A screening test utilises established immunoassay technology to measure the amount of a particular drug or drug group in the sample. Antibodies bind to the drug by recognising a distinct three dimensional shape. In simplest terms this can be thought of as a key in a lock. The key (drug) has a distinct shape which will only fit into a suitably matched lock (antibody). Once the antibody is bound to the drug a colour change occurs, the greater the intensity of this colour change the larger the volume of the drug.
Some drug families share a common shape and so an antibody raised to recognise a drug within that group will detect or cross react with all of the drugs within the family.
There are many hundreds of thousands of drugs available worldwide, many of which are broken down by the body into drug metabolites. With so many over the counter (OTC) and prescription only medicines (POM) available, there is a possibility that some of these drugs and their metabolites might have a similar 3-dimensional shape to other drugs and will be detected by the immunoassay. For this reason it is standard practice to perform a confirmation test and request details of a donor’s declared medication after a presumptive positive screening result.
Figure 1: A drug family can share a common shape to which an antibody will bind. Here four different drugs share a triangular section and therefore bind to the top portion of the antibody.
A confirmation test is able to definitively detect individual compounds by matching them to a commercially prepared reference standard. Confirmation tests provide qualitative results of drugs and/or drug metabolites present in the sample.