Background: Urinalysis is a fundamental diagnostic tool, yet its reliability is heavily dependent on the pre-analytical phase. International guidelines recommend processing within two hours. This study evaluates the stability of urinary sediment elements over 72 h using BD Vacutainer® UAP preservative tubes. Methods: A total of 150 urinary specimens were collected into BD Vacutainer® UAP tubes (containing chlorhexidine, ethyl paraben, and sodium propionate). Samples were analyzed at 0, 24, 48, and 72 h using the Sysmex UF-5000 fluorescence flow cytometer at room temperature. Stability was assessed using Cohen’s Kappa (κ) statistics, with κ > 0.80 defined as the threshold for good clinical agreement. Results: Cellular elements showed stability up to 24 h, but significant time-dependent degradation after 24 h. White Blood Cells were the most labile, showing a 36.7% loss of detectability at 48 h (κ = 0.62). Red Blood Cells (RBCs) and transitional cells exhibited a critical inflection point at 48 h with a 22.7% loss (κ = 0.77). In contrast, squamous epithelial cells and casts (cellular and non-cellular) remained highly stable up to 72 h (κ ≥ 0.97). Non-cellular analytes, including bacteria, crystals, yeasts, and spermatozoa, showed absolute stability (0% loss, κ = 1.00) throughout the entire 72-hour period. Conclusions: The use of BD Vacutainer® UAP tubes successfully mitigates osmotic lysis and alkaline degradation, extending the reliable analytical window up to 24 h.
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