The United States is experiencing an opioid overdose epidemic. Point-of-care (POC) drug of abuse testing is a useful tool to combat the intensified opioid epidemic.
To review commercially available POC drug of abuse testing involving opioids, to review opportunities and challenges for POC opioid testing and emerging testing methods in research literature, and finally to summarize unmet clinical needs and future development prospects.
The Google search engine was used to access information for commercial opioid POC devices and the Google Scholar search engine was used to access research literature published from 2000 to 2019 for opioid POC tests.
The opioid epidemic provides unprecedented opportunities for POC drug testing, with significant clinical needs. Compared with gold standard tests, limitations for commercially available opioid POC testing include lower analytical sensitivity, lower specificity, and cross-reactivity. In response to unmet clinical needs, novel methods have emerged in research literature, such as microfluidics and miniature mass spectrometry. Future prospects include the development of quantitative POC devices and smarter and real-time drug testing.
Opioids are a class of drugs that can produce analgesic, depressant, and euphoric effects on the central nervous system.1 Generally, opioids can be classified as natural opiates, semisynthetic opioids, or synthetic opioids. Natural opiates, such as morphine and codeine, are found in the opium poppy; semisynthetic opioids, such as oxycodone, oxymorphone, hydrocodone, and hydromorphone, are derived from chemical modifications to natural opiates; and synthetic opioids, such as methadone and fentanyl, are manufactured chemically from other molecules. Opioids are also divided into illicit opioids (eg, heroin and illicitly produced fentanyl) and prescription opioids used as pain relievers (eg, morphine, codeine, oxycodone, hydrocodone, fentanyl, and methadone). Maladaptive use of prescribed or illicit opioids can decrease consciousness level and blood pressure, depress respiration, and constrict pupils.2 The impairment or distress caused by the opioid toxidrome within a 12-month period is clinically defined as opioid use disorder.3
Currently, the United States is experiencing a national opioid epidemic. From 1999 to 2014, opioid overdose was the leading cause of death from poisoning.4 In 2017, 67.8% (47 600) of the 70 237 drug overdose deaths involved opioids, a 90% increase from 2013 (25 052); on average, more than 130 Americans die every day because of opioid-related overdoses.5 The Centers for Disease Control and Prevention data of drug overdose deaths involving opioids in the United States between 2000 and 2017 are shown in Figure 1.6
There are 3 distinct waves in the rising of opioid-related overdose deaths, marked with red stars separately in Figure 1. The first wave synchronized with the trend of extensively prescribed opioid analgesics (natural and semisynthetic opioids and methadone) for the management of chronic and acute pain in the 1990s. The number of opioid prescriptions increased7 from fewer than 3 million per year in 1990 to 11 million in 1999. Approximately 57 million people filled at least one opioid prescription8 in 2017. Although prescription opioids play a vitally important role in the treatment of pain, their overuse and misuse have contributed to the opioid epidemic. It is estimated that between 2008 and 2011, of opioid users age 12 years or older, 12 million per year were nonmedical users, and 91.8% of them reported a source of a prescribed opioid pain reliever.9 Seventy percent of prescription opioids were obtained from a friend or relative, a much higher percentage than obtained directly from a clinician (19.7%).9
The second wave was due to the rapid increase of heroin overdose deaths. Individual states and cities have reported substantial increases in deaths from heroin overdose10 since 2010. As shown in Figure 1, the mortality rate from heroin overdose increased nearly 500% between 2010 and 2016. In 2015, 5.1 million persons were estimated to have used heroin in their lives,11 and 16 770 hospitalizations and 81 326 emergency department visits occurred because of heroin-related poisoning in the United States.8
The third wave of drug overdose deaths began in 2013, driven by the dramatic increase in synthetic opioid–related overdoses (eg, fentanyl, fentanyl analogs, and tramadol). The average annual mortality rate of synthetic opioid overdoses increased by 71% from 2013 to 2017.5 Illicitly manufactured fentanyl and/or fentanyl analogs have become the major contributing factor to the current opioid epidemic. An increase of more than 300% in fentanyl encounters was reported12 from 2014 to 2015. Most fentanyl-associated deaths are caused by illicit fentanyl and/or fentanyl analogs either alone or doped in other abused substances, such as heroin or cocaine.13 On one hand, the combination drugs adulterated with powerful fentanyl make substance users—with or without realizing—more likely to overdose. On the other hand, it is often difficult to recognize the presence of fentanyl laced in the presumed substance(s) and administer appropriate treatment immediately based on clinical symptoms alone, which further increases the risk of overdose death.
Strong efforts have been made in the last few years to counter the opioid epidemic. Opioid prescribing rates declined by 13.1% from 2012 to 2015 and has continued to decline in recent years because of awareness of physicians and implementation of new policies,8,14 including prescription opioid monitoring programs. One study indicated that the implementation of a prescription drug monitoring program was associated with a more than 30% reduction in opioid prescriptions for pain management.15 The rate of overdose deaths involving heroin and natural and semisynthetic opioids remained stable from 2016 to 2017 (Figure 1). The latest data, from July through December 2017 to January through June 2018, indicate that overall opioid-involved overdose deaths declined by 4.6% in 25 states and prescription opioid–involved overdose deaths declined by 10.6%.16 Provisional data from 2018 to 2019 also indicate some improvements.17 However, final data are needed to confirm this trend.
OPIOID POINT-OF-CARE TESTING
Drug testing is an essential tool to counter the opioid epidemic by providing critical information about opioid use.4,18,19 Several biological specimens can be used for opioid testing, including urine, saliva, sweat, blood, hair, and breath.20 The advantages and disadvantages of each specimen type are summarized in other review articles.4,20,21 Urine remains the predominant specimen for clinical opioid testing, although it is susceptible to adulteration or substitution.4
There are 2 major types of drug tests: screening and confirmatory. Screening testing, also called presumptive testing, uses qualitative technique to identify certain targeted drug classes. Immunoassay is the most commonly used method for the screening of opioids. There are several types of laboratory immunoassay techniques for prescription opioid monitoring.22,23 Various point-of-care (POC) immunoassay platforms are widely available for rapid opioid screening in the field or in the emergency department.24,25 Confirmatory drug testing, also called definitive testing, uses quantitative technique to confirm screening results, or to test for substances without screening options. Confirmation testing uses highly specific and sensitive analytical methodology such as gas chromatography–mass spectrometry (MS) and liquid chromatography (LC)–tandem MS (MS/MS).26,27
Compared with cumbersome laboratory instruments, POC devices have many advantages in meeting the challenges of the opioid epidemic. First, POC tests are able to detect opioids and/or metabolites rapidly within minutes and enable immediate treatment decisions. Second, POC tests are relatively inexpensive (approximately $1–10; costs vary with respect to methodology, device needs, and the number of tests in a cartridge). Finally, POC tests are simple to perform in a practitioner's office or at home.
Most commercially available POC devices for drug of abuse testing involving opioids are based on lateral flow assay (LFA) technology, as shown in Table 1. These LFA-based devices include urine cassettes, urine dipsticks, combination urine collection/test cups, and saliva swabs, as shown in Figure 2. Two novel platforms are based on fingerprint and programmable bio-nano-chip technology, using a microfluidic cartridge for loading sweat or urine samples. Almost all LFA-based POC tests are performed in one step after depositing the urine or saliva sample.
OPPORTUNITIES AND CHALLENGES FOR POC DRUG TESTS IN THE OPIOID EPIDEMIC
The opioid epidemic provides unprecedented opportunities for POC drug testing, with significant clinical and market needs. Medicare data reveal28 that the number of POC drug tests reimbursed increased from 101 in 2000 to more than 3.2 million in 2009. As results can be obtained within minutes, POC opioid testing may be preferred in many clinical settings, including emergency departments, detoxification clinics, and drug treatment clinics, as well as in settings related to chronic pain management and maternal fetal medicine.25 Example clinical uses include screening for opioid use, detecting fentanyl doping, verifying self-reports, making preliminary treatment decisions in emergencies, and validating adherence in taking prescribed controlled opioids.29
Opioid POC testing can be used in primary care settings for screening and brief intervention, and is usually used in combination with standardized substance use and addiction screening questionnaires.30 Bach et al31 used a urine fentanyl-screening strip (BTNX Inc) to investigate the prevalence of fentanyl exposure and found 1 in 3 emergency department patients with active opioid use history in Baltimore, Maryland. On-site screening allows clinicians to administer necessary treatment or adjust therapy immediately, especially in emergency departments and detoxification clinics. It can also provide an objective source of information for comparison with the patient's self-reported opioid use. Individuals with addiction may underreport substance use or give inaccurate or incomplete histories, making some self-reports unreliable.32 In another emergency department–based study,33 drug testing identified almost 6 times as many patients as having used opiates in the previous 24 hours (10 of 100; 10%) relative to self-report (11 of 641; 1.7%). Opioid POC testing can also assist in early identification of possible toxins in high-risk populations, such as patients with histories of substance use and adolescents with altered mental status.34 Moreover, drug screening can increase the safety of opioid prescriptions by preventing potential overdose or dangerous drug-drug interactions.
Assessment and Treatment
The presence of opioids in patients affects clinical treatment decisions, and the information of any ongoing substance use is necessary for the clinicians to prescribe controlled opioids. Opioid POC testing is extensively used in opioid use disorder treatment programs, and is also appropriate in the assessment and treatment of medical conditions such as overdose, chronic pain, psychiatric disorders, and sleep disorders.29,35 To prevent potential adverse effects of pharmacotherapy, opioid screening (eg, for methadone, oxycodone, buprenorphine) is done prior to starting naltrexone to treat opioid use disorders.36 Prescribing opioids without careful drug testing may increase the risk of drug overdose and concomitant use of opioid analgesic with other depressants or sedatives (eg, benzodiazepines).37 An analysis found that 70.7% of deaths involving opioid analgesics in New York State also involved at least 1 other substance, most frequently a benzodiazepine.38
Opioid analgesics are the most potent and effective pain-relieving medications currently available.39,40 However, noncompliance with chronic opioid therapy is not uncommon. Michna et al41 observed noncompliance in 45% (211 of 470) of urine testing results, including presence of illegal substances (95 of 470; 20.2%) and additional prescription medications (68 of 470; 14.5%), absence of the prescribed opioids (48 of 470; 10.2%), and evidence of urine specimen tampering (11 of 470; 2.3%). Studies have illustrated high drug abuse rates of 18% to 41% in patients receiving opioids for chronic pain treatment.42 Toxicology testing can identify more nonadherent patients with opioid therapy when compared with the use of self-reporting or behavior monitoring alone.43,44 Drug testing may also act as a deterrent to illicit/nonprescribed drug use in the process of opioid therapy.18 Periodic opioid POC testing provides rapid results for evaluating opioid compliance and exposing the risks for drug misuse, abuse, and diversion. The range of available POC tests in pain clinics has expanded to include tests for morphine/codeine, oxycodone, buprenorphine, methadone, and fentanyl.45–49 After opioid POC testing, confirmatory testing should be performed for all samples negative for prescribed opioids, positive for nonprescribed opioids, or positive for illicit drugs. Manchikanti et al47 reported that agreement between POC testing and LC-MS/MS in monitoring prescribed opioids was as high as 80.4% (740 of 920), and nonprescribed opioids were used by 5.3% (53 of 1000) of chronic pain patients. Opioid POC testing is also very useful for monitoring relapses in patients with histories of opioid use disorder during/after the addiction treatment period.44 Additionally, it can be used to monitor treatment effect and update a treatment plan.
Compared with gold standard tests, although opioid POC screening has many advantages, it faces several challenges, including (1) lower analytical sensitivity and higher detection thresholds, which can lead to false-negative results; (2) lower specificity and difficulty in distinguishing the parent opioid and its active metabolites; and (3) cross-reactivity with other substances, which can lead to false-positive results.
Analytical sensitivity is usually better for MS-based methods, and POC devices are less sensitive. For example, central laboratory and MS-based methods are able to definitively quantify fentanyl as low as 1 ng/mL,50 whereas commercial LFA strips (eg, from BTNX Inc, DrugCheck, NarcoCheck, Instant-view, SureScreen, and Alcopro) are available with cutoffs of 10 to 20 ng/mL for fentanyl, which would yield false-negative results in urine samples with fentanyl concentrations below these cutoffs. This challenge could be overcome with carefully optimized design of the POC method. For example, our group recently developed a fentanyl-screening LFA strip that was able to detect fentanyl at 1 ng/mL and/or norfentanyl at 10 ng/mL in urine within 5 to 10 minutes. Its clinical sensitivity and specificity were 100% and 99.5%, respectively, with good concordance between the fentanyl strip and gold standard LC-MS/MS.51 A diagnostic accuracy study of 1000 patients taking chronic opioid therapy compared opioid POC testing with LC-MS. Morphine, methadone, and oxycodone were tested with cutoff values of 300, 300, and 100 ng/mL in POC testing and 50, 100, and 50 ng/mL in LC-MS, with false-negative rates of 7.8%, 3.9%, and 25% in POC results, respectively.52
In addition, opioid POC screening lacks the ability to distinguish the parent opioid from its active metabolite. Interpretation of drug screening results must consider knowledge of opioid metabolism. Metabolism of common opioids has been summarized in other reviews.4,53,54 Both this and the next limitation are due to the immunoassay nature of the POC assays, not specific to the POC format itself.
Cross-reactivity differs among opioid POC devices because of the differences in drug antibody specificity. Compounds that may cause false-positive results on an immunoassay screen have been reported in some literature.4,55 In some cases, cross-reactivity with the metabolites may be by design in order to pursue a longer detection window. For example, a fentanyl rapid-screening strip demonstrated cross-reactivity with the major metabolite norfentanyl and no cross-reactivity with several common drugs of abuse (morphine, cocaine, methadone, etc) or treatment drugs (acetaminophen, naloxone, etc); minor cross-reactivity was observed for risperidone (reactivity 0.4%) and its major metabolite 9-hydroxyrisperidone (reactivity 0.05%).51 The list of cross-reacting substances and their cross-reactivity should be consulted carefully to help clinicians respond appropriately to a presumptive positive test result.
RESEARCH PROGRESS OF OPIOID POC TESTING
Various novel opioid POC testing technologies, including LFA strips, microfluidics, optical biosensor, and miniaturized enzyme-linked immunosorbent assay (ELISA)/MS, have been reported in research literature. They are summarized in Table 2 and Figure 3.
Lateral flow assay is a portable, low-cost, user-friendly, and well-established platform for drug screening in POC settings. Lateral flow assay strips are based on competitive immunoassay reactions because there are no pairs of antibodies for the small drug molecules to form a common sandwich structure.56 The entire immunoassay reaction process is integrated into one capillary paper–based LFA strip and started by adding a liquid biological sample.57 To achieve quantitative colorimetric readout of LFA strips, various optical strip readers (eg, smartphone, scanner) and image-processing algorithms are designed to measure the intensities of test/control lines.58–61 For example, Taranova et al60 reported a lateral flow microarray with multiple immuno-spots on the test zone (Figure 3, A) for the 1-step quantitative detection of several drugs, including morphine, amphetamine, methamphetamine, and benzoylecgonine (the major cocaine metabolite), using a scanner. Carrio et al61 developed an automated smartphone-based strip reader for drug of abuse LFA tests (Figure 3, B). Additionally, our group recently developed a sensitive fentanyl-screening LFA strip that was able to detect urine fentanyl at 1 ng/mL and/or norfentanyl at 10 ng/mL within 5 to 10 minutes (Figure 3, C).51 Yang et al62 developed a stacked paper-based assay for the simultaneous detection of multiple drugs, which combined the LFA method with a bar code technology (Figure 3, D). Hence, its qualitative results can be read out conveniently by a bar code scanner.
The microfluidic chip is another promising and competitive POC platform for rapid drug screening. Zhang et al63 reported a DVD-based microfluidic platform for the quantitative and multiplexed detection of drugs of abuse in saliva, obtaining digital signal readout with a conventional optical DVD drive (Figure 3, E). The limit of detection of this assay is as low as 1.0 ng/mL for morphine and 5.0 ng/mL for cocaine. Li et al64 developed an integrated competitive volumetric bar-chart chip to detect multiple drug targets (eg, cocaine, opiates) in urine, serum, and whole blood (Figure 3, F). Platinum nanoparticle–catalyzed oxygen generation in H2O2 solution pushes the red ink to advance in the glass microfluidic channel. Visual qualitative bar-chart readout results are displayed based on the direct competition of oxygen generated by the sample and the internal control. The competitive volumetric bar-chart chip test results showed good agreement with an LC-MS/MS method, with a clinical sensitivity and specificity of 94% and 100%, respectively.
In addition, several novel optical biosensors have been developed and used as POC drug screening methodologies. Bonanno and DeLouise65 demonstrated a label-free porous silicon photonic sensor for analyzing opiates in urine. It offered a wide dynamic range for morphine (5.0−3077 ng/mL), covering the current positive cutoff value (300 ng/mL) in clinical opiate urine screening. Furthermore, the porous silicon sensor showed desirable high cross-reactivity with other common opiates (morphine-3-glucuronide and 6-acetyl morphine), in addition to the semisynthetic opioid oxycodone, while low interference from cocaine metabolite was maintained. A new approach for the rapid identification of drug metabolites in fingerprint sweat was developed by using antibody-functionalized nanoparticles (Figure 3, G).66–68 The fingerprint sweat sample is collected by simply pressing the finger onto a cartridge (within 5 seconds); then, the sample collection cartridge is inserted into a portable reader. The reader analyzes the fingerprint sweat and provides a qualitative test for different drugs in less than 10 minutes.68
The traditional ELISAs or gold standard MS analyzers have also been miniaturized so that they could be used at POC to detect drugs rapidly. Integration of sample preparation, microfluidic, and control/detection instrumentation components into a miniature system remains a huge challenge for POC diagnostics. Miyaguchi et al69 reported a portable microchip-based ELISA system for quantitative detection of drug in hair. Sample preparation (micropulverized extraction) and quantitation of microchip-based ELISA can be accomplished in less than 30 minutes. A programmable bio-nano-chip platform was developed for the rapid (approximately 10 minutes) quantitation of drugs of abuse in POC settings (Figure 3, H).70–72 The programmable bio-nano-chip analyzer along with customized disposable cartridges had the proven multiplexed and sensitive detection capacity of 12 drugs in oral fluid.72 The emergence of miniature MS made the prospect of confirmatory POC drug testing a possibility.73,74 Ouyang's group75,76 developed a benchtop miniature MS/MS system (25 kg, 19.6 × 22.1 × 16.5 in [49.8 × 56.1 × 41.9 cm]) with digital microfluidics and ambient ionization source capabilities (Figure 3, I). Multiple drugs could be quantified from 4 dried urine samples in less than 15 minutes. The limits of quantitation for cocaine, benzoylecgonine, and codeine were 51, 21, and 39 ng/mL, respectively.77 These limit-of-quantitation values were approximately 10 times higher than those of conventional MS methods, but they were well below the typical confirmatory cutoff values.78 Most of these emerging opioid POC platforms in research literature are not yet commercially available, with the exception of the fingerprinting technology and the p-NBC technology, which have been commercialized by Intelligent Fingerprinting and SensoDX, LLC, Inc, respectively.
With existing technologies, there are still several unmet clinical needs for opioid POC testing, which may be the directions for future technology development.
Quantitative Opioid POC Detection
The existing commercial LFA-based POC opioid analytical platforms (Table 1; Figure 2) often generate results rapidly within minutes. However, almost all of them are qualitative, whereas quantitative drug concentration results are important for compliance assessment and dose adjustment. Providing rapid, accurate, and precise quantitative results is still a challenge that needs to be addressed in future development of opioid POC platforms. Efforts have been made in some research literature (Table 2; Figure 3). For example, to achieve quantitative colorimetric readout of LFA-based assays, smartphone-based optical strip readers and image-processing algorithms have been specifically designed to measure the color intensities of test/control lines, which could be correlated with analyte concentrations.58,60 In addition to paper-based platforms, microfluidic chips have been used to provide rapid quantitation results of opioid POC screening, such as in competitive volumetric bar-chart chip64 and DVD-based microfluidics.63 The accuracy and precision of these platforms remain to be validated using a large number of real-world clinical samples. Another potential quantitative solution is to bring the gold standard instrument into the field, which also effectively eliminates the need for prescreening with error-prone immunoassays. The emergence of miniature MS in research literature made this prospect a possibility.73–77
Smarter and Real-Time Drug Testing
When drug testing is conducted according to predictable schedules, it is much easier for drug users to cheat and “pass” the tests by avoiding the detection window or taking substances that are not in the drug test panels.29,79 Therefore, smarter drug testing uses random instead of scheduled testing based on clinical indication, with a broad and rotating panel of drugs. The American Society of Addiction Medicine emphasizes the need for smarter drug testing: “The most important challenge in drug testing today is not the identification of every drug we are technologically capable of detecting, but to do medically necessary and accurate testing for those drugs that are most likely to impact clinical outcomes.”29
None of the current POC drug testing can offer real-time information about substance use behavior and trigger real-time analysis or intervention.80 Given the rapid development of artificial intelligence and mobile technologies, there has been significant interest in the development of behavioral interventions delivered through computer and mobile technologies to people with substance use disorders,81,82 such as Web-based self-help interventions and virtual therapeutic software.83,84 Wearable platforms can play a great role in smarter drug testing and real-time drug use monitoring. The rapid development of wearable bioelectronics in recent years85,86 has led to continuous monitoring applications for several target drugs, such as alcohol87,88 and caffeine.89 Several commercial wearable platforms are available and capable of transdermal monitoring of alcohol consumption using insensible sweat,90 such as the SCRAM Secure Remote Alcohol Monitor (Alcohol Monitoring Systems, Inc), the WrisTAS Wrist Transdermal Alcohol Sensor (Giner, Inc), the BACtrack Skyn (BACtrack Breathalyzers/KHN Solutions, Inc), and the Proof alcohol tracking wearable (Milo Sensors, Inc). In addition, there are some wearable platforms for drug use monitoring and opioid overdose detection via physiologic parameters (eg, acceleration, skin temperature, pulse, electrodermal activity, and blood oxygen),91,92 such as the Q Sensor (Affectiva) and a prototype wristband (Pinney Associates, Inc).93 However, the menu of target drugs in these on-body detection methods currently is very short. Immunoassay-based drug testing may be challenging in wearable biosensors because the biological recognition elements, such as antibodies and aptamers, are very sensitive to their local environments.94 Novel recognition interface and molecularly imprinted polymers might provide a solution to this challenge.95,96
Smarter drug testing might generate more information about dose and time of administration from a series of snapshot-based tests, which may be achieved using big data and machine learning. Combining drug concentration and physiological parameters may offer more accurate information. For example, respiratory rate might be monitored using vital sign monitors, and pupil diameter might be monitored using a high-speed, infrared eye-tracking camera. When these parameters are assessed simultaneously with conventional opioid POC test results, a more comprehensive and accurate drug test might be achieved.
Opioid POC testing is an important tool to counter the intensified opioid epidemic in the United States. Various commercial tests for POC opioid screening and confirmation are available, mainly based on immunoassays and chromatographic methods. Several unmet clinical needs for opioid POC testing remain. In response to these unmet clinical needs, novel methods have emerged in research literature, such as microfluidics and miniature MS. Future prospects include the development of quantitative POC devices and smarter and real-time drug testing.
The authors have no relevant financial interest in the products or companies described in this article.