- A drug screen tests for a wide range of substances. It may be performed on urine or blood, depending on the specific drugs and techniques that are used. Unfortunately, the substances detected by a drug screen vary among hospitals.
- The drug screen is an adjunct to the history and physical examination in the evaluation of the poisoned patient. It rarely provides critical information but may be useful to evaluate patients in whom clinical information fails to provide a working diagnosis.
- Urine is the sample of choice for drug screens because most drugs and their metabolites are excreted and concentrated in the urine.
- The two major methods used in drug screens are immunoassay and chromatography. Mass spectrometry can be used in conjunction with chromatography to verify drug identity more precisely.
Immunoassay
- A drug-specific antibody binds to the target drug. The antibody-drug complex formed is then detected by one of several techniques (enzymatic reaction, fluorescence, or radioisotope labeling).
- The advantages of using this method include rapid turnaround and the need for minimal specimen preparation and handling. This keeps time and costs relatively low.
- This is the method used in most laboratories. However, several bedside assays also use this method (e.g., Triage and Verdict).
Chromatography
- The differential solubility of drugs in polar and nonpolar solutions is used to separate the different compounds in a sample. The most common technique is thin-layer chromatography (Toxilab), but liquid or gas chromatography columns are used as well, depending on the drug to be identified.
- After separation, the compounds are then detected by color development.
- Chromatography can detect a wide array of substances but has several drawbacks. The process involves substantial specimen preparation, and interpretation requires well-trained and experienced technicians. In addition, cross-reactivity among compounds may occur, thus providing only probable identification of drugs in the urine.
Mass Spectrometry
- Mass spectrometry (MS) is usually combined with gas chromatography (GC/MS) and is used to confirm the drugs identified by other methods.
- The drug molecule is bombarded, breaking it into fragments. These fragments are then identified by comparison with a computer database of fragments of known compounds.
- The advantage of this system is high accuracy in identification. However, this process is relatively time consuming and expensive and requires experienced technicians.
For some substances (acetaminophen, anticonvulsants, ethyl alcohol, ethylene glycol, isopropyl alcohol, lithium, methanol, salicylate, theophylline), screening is best performed on a serum sample.
There are no absolute indications for toxicology screens. Routine urine toxicology screening in overdose patients is expensive and should be discouraged.
Recommended Uses of Drug Screening
- The patient has an altered mental status of undetermined etiology. Testing in this scenario may provide information and confirm the etiology of altered mental status.
- Reported ingestants do not correspond to clinical findings. For example, if a patient has reportedly ingested a benzodiazepine, but has a heart rate of 120 beats/min, further evaluation should include a drug screen to assess for the presence of stimulants, tricyclic antidepressants, or other drugs.
- Athletics. Drug screening is used in athletics to detect illegal methods of enhancing performance.
- Brain Death. Declaration of brain death should not be made until the presence of CNS depressants has been excluded.
- Poisoning. Drug screens may be helpful if intentional poisoning is suspected.
- Pregnancy. If drug abuse is suspected, drug testing of pregnant women at the time of delivery may pave the way for a social service intervention.
- Psychiatric. Drug screening in patients with psychiatric symptoms may reveal a toxic cause.
- Treatment Programs. Drug screens may be helpful if noncompliance with drug treatment programs is suspected.
- Workplace. Drug screening in the workplace is used to detect high-risk potential employees and to reduce risk of occupational accidents.
- It should be determined how long it will take for test results to return. In order to have a meaningful impact on immediate patient care, results should be available within 1 hour.
- Drug screens should be ordered only after the treating physician has determined what actions will be taken when the results are obtained. For example, serum drug levels of unusual compounds are rarely of clinical use because the correlation of blood level to toxicity or prognosis is unknown. Furthermore, these test results may not be available for several days.
- Contacting a medical toxicologist for advice concerning the usefulness of drug screening should be considered.
- Prior to collecting the specimen, consider contacting the laboratory to assure that the specimen is correctly collected and handled.
Collection
- Specimen Collection. A urine sample of 20 to 100 cc should be obtained and sent to the laboratory. Be alert to techniques used to obscure the presence of drugs in urine. Ambulatory patients providing a sample for analysis should be observed to prevent tampering or substitution of another sample.
- Specimen Handling. The specific drug being sought or the clinical situation should be communicated to the laboratory. This will allow the technician to better interpret the results or to inform the clinician that the drug may not be detected by the assays that are available.
Blood
- Specimen Collection. Prior to collecting the specimen, consider contacting the laboratory to assure that the appropriate type of collection tube is used.
- Specimen Handling. The specific drug being sought or the clinical situation should be communicated to the laboratory. This will allow the technician to better interpret the results or to inform the clinician that the drug may not be detected by the assays that are available.