how long does it take for urine to sit without refrigeration until it cannot be used in a drug test?

i will be bringing in a sample to a center and will be traveling for a few hours with the unrefrigerated sample. really need to know how long it takes until it becomes unusable. the urine test will be done by a pregnancy-type test and not in a lab.

what can i do to improve chances of the sample surviving a couple of hours until it needs to be tested?

thanks in advance