Kidney diseases are a major health concern worldwide. Currently there is a large unmet need for novel biomarkers to non-invasively diagnose and monitor kidney diseases. Urinary cells are promising biomarkers and their analysis by flow cytometry has demonstrated its utility in diverse clinical settings. However, up to date this methodology depends on fresh samples, as cellular event counts and the signal-to-noise-ratio deter over time. Here we developed an easy-to-use two-step preservation method for conservation of urine samples for subsequent flow cytometry. The protocol utilizes a combination of the formaldehyde releasing agent imidazolidinyl urea (IU) and MOPS buffer, leading to gentle fixation of urinary cells. The preservation method increases acceptable storing time of urine samples from several hours to up to 6 days. Cellular event counts and staining properties of cells remain comparable to fresh untreated samples. The hereby presented preservation method facilitates future investigations on flow cytometry of urinary cells as potential biomarkers and may enable broad implementation in clinical practice.

I have read the journal's policy and the authors of this manuscript have the following competing interests: the preservation method of urinary cells was filed as a patent (US2022/0003748A1 and EP2019/086433). Apart from this, all authors assured they have nothing to disclose.

The author(s) received no specific funding for this work.

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This study was approved by the ethics board of Charité University Hospital, Berlin (approval number EA1/284/19). All patients and healthy donors gave their informed written consent for study participation and scientific publication of the results.

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