SPECIALIST PHARMACOLOGICAL & BIOCHEMICAL INTERVENTIONS

Testing of biological matrices such as urine, plasma and hair for substances of misuse is used largely to test for concordance with a course of treatment. It is important to avoid becoming over-reliant on such techniques in clinical practice; the emphasis must be on developing an honest and trusting relationship with the client - overuse of biochemical monitoring can act to undermine this. There is no clear evidence in support of frequent testing as an enhancer of outcomes and as a stand-alone measure without historical data, interpretation is often difficult or meaningless. Laboratory testing in particular may become extremely costly if used excessively by services; it is thus imperative that specialist services develop local protocols regarding the appropriate use of biochemical screening tests. A measured use of monitoring can however act both to test the honesty of self-reporting, as a tool for outcome monitoring and as a rewarding experience for clients who are providing clean samples.

INTERPRETATION OF RESULTS

The interpretation of results will be dependent on 3 major factors:

• The client's statement regarding the class, route, amount and timing of drug use.
• The technology used to perform the test.
• The body matrix used for testing (e.g. urine, saliva, hair).

TESTING TECHNOLOGY

There are two main classes of technique - chromatography and immunoassay. Immunoassay may be an easier and cheaper alternative and is the mainstay of on-site testing techniques. Its main disadvantage is that it can usually only detect the class of drug rather than the actual drug itself. This leads to problems with interpretation, especially in the case of opiates where the same positive result may occur in response to heroin use or over-the-counter codeine use.

Chromatography techniques are able to detect the actual drug and the gold standard laboratory test is Gas Chromatography/Mass Spectrometry (GC/MS). If there is doubt about the validity of a test result, a confirmatory test using GC/MS should usually be performed in the laboratory. The other commonly used laboratory test is High Pressure Liquid Chromatography (HPLC), the accuracy of which approaches that of GC/MS. Some laboratories will also employ immunoassay techniques, or cheaper forms of

chromatography (such as TLC) to screen samples for positives, before confirming the positive samples with GC/MS or HPLC.

Most on-site rapid tests use immunoassay technology, although there are paper chromatography kits available. Paper chromatography tests are inaccurate, labourintensive and time-consuming and their use is not recommended in any setting. Most immunoassay tests operate by the binding of drug to an antibody, thus preventing the binding of a drug-conjugate (competitive immunoassay) to the antibody which in turn prevents the formation of a visible band, indicating a positive test result. (However, some tests operate in a reverse fashion where the formation of a visible band indicates a positive result: instructions must be carefully read). As a generalisation, these tests will be slightly less accurate than laboratory testing using GC/MS due to i) higher cut-off points leading to false negatives and ii) greater potential for giving false positive results. As mentioned above, they are also limited by their inability to detect a specific drug, reporting only on the class of the drug.

Next page .. BODY MATRIX

Alcohol Guidelines index

The above information is copyright of Dr Bruce Trathen MBBS MRCPsych (2006). ISBN 0-9545164-0-0. The author grants permission for these guidelines to be downloaded, copied and distributed freely, but does not grant permission for their sale.