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How to use toxicology screening tests

The evaluation of poisoned patients presenting to the hospital predominantly relies on clinical assessment based on the history of ingestion/exposure and clinical symptoms/signs.1 The role of toxicological screening is controversial. In the emergency department (ED), a toxicology screen (TS) can be used to detect a substance that causes the clinical features in an individual. Therefore, as a diagnostic test, the term ‘screen’ can be misleading. A toxicology screen generally involves qualitative detection of specific drugs or metabolites in biological specimens. Detection indicates that a substance is present; however, it does not indicate whether a patient's symptoms/signs are caused by the presence of this substance, and it is important to exclude other causes. Therefore, a toxicological screen performs poorly as a diagnostic test.2

In addition to a general TS, there are specific drug/chemical assays that can be used to guide clinical management of poisoned patients (see box 1).3 These are used to measure the concentration (or ‘level’) of a drug/chemical, and are the most useful type of screening used to manage a poisoned patient. The poisonings for which these are useful are those in which the serum concentration of the drug/chemical correlates to the clinical effects and toxicity, and will either determine treatment (eg, N-acetylcysteine treatment in paracetamol poisoning) or help guide it (eg, urinary alkalinisation in salicylate poisoning) (box 1). These tests are ordered selectively when particular drug exposures are suspected, either on the basis of the history, or through characteristic clinical features.

The decision to use a toxicological screening test depends on clinical features, the suspected substances involved and the influence it may have on patient management. This paper presents the evidence for the role of toxicological screening in paediatric practice. #### Box 1 Quantitative drug assays that determine or guide treatment

Drug concentrations that determine …