I was asked a question the other day that I was completely unprepared to answer, and to the customer’s dismay they were left with an unanswered question. Their question was this:
“On the day of the test I was so afraid of dilution that I did not drink anything, but I realized that might mean I was not going to be able to urinate for the sample, so I grabbed some old urine I had stored from a previous test a few weeks before and added that to my sample. I know the old sample is clean as well as my current sample, but will the old sample go bad and make the test come up positive for some reason?”

After doing a bit of research I am finally able to answer this question.

While the old human urine sample should have no effect on the current sample, human urine does technically ‘go bad’. To remain useful for a testing facility to be able to perform any tests on the sample the human urine must be properly stored to remain ‘good’. Proper storage includes a vacuum sealed container and a freezer. However, not all of us have a vacuum sealed container. In that case it is recommended that you use a clean and dry container (preferably one that has not been used before) to store the human urine.

Storage of human urine specimens should not exceed 2 hours at room temperature or 4 hours refrigerated (2-8°C). It is possible to refrigerate human urine for up to two weeks, if you have properly prepared the human urine for storage with a vacuum sealed container. However, any longer than that (or if you will be using a regular container) and you should put it into the freezer in an airtight container. In the freezer, the human urine will keep for about a year – but it should be noted that any period of time in a freezer and the human urine sample can slowly begin to deteriorate in color and creatine levels – so it is best to use the sample as soon as you can – human urine freezes at 32°F, just like water. It’s best not to transfer the human urine from container to container, especially if you’re not using brand new containers, to prevent any contamination of the human urine that might trigger a false positive or invalid test result.

And no, it is not that hard to unfreeze human urine on short notice. Take the human urine out of the freezer and when you are ready to use it, place the airtight/watertight container inside a larger glass, and fill that glass with very hot tap water. Even if the human urine is frozen solid, within 5 minutes you have human urine that is thawed and warmed to at or above body temperature.

Microwaving human samples has little or no effect on the constituents being tested. The reason why microwaving human samples is not recommended is because human urine is bio-hazardous, and should be kept away from areas where food and utensils get involved.

A variety of urine preservatives (tartaric and boric acids being the most common) are available that allow human urine to be kept at room temperature while still providing results comparable to those of refrigerated urine. Generally, the length of preservation capacity ranges from 24 to 72 hours.

The Mayo Clinic prepared a chart on additives that can be used to preserve urine for extended periods of time. Here is a link to that chart:

http://webserver01.bjc.org/labtestguide/special%20instructions%5CUrinePreservatives.htm