Doing some research into immunoassay tests as FBR suggested would probably be the best course of action to find out for sure. I haven't had enough time to find reliable sources on that yet, but I'm sure once you understand the complexities of how the test actually detects the metabolites it would help explain whether or not the reaction could result in differing faintness of lines or if that is affected by a variable of its own. If I ever find anything Ill keep you posted.