A variation in test results is almost always attributable to sample contamination or testing technique. The tests seem to work far beyond when you might expect. the ammonia test is theoretically only good for 3 years but even my 6 year old bottle gives me a good reading. It is not an organic that will decay over time so as long as you keep things clean, expect your test chemicals to work almost forever. Even if the chemicals were stale, the results would be consistent if your technique and samples were consistent. Caution,v my own, and I assume other people's, water contains ammonia as a part of the chloramine used by my water supplier. That means that ammonia results can be skewed by contamination of a sample by my tap water. What I do is rinse out a test tube thoroughly in my sink, then I rinse the tube with tank water before filling it. By using tank water as the final rinse agent, I eliminate the tap water as a source of sample contamination. Since the test tube has already been rinsed well in the sink, I do not contaminate my tank water with reagents by using it to rinse out my test tube.
I'm sure not rinsing the vials out can cause a little issues, though. kinda common sense
