In order to ascertain whether an individual has administered drugs of abuse or not, drug testing is commonly employed to check for the presence of any drug(s) of abuse or their metabolites in the donor’s biological specimen such as blood, urine, oral fluid, sweat or hair.
Urine has been and remains the mostly widely used body fluid specimen for routine testing for drugs of abuse, but oral fluid, sweat and hair are gaining scientific credibility as alternative specimens following advancement of testing technology. Lets review the uses and limitations of different specimens for testing below:
Purposes of drug testing
Over the past several decades, drug testing has been used worldwide in a variety of disciplines including criminal justice, emergency medicine and clinical toxicology, and workplace.
Drug testing plays an important role in facilitating the judicial sentence of drug abusers in courts, drug surveillance programmes of inmates who are detained under the custody of drug treatment centres, as well as the enforcement of the legislation of driving under the influence by police.
Emergency medicine and clinical toxicology
Timely and reliable drug test results are of prime importance in the field of emergency medicine and clinical toxicology. The objective of testing is focused on determining the class of drugs that has been inadvertently or purposely ingested or exposed to the patients. Mortalities and morbidities would then be greatly reduced by effective, appropriate and prompt antidotal treatment or supportive care.
Pre-employment and workplace drug testing has increased rapidly over the last decade in western countries such as United States of America and United Kingdom. Federal organizations, government agencies, military and private corporations exercise drug testing either under mandatory legislation or corporate commitment as a measure to improve safety within the workplace.
Types of specimens for testing
Blood is widely used for drug testing in clinical and emergency toxicology because it offers the best correlation between drug level and pharmacological impairments to the body. The time window for drug detection in blood is shorter, mostly within several hours, than in urine. For example, at a given dosage of cocaine, blood testing can detect use within 12 hours while urine testing can detect use within 48 to 72 hours. Even though blood is a good specimen for determining the presence of drugs, the concerns about invasiveness of the collection, ease of transportation and storage, and specimen stability greatly hamper its popularity in other fields of application even though substitution and dilution of specimen to tamper with drug testing are considered impossible.
By far, urine is the most widely used specimen for drugs of abuse testing because of the advantages of large specimen volume and relatively high drug concentrations that render drug detection comparatively easier than in other specimens. In addition, the technology used in urine testing is well developed and has withstood legal challenges. Furthermore, urine collection is considered non-invasive, and specimens can be collected by non-medical personnel. Urine is a matrix that remains stable over time and can be frozen to maintain the integrity of the sample. Drugs in urine are normally detectable up to 1-3 days. However, unless the urine sample is obtained under direct observation, adulteration, substitution or dilution to circumvent drug testing is possible.
Following the advancement of technology in detecting trace quantity of drug(s) in hair, hair testing has gained attention because of its ability in providing a longer window of detection from months to years when compared to other specimens. In contrast to providing short-term drug abuse profile through blood and urine testing, hair testing provides complementary information about the long-term drug abuse history of a donor. Furthermore, sampling head hair specimen is considered non-invasive and the drugs incorporated in the hair remain stable and bound for a long time leading to no concern about specimen adulteration. Head hair sampled from the scalp is preferred in order to obtain the retrospective chronological drug abuse history of a donor. Head hair tends to grow at a rate of about 1 cm per month, so a 3 cm section of hair would represent a 3 month history. However, testing for drug in hair is comparatively time consuming and costly, and it must be performed in the laboratory because of unavailability of on-site screening kits.
Oral fluid is increasingly used for drug testing because the concentrations of many drugs in oral fluid correlate well with blood concentrations. Advancement of instrumental sensitivity makes oral fluid a suitable alternative to blood. Oral fluid is a non-invasive specimen that can be sampled under direct observation to prevent adulteration or substitution. The main disadvantage of oral fluid testing is its short window of detection, with most drugs being detectable within several hours only. This characteristic renders it suitable for determining very recent drug abuse, but weakens its ability in detecting use over time. For example, someone administered heroin a day ago is likely to be tested negative by oral fluid test, but positive by urine test.
Collection of sweat is undertaken by attaching a tamper evident patch, with underlying adsorbent pad inside, to the skin over a relatively long period of time (10-14 days). Analysis of sweat must be performed in a laboratory and on-site test kits are not available. Sweat testing has not widely been used because of challenges of the potential contamination from the environment and from residual levels of drug in the skin from prior use.
Characteristics of different specimens for drug testing
The advantages, disadvantages and time window of detection of different specimens are summarized below.
Screening test versus confirmatory test
In order to undertake drug testing, there must be a cutoff level for each type of drugs to be tested, and such cutoff point serves as an administrative breakpoint in distinguishing a positive or negative result. Any sample that contains the drug/drug metabolite of interest at the concentration levels equal to or greater than the designated cutoff level is reported as positive, whilst a negative is reported for the concentration level less than the cutoff. Generally, a drug test can be categorized as either a screening test or a confirmatory test, with respect to the detection method and testing principle being employed.
Screening test refers to the initial test undertaken to test for a broad class of drugs and their metabolites in the specimen with presumptive result, i.e. positive or negative. Generally screening test is rapid, sensitive, inexpensive with acceptable levels of precision and reliability, however, it lacks precise specificity and may be subject to a false positive result due to cross-reactivity with other non-targeted drugs of similar chemical structure.
On-site screening test
Use of on-site immunoassay screening kits are highly popular in the fields of workplace testing and emergency toxicology because results are available within several minutes, with reliability similar to laboratory screening, at the site of specimen collection. Furthermore, these kits involve no calibration or maintenance, and no special skills are needed to perform the screening test. Most kits have built-in quality control zones in each panel, which ensures reagent integrity and testing validity. Nowadays, commercially-available on-site screening test kits are usually designed for urine and saliva specimens only, but not for sweat or hair as yet.
Laboratory screening test
Instead of on-site testing, drug screening may also be performed by instrumental immunoassay method in the laboratory by automated, sophisticated and high throughput analysers. Generally, laboratory drug screening has to take at least 1-2 days before the results are ready for collection because of time taken in delivering the specimens to the laboratory, running the tests and preparation of test reports.
Any specimen, which has been presumptively screened positive, should be subject to confirmatory testing in order to eliminate false-positive results that arise from cross reactivity. Confirmatory testing should employ highly specific and alternate chemical technique in order to obtain unequivocal and accurate analytical results.
Kits for on-site testing
Over the past 10 years, testing kits of different designs have been marketed in order to meet the growing demand for drug screening at point of collection. These on-site test kits are commonly used by healthcare professionals, and drug treatment and rehabilitation programme supervisors to help deter drug use by the patients and supervisees, respectively. Dipcards and cassette kits that employ lateral flow immunoassay technology have been proven to be reliable and easy to use. Recently, newly designed testing cups, also employing the same technology, with integrated test strips in the interior surface have grown in popularity because of their ease of testing and sanitary protection to the test operator, with the elimination of specimen transfer or direct contact with the specimen.