Why do some specimens screen positive but not confirm?
The preliminary screening, whether POC or in the laboratory, is accomplished by using antibodies to the drugs. While the manufacturers take great care to make the antibodies as selective as possible, sometimes other compounds in the specimen will cross-react and cause a false-positive screen. Most of the time the preliminary screening is quite accurate. But high concentrations of other prescribed medications, or other non-prescription agents may cause a positive screen reaction. This is why confirmatory analysis is essential. In that process, specific chemical compounds are uniquely identified and quantitated. The preliminary screen is just an initial snapshot to provide guidance in confirmatory analysis.
