I had some nitrite/nitrate test strips that were WAY off-base. I checked my big tank with one of the test strips and they showed the nitrites WAY into the dangerous range, over 5ppm. I didn't think it could spike that badly that quick, so I pulled out the regular dropper kit and tested it.... barely registered... 0.25. It seems to measure nitrates correctly, but it's horribly alarmingly wrong on nitrites.
For my pool though, I use 5-in-1 test strips that look almost identical... quite possibly manufactured by the same company in all actuality. They're wonderful. So easy, so convenient, and if there's a tiny bit of inconsistency with results, it's not likely to kill anything.