Urine Drug Screen

OVERVIEW

The urine drug screen, or urine toxicology screen, is a qualitative assay performed to indicate the presence or absence of a suspected drug

• Most doctors have little understanding of the limitations of the urine toxicology screen used by their hospitals (Durback et al, 1998)

TYPES OF URINE DRUG ASSAYS

Spot tests

• Rapid bedside qualitative testing
• result in colour change
• rarely used today

Spectrochemical tests

• e.g. co-oximetry
• rarely used

Limited urine screen (e.g. Toxlab)

• Laboratory urine drug screens done locally usually look for a limited number of toxins using thin layer chromatography and/or a variety of drug-specific antibodies (ELISA)
• can be rapidly performed in most centres and are widely available
• typically used for drugs of abuse
• immunoassays have better sensitivity and specificity than spot tests and spectrochemical tests
• immunoassays are still limited by interference from cross-reactivity resulting in false positives

Comprehensive urine screen

• A more complete drug screen to detect virtually all possible ingested drugs
• techniques include high performance liquid chromatography (HPLC), Gas chromatography-Mass spectrometry (GC-MS) and LC tandem mass spectrometric analysis (LC/MS/MS)
• typically used by forensic toxicologists for determining cause of death
• very expensive and found in reference laboratories
• typically takes two to three weeks to get a result, thus unhelpful in the acute management of patients

USE

• identification of drug exposure when history is lacking identification of drug use in known or suspected drug users

PROBLEMS WITH QUALITATIVE URINE DRUG SCREENS

In general

• rarely change the acute management of suspected poisoning or overdose
• assays vary from hospital to hospital and may not include the same drug panel
• high rates of false positives
  • e.g. quinolones can cause false positive results for opioids
  • e.g. PPIs can cause false positive results for THC
• high rates of false negatives
  • e.g. clonazepam, lorazepam, alprazolam, midazolam (CLAM) are missed by many assays
• only indicate the presence of a drug, not the time of ingestion (assays may remain positive for extended periods post-drug exposure in some cases, e.g. days or weeks), the concentration or whether there is clinically significant intoxication
• results require confirmation with a different assay (usually quantitative, such as gas chromatography/ mass spectrometry)
• clinicians often have a poor understanding of the type of assay used at their institution, the limitations of the assay or the cost

Drugs of interest that are commonly not included in urine drug screens (Nelson et al, 2015):

• Non-benzodiazepine hypnotics: zolpidem, eszopiclone, zaleplon
• Ketamine (‘‘special K’’)
• Mescaline (‘‘peyote’’)
• Psilocybin (‘‘magic mushrooms’’)
• Gamma-hydroxybutyrate (GHB) and 1,4-Butanediol (precursor to GHB)
• Chloral hydrate
• Synthetic and designer cannabinoids (‘‘spice’’ and ‘‘K2’’)
• Tryptamines
• Phenethylamine derivatives (synthetic stimulants, ‘‘bath salts’’, ‘‘2C’’ drugs)
• Imidazoline receptor agonists (clonidine, tetrahydrozoline, oxymetazoline)

Nelson et al (2015) has appendices listing the estimated detection times in the urine of commonly used drugs of abuse as well as common sources of inappropriate results on urine immunoassays

AN APPROACH

In emergency medicine and critical care settings, urine drug screens are rarely useful and should only be performed if they will potentially change acute management

• Clinicians need to be aware of the type of urine drug screen available to them, and its limitations, before ordering a urine drug screen
• Urine drug screens have high rates of false positives and false negatives; for definitive results, further quantitative assays are required

References and Links

Journal articles

• Bhalla A. Bedside point of care toxicology screens in the ED: Utility and pitfalls. International journal of critical illness and injury science. 4(3):257-60. 2014. [pubmed]
• Durback LF, Scharman EJ, Brown BS. Emergency physicians perceptions of drug screens at their own hospitals. Veterinary and human toxicology. 1998; 40(4):234-7. [pubmed]
• Moeller KE, Lee KC, Kissack JC. Urine drug screening: practical guide for clinicians. Mayo Clinic Proceedings. 83(1):66-76. 2008. [pubmed]
• Nelson ZJ, Stellpflug SJ, Engebretsen KM. What Can a Urine Drug Screening Immunoassay Really Tell Us? Journal of pharmacy practice. 29(5):516-26. 2016. [pubmed]
• Tenenbein M. Do you really need that emergency drug screen? Clinical toxicology. 47(4):286-91. 2009. [pubmed]

FOAM and web resources

• HQMEDED — Pitfalls of the urine drug screens by Jon Cole (2012)