Preservation of urine for flow cytometric and visual microscopic testing

Preservatives that could prevent destruction of cells, casts, and bacteria in urine are of great practical importance because they allow centralization and improvement of accuracy of urine particle counting. We compared two in-house mixtures and one commercial solution, as well as refrigeration, for their ability to preserve urine for both automated analysis (flow cytometry) and visual microscopy.

Urine specimens were preserved by refrigeration at 4 degrees C without preservatives (procedure 1); in a lyophilized solution intended to preserve specimens for bacterial culture (Urine C&S tubes; BD Preanalytical Solutions; procedure 2); in 10 mL/L formalin-0.15 mol/L NaCl (procedure 3); in 80 mL/L ethanol-20 g/L polyethylene glycol (procedure 4); and by storage at 20 degrees C without preservatives (procedure 5). Test strip measurements were used to select specimens positive for leukocyte esterase, hemoglobin, albumin, or nitrite. For 106 consecutive strip-positive specimens, urinalysis was performed by UF-100 (Sysmex) and by phase-contrast microscopy after Sternheimer supravital staining. Automated analysis was performed at arrival in the morning, on the same day in the afternoon, and after 1 and 3 days. Visual microscopy was performed at arrival and 3 days later.

Urine bacterial counts were well preserved with procedures 1-3, with a false-positive rate of 0.0-3.4% at day 3 vs 28% without preservation (procedure 5). Erythrocytes were poorly preserved for 3 days (kappa coefficients, 0.24-0.61); after 1 day, fair preservation was seen with procedure 2 (kappa = 0.78), compared with less favorable preservation with procedure 1 (kappa = 0.61) or procedure 5 (kappa = 0.66). Leukocytes were well preserved by all five procedures in the acidic adult urines investigated. Counts of casts and large epithelial cells were artifactually increased by procedure 3. Procedure 2 performed at least as well as refrigeration for specimens analyzed with visual microscopy.

Urine specimens from adults can be stabilized at room temperature for both automated particle analysis and visual microscopy.