Introduction: Urine diagnosis by light microscopy is considerably more difficult when specimens are analyzed after a certain period of time. Objectives: (1) To investigate whether this change effectively exists at a significant level of 12 h. (2) To apply measures, once the above has been done, allowing for the analysis of samples beyond 12 h in similar conditions. Materials and Methods: Both freshly produced urine and pathological samples were used under certain experimental conditions: initial, 12 h, 12 h + fridge at 4°C (F), 12 h + chemical preservation (S) and 12 h + SF. The chemical preservative was prepared at a 50/50 ratio with 3% formaldehyde and 2.5% glutaraldehyde by the addition of a buffer of pH 7.2–7.4, resulting in a solution of pH 7.35 at 25°C room temperature. Urinalysis was carried out on all samples: glucose (enzymatic method of hexokinase) and total protein in liquid (red pyrogallol method). Centrifugation was followed by sediment analysis with light microscopy. Statistical analysis was done with the Kolmogorov-Smirnov normality test, Friedman nonparametric test and multiple comparisons. Results and Conclusion: Urine samples tested 12 h after having been produced changed significantly (p < 0.0001), making it necessary to adopt certain measures to maintain their initial conditions. In our case, after the addition of the chemical preservative, samples did not present any changes (p > 0.10) in relation to the initial conditions and were seen to be reliable, therefore indicating the suitability and effectiveness of the analytical conditions (urinalysis in particular, sediment analysis).