Urine testing typically involves a screening test followed by a confirmation test. Confirmation is usually performed on the sample taken for screening.
Screening tests
Most drugs of interest are first detected by simple immunoassays. These are broad screening tests that are quick, often cheap and effective for showing a positive or negative result. However, as with all screening tests there are limitations to the degree of interpretation that can be inferred from the result. The limitations of immunoassay techniques include false positives as well as false negative results.
A false positive is a screening test that fails to be confirmed using other more sensitive and specific techniques such as gas chromatography/mass spectrometry or liquid chromatography/mass spectrometry. This means that the immunoassay has cross-reacted with some other substance in the urine leading to a false positive result for the substance of interest. Other drugs can trigger such false positive results and the laboratory should have a list of compounds which can cross-react with the screening test. For example, ranitidine can produce a false positive result for amphetamines. It is also worth noting that some foodstuffs can also produce positive results such as poppy seeds for opioids.
A false negative result is possible when the screening test is negative but the confirmatory test is positive. This is less common as negative screening tests are not usually confirmed. When a screening test is negative that is usually the end of the investigation. In a workplace, a false negative test can have far-reaching ramifications if an incident occurs after screening and a urine sample test then finds drugs which were missed by the initial screening process. On-site or point-of-care devices must therefore be rigorously tested and validated before use in the field. AS/NZS 4308 states that on-site screening devices be evaluated at 25% above and 30% below the level considered positive (these are typically referred to as cut-offs). The Standard also specifies that failure of no more than 10% of on-site devices is permitted.
When using immunoassay techniques, samples can easily be adulterated to provide a false result. Adulterations are common in patients who undergo clinical compliance testing, for example abstinence control in drug users. Adulterations can include water (leading to dilution of urine), bleach and masking agents (such as diuretics) or other substances that interfere with the screening test. Adulterant checks are also part of a laboratory's capability to detect an invalid specimen. The Standard provides guidance on what to do to avoid adulteration and how to test for adulterants (for example temperature and creatinine checks).
Confirmatory tests
Confirmation tests are usually required for medicolegal purposes when drug testing is used in the workplace or for family custody disputes in which parents are allegedly using drugs at home in the presence of children. An initial urinary screening test must be confirmed for evidence of drug use. Confirmatory testing is more sensitive and specific than screening tests and confirms the drug of interest as opposed to the drug class.
Clinical laboratories have relied on gas chromatography/mass spectrometry for confirmation, however developments in liquid chromatography/ mass spectrometry technologies over the last 10 years have meant a wider range of compounds can be confirmed simultaneously. Laboratories must demonstrate compliance with requirements to either International Organization for Standardization (ISO) 17025 (for chemical/forensic testing) or ISO 15189 for medical/pathology testing and must be accredited by the National Association of Testing Authorities.