Why are screens sometimes positive, but confirmations negative?
Two possible reasons emerge for this. First, drugs may be metabolized to a number of different compounds which retain the ability to be recognized by the screening antibody, and screen positive, while the target drug or metabolite of confirmation is far below the cutoff concentration. Secondly, other substances or drugs may be cross-reactive to give a false positive screen, while the target drug itself is not present in the specimen.
