Standard drug testing is regularly carried out using urine, blood or oral fluid. However, fingerprints present a good alternative, as the sample collection is non-invasive, rapid and safe. Herein, we describe the application of two different testing methods for the detection of cocaine in fingerprint samples.
by Dr Catia Costa, Dr Mahado Ismail and Dr Melanie J. Bailey
Drug abuse in the United Kingdom is on the rise and it is a cause for concern, with widespread financial and social implications [1, 2]. The ever-growing drug and alcohol culture in the UK has led to the implementation of workplace drug testing in many industries, especially those in high-risk operational environments. Consequently, there has been a surge in the demand for drug-screening suppliers to develop faster and more reliable testing. This demand is set to increase the market value of drug and alcohol testing in the UK from £167 million to £231 million by 2019 .
Conventionally, drug testing is carried out using biological matrices such as blood, urine and, more recently, oral fluid. These matrices and methods of analysis, although established, present a few problems relating to sample collection and transportation. The collection of blood requires medically trained personnel and sample collection is considered invasive, whereas urine carries privacy concerns. Oral fluid is an alternative matrix used for non-invasive drug testing, although sample collection can be time-consuming. All these three matrices are also biohazardous, making sample storage and transportation a potential issue. The potential use of fingerprints for drug testing has become the subject of many recent publications. Fingerprint samples present a good alternative for drug testing as collection is non-invasive and rapid, and there are no known biohazards associated with the sample. Additionally, the fingerprint pattern can be used for donor identification.
Chemical analysis of fingerprints
The chemical information embedded in a fingerprint sample has been reviewed by many, and several publications have explored the detection of substances such as cocaine [4–6], heroin , methadone , lorazepam , methamphetamines , caffeine  and cough medicine  in fingerprints after administration of the substances. These reports are predominantly based on liquid chromatography-mass spectrometry (LC-MS), which is very well established in the field of toxicology for its quantitative potential as well as its sensitivity and reliability. New advances in the field of mass spectrometry saw the rise of ambient ionization mass spectrometry techniques that allow the sample to be analysed in its native state, under ambient conditions. Examples include desorption electrospray ionization (DESI), liquid extraction surface analysis (LESA) and paper spray, which have been applied to the detection of cocaine and metabolites in fingerprint samples [4–6].
Most of these reports in the literature have looked at fingerprint samples collected after administration of the substances. However, no research has investigated the significance of the detection of these substances compared to a large background population of non-drug users. This is of particular importance as a positive test result may be the outcome of contamination by contact with contaminated surfaces or handling the parent drug rather than ingestion. This directly highlights the need for a sampling strategy that removes any contact residue while providing enough fingerprint material for the analysis.
Detection of cocaine in fingerprints
The detection of cocaine in fingerprints has been studied and reported by Ismail et al. . This study looked at fingerprints collected from the background population (i.e. non-drug users) and from patients at a drug rehabilitation clinic. Both sets of samples were collected as presented and after handwashing, followed by wearing nitrile gloves for 10 minutes. Fingerprint results were supported by oral fluid analysis and patient testimony. Analysis of samples collected from patients (n=13) at the rehabilitation clinic yielded a 100% detection rate for cocaine for samples collected as presented and after handwashing. However, the detection of the cocaine metabolite, benzoylecgonine (BZE), decreased from 94% from samples collected as presented, to 87% for samples collected after handwashing. To evaluate the significance of the results above, fingerprint samples collected from the background population were analysed to investigate the prevalence of these substances in non-drug users. Samples collected as presented (n=99 samples) returned a 13% and 5% detection rate for cocaine and BZE, respectively. After handwashing, cocaine was only detected in 1% of the samples analysed (n=100) and no BZE was present. These findings suggest that cocaine can be detected in the background population owing to environmental exposure (e.g. contact with bank notes). However, after using a handwashing procedure, cocaine and benzoylecgonine were not prevalent. Collection of fingerprint samples after a hand-cleaning procedure is therefore advantageous to reduce potential false-positive rates that can be observed from environmental exposure.
As previously mentioned, the use of chromatographic methods is well established in the field of toxicology. However, such methods often rely on extensive sample preparation and analysis. To overcome this issue we have developed paper spray-mass spectrometry (PS-MS) for the detection of cocaine in under 4 minutes from fingerprints collected from patients seeking treatment at a rehabilitation centre . For this method fingerprints are collected on a triangular piece of paper, which is in turn placed on the paper spray source for analysis. An internal standard, solvent and voltage are applied to the paper, resulting in the extraction and ionization of the fingerprint residues before detection on the mass spectrometer (Fig. 1). The method was evaluated with 239 fingerprint samples collected from drug users at the National Health Service (NHS) rehabilitation clinics and from the background population. A positive result was based on the detection of cocaine or one of its two main metabolites, BZE and ecgonine methyl ester (EME). A 99% true-positive rate was achieved on the samples collected from patients at drug rehabilitation centres, which was supported by standard saliva drug testing and patient testimony. Analysis of samples collected from the general population yielded a 2.5% false-positive rate. This follows from the work by Ismail et al.  described above, where in the absence of a hand-cleaning procedure cocaine was detected in the background population. Both studies highlight the need for a well-defined sample collection procedure to eliminate false-positive results while maintaining true-positives.
This method has since its publication been shortened to 30 seconds and it has also been applied to the detection of heroin, morphine, codeine, 6-AM and explosive materials. This highlights the potential for the technique to be on a par with current testing methods that target a wide range of substances.
Another advantage of using fingerprints for drug testing is the possibility to integrate a fingerprint visualization step for donor identification. This would be of particular benefit for preventing cheating and also in cases of disputed results where one would be able to prove that the results were derived from the correct person. Silver nitrate was used to visualize fingerprint samples collected from drug users by treating the substrate before sample collection. Upon collection, samples were exposed to ultraviolet light to bring out the fingerprint pattern (Fig. 2). Analysis of fingerprint samples collected from drug users after silver nitrate development yielded a 100% detection rate for cocaine, showing great potential for this development step to be included in the fingerprint testing routine.
The future: treatment adherence monitoring
Treatment non-adherence is a well-known problem in the NHS and it is estimated that it can cost over £500 million each year . Thus, the establishment of an adherence monitoring tool could result in substantial savings for the NHS. Fingerprint testing offers the opportunity for remote testing where the samples can be collected by the patient at home and sent to the laboratory for analysis. In cases of non-adherence, medical professionals may intervene and ensure the patient is receiving adequate treatment. This is of particular interest for conditions known to have poor adherence rates such as diabetes, cardiovascular diseases and mental health disorders  or for highly infectious diseases such as tuberculosis.
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Catia Costa*1 PhD, Mahado Ismail2 PhD and Melanie J. Bailey2 PhD
1Ion Beam Centre, University of Surrey, Surrey, GU2 7XH, UK
2Department of Chemistry, University of Surrey, Surrey, GU2 7XH, UK