Abstract:Objective To explore the clinical and cytopathological features of BK polyomavirus (BKV) infection in urine cytological samples and the significance of immunocytopathological analysis. Methods Thirty-four urine samples of 3 235 cases diagnosed as BKV infection by cytology from October 1, 2020 to August 31, 2024 in the Pathology Department of Xi’an Daxing Hospital were collected as study samples. The pathological specimens were re-read, the morphological characteristics of the cells were summarized, and the pathological analysis of immune cells was performed (through the counting of CD4⁺ , CD8⁺ and CD3⁺ T cells, and the calculation of the CD4⁺ /CD8⁺ ratio, the immune function was judged to be balanced, reduced, low or severely low), and the disease changes of the patients were followed. Results Among the 34 cases, 29 cases were males and 5 cases were females, with median age of 65 years. The clinical symptoms were non-specific, and all of them were mainly primary symptoms; 3 cases were kidney transplantation, 22 cases were malignant tumors, and 9 cases had no history of malignant tumors or transplantation. The common feature of BKV infection cytology was the high nucleus-to-plasma ratio, and the nuclei stained by immunocytochemistry (ICC) was uniform and dark brown, which could be easily confused with high-grade urothelial carcinoma (HGUC) cells. Twenty-seven patients underwent pathological analysis of immune cells, and the results showed that 96.3% were immunocompromised and 3.7% were immune balanced. There were 32 cases of HGUC - negative and BKV infection, and 2 cases of HGUC combined with BKV infection. Among them, 3 cases were diagnosed with BKV infection after this re-reading the films, and the initial missed diagnosis rate was 8.8%. The 3 cases were diagnosed as BKV infection only after two times of film reading and supplementary ICC staining. After 3-72months of follow-up, 10 cases were repeatedly infected, and one patient died of multiple organ system failure after infection. Conclusion Urine cytology BKV infection is easily confused with the diagnosis of HGUC, and most patients are immunocompromised and have a poor prognosis. Understanding the cytological and immune cell pathological features of BKV infection can help pathologists make more Objective and accurate interpretations.