Detection of FGFR Alterations in Urothelial Carcinoma: Current Diagnostic Methodologies, Technical Considerations and Future Perspectives.

Fibroblast growth factor receptor (FGFR) alterations have emerged as clinically actionable biomarkers in urothelial carcinoma (UC), particularly following the introduction of FGFR-targeted therapies for patients with advanced disease. Activating mutations, gene fusions, copy number alterations, and expression changes involving FGFR2 and, especially, FGFR3 contribute to urothelial tumorigenesis through persistent activation of oncogenic signaling pathways and are enriched in specific molecular subtypes. Because these alterations occur at different molecular levels, their accurate detection requires complementary diagnostic methodologies rather than a single analytical platform. This review summarizes the current landscape of FGFR alteration testing in UC, with a particular focus on the analytical principles, strengths, limitations, and clinical applications of tissue- and liquid biopsy-based approaches. Conventional techniques, including DNA- and RNA-based next-generation sequencing, PCR-based companion diagnostic assays, fluorescence in situ hybridization, and immunohistochemistry, are critically discussed together with emerging liquid biopsy strategies for longitudinal disease monitoring and resistance assessment. We also review the current clinical indications for FGFR testing, its role in patient selection for targeted therapies such as erdafitinib, and the molecular mechanisms underlying acquired resistance. Finally, future perspectives involving artificial intelligence, single-cell and spatial transcriptomics, multi-omics integration, and novel biomarker discovery are examined as potential strategies to further refine precision oncology in UC. Collectively, accurate and standardized detection of FGFR alterations represents a fundamental component of contemporary molecular pathology and is expected to play an increasingly important role in guiding personalized therapeutic strategies for patients with UC.