Note: reagent field test reactions can vary on the basis of temperature, minor contaminants in a sample, the age of the testing reagent, lighting, and a number of other factors. Field reagent tests should be considered rule in / rule out tests rather than a positive identification method. If a particular drug is known to change color in the presence of a field reagent and no reaction occurs when testing a sample, that's evidence that the drug is not present in the sample. However if the sample does cause a color change consistent with what is expected for a specific drug, it is not proof that the target drug is present or pure. See the Testing Kit FAQ for more information.