I would like to add that for the nitrate test, in common with all other liquid testers, the second bottle should be shaken well before using and the tube shaken well after the bottle #2 drops are added. The shaking times are given in the instructions. Failure to shake as described gives false results.
In addition, when the kit is new or if it hasn't been used for some time, tapping the bottle on the worktop as part of the pre-addition shaking helps to break up any lumps of sediment on the bottom of the bottle.