Presumptive drug testing—The importance of considering prior probabilities

Abstract

Presumptive drug testing is commonly used in both the clinical and forensic fields to allow rapid identification of the presence and/or usage of drugs. Because the tests generally have a high sensitivity and specificity (often >90%), then a positive test result may be taken to mean there is a high probability that a targeted drug is present. This assumption is, however, incorrect. This paper demonstrates how, in order to assess the positive predictive value (PPV) of a test, it is necessary to take into account, along with the sensitivity and specificity of the test, the prevalence of the drug in the population being investigated. We demonstrate how an alternative, Bayesian approach to assessing the posterior probability of a drug being present mimics the conventional calculation of PPVs but, because a Bayesian approach requires case‐specific prior probabilities, the posterior probabilities are more meaningful than PPV in any one specific case. The effectiveness of presumptive test results in cases such as drink‐spiking, drug‐driving, testing of drugs during seizures and the confirmation of initial presumptive test results is explored. In order to exploit the potential of presumptive drug testing, it is important that the prevalence of the targeted drugs in relevant populations is understood but, more importantly, it is important to consider using a Bayesian approach in order to tailor results to the specific individual or drug batch being tested. This article is categorized under: Toxicology > Analytical Toxicology Jurisprudence and Regulatory Oversight > Expert Evidence and Narrative

The effect of contextual information on decision-making in forensic toxicology

“…As mentioned earlier, where raw data is close to a cut-off value, cases may be deemed ‘inconclusive’ or ‘borderline’. Although the mathematically correct answer is clear, immunoassay is known to produce both false positives and false negatives [ 27 , 39 ]. Given this, a valid scientific strategy could be to repeat or confirm all borderline cases (e.g., those within a certain specified ± of the cut-off value) but only if this is the pre-documented procedure in the laboratory, not post-hoc and dependent on whether or not the ‘expected’ or ‘wanted’ results were achieved.…”

“…Immunoassay results are presumptive , which means that they do not provide unequivocal evidence of the presence of a drug and any positive findings should be confirmed by another more sophisticated technique, such as gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) [ 26 ]. The secondary technique is used firstly to confirm the positive result of the immunoassay, because immunoassays are known to suffer from false-positives [ 27 ], and secondly, if the result is a true positive, to identify the member(s) of the drug family present. Like many tests in forensic toxicology, immunoassay screening is carried out in batches e.g., multiple cases are analysed together, sometimes with repeats of samples from each case, as well as positive and negative control samples.…”

On the Application of Microfluidic-Based Technologies in Forensics: A Review

“…In review papers by Virkler et al and An et al, the current and emerging methods for the identification of all BFs are extensively discussed [8,10]. The possible results of the tests can be categorized into (a) true positive (or negative) where the species of interest are (or are not) present and the test outcome indicates a positive (or negative) result; (b) false positive (or negative) wherein the species of interest are not (or are) present, but the test outcome indicates a positive (or negative) result [11]. The advantages of presumptive tests include simplicity, ease of interpretation, narrowing the options for the subsequent test, possibility of utilization on larger areas, and locating evidence not visible to the naked eye.…”

Combining presumptive color tests, pressure-sensitive adhesive-based collection, and paper spray-mass spectrometry for illicit drug detection

“…12 Because of the lack of specificity exhibited by color tests, scientists and the public alike are concerned about false positive that occur. 13 Falsely positive color tests have led to arrests and prison time on numerous occasions. [14][15][16] Steps have been taken to improve the specificity of color tests.…”