Urine specimens retained for urine testing may be left for a maximum of four hours in the winter months when temperatures are cooler, and discarded after four hours. In the summer months, it should not be left for more than two hours, as too long a period of time may lead to bacterial growth or exogenous contamination, which may lead to inaccurate results. For 24-hour urine protein measurement or for patients who are keeping a 24-hour urine specimen volume, a small amount of preservative is added every 2-4 hours. Usually, the preservative consists of dilute sulfuric acid, which can be picked up normally in the hospital laboratory. The preservative can be left in the urine specimen for 24 hours or more, depending on the time of day and the requirements of the test. If the preservative is not added to the urine, more than 2-4 hours need to be discarded and retained, so as not to cause false-positive results or inaccurate results and mislead the clinical judgment and treatment.