Why should glucose test be performed fresh urine specimen?

For some tests, the values obtained from old urine may not be accurate: some of the chemicals in urine are volatile, and will evaporate (as will the water unless it is in a tightly sealed container). Also, some chemical components begin to oxidize as soon as they are exposed to the air, making their measurement increasingly inaccurate over time. At the same time, bacteria may begin to break down the urine.