Why was it useless? Do you have it unrefrigerated longer than 8 hours? We say it starts to go bad after about 8 hours, not that it is bad at 8 hours. It can take a little standing around at room temperature you just want to use that sample usually that day. The collectors don't always stick it in the frig, they sometimes keep it sitting in a pile with other samples until they get sent out, which is usually at the end of the day. You want all this time that you have it out and they have it out to be under a few days like 5 days. Like a worst case scenario would be giving a sample late on a Friday and it not going out till the next week and there was a holiday on Monday and you had it out all day Thursday. Other than something like this (it would probably still be ok), one day on the sample should be fine.

DOT uses 50 ng/ml on immunoassay and instant tests usually are 50 ng/ml and use immunoassay technology. If you pass using the first void of the day, you are fine like 99% of the time.