I've seen API test claim 40ppm of NO3 when in fact, it was 0.0ppm.
I've never seen any deviations of note from Lamotte's.
Well, if you think so and are willing to accept it as good enough and not want accuracy, I guess, but then why bother testing at all if you are just ball park guessing in the first place?
I can do that with a simple water change and known weight of ferts.
If you test, do it right.
If not, do not bother. There's no 1/2 way here.
If you do not bother to make a reference, then do not bother to use them and do not assume that someone else's test kit is the same as yours.
Your method, the chemicals, the expiration dates etc all vary.
Do not rely on "faith" and "hope" they are accurate.
Make sure they are.
Most folks that do check, check the test kit once, maybe a few times over a short time frame...then they assume that from then on, forever......those test kits are good.
Most pH/KH/Gh test kits are fine in the cheapy ranges, but the Lamotte's are not that much more for those either(5-10$ at most).
I've tested friend's test kits against known standards, Seachem, API, Lamotte, Hach, Tetra, RedSea...........
The Hach and Lamotte where never off by more than a unit in their scales.
All the other cheapo brands have been way way off(not always, but far more frequently than I assume/or risk).
Do yourselves a favor, make some reference solutions.
Make a set of PO4(0.2ppm, 1ppm, 2ppm), NO3(2ppm, 5ppm, 15ppm), KH(17.86ppm, 71.44ppm), GH(17.86ppm, 71.44ppm) solutions.
Make 500mls of each and then get some good bottles and keep them around.
Check the test kits when you test your tank water about once every 3-6 months. Not hard, then you are lot more certain.
"Faith based testing" is poor method to test anything in this hobby.