Drug abuse screening with immunoassays: unexpected cross-reactivities and other pitfalls.

Immunoassays are the only practical means of coping with the increasing demand for drug abuse screening because of their simplicity and ease of automation. However, it is essential that the analyst understands the strengths and weaknesses of such assays to enable correct interpretation of results. This is especially important with the increasing popularity of near-patient testing where assays are liable to be carried out by inexperienced and untrained staff. False-positive and false-negative results, unexpected cross-reactivities and other pitfalls are discussed, together with an attempt to explain the reasons for these problems.