For instance, if a test kit measurement is at the 0.25 mark on its color scale, the true value will be somewhere between 0.2375 and 0.2625. An individual kit's reliability is dependent upon several factors. Some are controlled by the manufacturer and some by the user. The manufacturer controls the quality and quantity of the reagents. Too little or too much reagent can often cause significant differences in readings.
Improper packaging can also cause differences in readings.
Liquid reagents are subject to increased reagent concentration and degradation because of evaporation and absorption of air into the solution. Poor color comparison schemes are also significant contributors to inaccurate and non-reproducible readings. The kits for nitrite require a minimum reaction time for complete color development. Exposure to sunlight will cause nitrite color samples to change enough that a meaningful comparison is impossible. Each kit should specify the light conditions under which comparisons are to be made.