosis. Panel E (LGUN): A papillary cluster of mildly atypical urothelial cells with fibrovascular core, mild nuclear enlargement but fine chromatin and smooth membranes.</image>
<image>A medical illustration comparing polyomavirus (BK virus) cytopathic effect with high-grade urothelial carcinoma in urine cytology. Left panel (BK virus): Enlarged decoy cells with smooth round nuclear membranes, ground-glass homogeneous intranuclear inclusions, and an inset showing positive SV40 immunostain with strong nuclear brown staining. Right panel (HGUC): Malignant cells with irregular angular nuclear membranes, coarse clumped chromatin, markedly increased N:C ratio, and an inset showing negative SV40 immunostain. A comparison table below highlights the key distinguishing features including nuclear membrane contour, chromatin pattern, N:C ratio, and SV40 IHC result.</image>
Clinical Pearls
The Paris System is deliberately focused on HGUC detection because this is where urine cytology has the highest diagnostic impact; cytology is an inadequate screening tool for low-grade papillary neoplasms, which are detected by cystoscopy. A NHGUC result does not exclude bladder cancer, as low-grade tumors are frequently cytologically bland. BK virus decoy cells are the most common mimic of HGUC, and SV40 immunohistochemistry should always be considered in immunosuppressed patients, especially renal transplant recipients. The AUC category should not exceed 10% of cases; excessive use dilutes its clinical utility and defeats the purpose of standardized reporting. Instrumented specimens have significantly higher cellularity and sensitivity than voided urine, and the specimen type should always be noted in the report as it affects interpretation. UroVysion FISH is most useful in the AUC/SHGUC gray zone and for post-BCG surveillance but should not be used as a standalone screening test. Nuclear membrane irregularity (angulation) is the single most important criterion distinguishing HGUC from reactive atypia and BK virus changes.
References
- Rosenthal DL, Wojcik EM, Kurtycz DFI, eds. The Paris System for Reporting Urinary Cytology. 2nd ed. Springer; 2022.
- VandenBussche CJ, et al. Improving the reproducibility and performance of urine cytology with The Paris System for Reporting Urinary Cytology. Am J Clin Pathol. 2017;148(3):230-237.
- Barkan GA, et al. The Paris System for Reporting Urinary Cytology: The quest to develop a standardized terminology. Acta Cytol. 2016;60(3):185-197.
- Meilleroux J, et al. The Paris System for Reporting Urinary Cytology: A meta-analysis. Cytopathology. 2020;31(3):184-192.
- Huyett P, et al. Fluorescence in situ hybridization (FISH) in urine cytology. Cancer Cytopathol. 2019;127(12):735-747.
