Real-World Diagnostic Yield and Longitudinal Dynamics of Fluorescence In Situ Hybridization in Hematologic Malignancies: A Five-Year Retrospective Analysis of 1357 Patient Samples

Background/Objectives: Fluorescence in situ hybridization (FISH) is an established component of the diagnostic and prognostic workup of hematologic malignancies. However, most evidence supporting FISH interpretation originates from treatment-naïve diagnostic cohorts, providing limited insight into how cytogenetic findings evolve during longitudinal disease monitoring and therapeutic exposure. We retrospectively analyzed 1357 consecutive FISH assays performed in 1092 patients with hematologic malignancies between 2019 and 2023. Methods: Multivariable logistic regression identified predictors of aberration detection, while longitudinal analyses explored temporal changes in cytogenetic profiles among serially monitored patients. Observed abnormality rates were compared with published diagnostic benchmarks. Results: The cohort had a median age of 68 years and was predominantly composed of patients with myelodysplastic/myeloproliferative neoplasms (27.2%), multiple myeloma (26.9%), and chronic lymphocytic leukemia (21.6%). Overall, 29.2% of informative assays revealed cytogenetic abnormalities. Clinical indication was the strongest predictor of aberration detection. Compared with diagnostic samples, follow-up specimens showed significantly lower odds of abnormal findings (adjusted OR 0.48, 95% CI 0.36–0.64; p < 0.001), whereas relapse samples demonstrated higher odds (adjusted OR 1.82, 95% CI 1.28–2.59; p = 0.001). Increasing age was independently associated with abnormal FISH results (adjusted OR per decade 1.24, 95% CI 1.12–1.38; p < 0.001). Several canonical abnormalities occurred less frequently than expected from historical diagnostic series, including BCR::ABL1 within the chronic myeloid leukemia testing pathway and del(13q14) in both chronic lymphocytic leukemia and multiple myeloma. In contrast, del(17p), involving the TP53 locus, remained consistently represented across disease categories and was enriched in chronic lymphocytic leukemia relative to published diagnostic cohorts. Conclusions: Longitudinal assessment revealed increasing cytogenetic heterogeneity over time, with common lesions progressively declining and rare or complex abnormalities expanding from 16.3% to 36.7% of the observed cytogenetic landscape. The cytogenetic patterns detected by FISH in contemporary hematology practice are strongly associated with clinical context, disease stage, and treatment history; however, the cross-sectional and retrospective design precludes inference of causal influence. The divergence between real-world positivity rates and historical diagnostic benchmarks highlights the need for context-specific interpretation of cytogenetic findings. The persistence of TP53 loss across disease boundaries and the progressive diversification of aberrations over time underscore the dynamic nature of clonal evolution in hematologic malignancies.