Emergency and Overdose Drug Testing

Emergency and overdose testing may be used to screen for, measure the quantity of, and/or confirm the presence of a drug or toxic substance in someone who is:

• Suspected of overdosing
• Thought to be poisoned
• Showing symptoms of toxicity

Drug testing may be used to help decide how to treat someone who is acutely ill. Once a diagnosis is made, testing may sometimes be used to monitor the effectiveness of treatment and/or monitor the level of the drug or substance in the blood or body.

Results from emergency and overdose testing are primarily used for purposes of treatment. Sometimes test results are used, however, in legal cases for establishing a cause of death or determining if impairment from drugs was a contributing factor in an accident, among other purposes. In these cases, clinical laboratories follow special legal (forensic) procedures for collecting samples, handling them, testing, and reporting of results.

A variety of different approaches may be used to evaluate someone who is suspected to have overdosed or ingested something toxic. Exactly which tests are performed will depend upon the situation and the standard practices of the emergency room (ER). Healthcare practitioners order tests according to their clinical utility, meaning choices are made based on what will be useful, available rapidly, and will help guide treatment. Decisions about tests may be influenced by the turn-around-time for results, tests' sensitivity and specificity, their ability to provide quantitative results, the availability of specific treatments, and the potential for managing the patient's symptoms effectively in the ER.

One approach to making decisions about emergency toxicology testing uses two tiers of testing. The first tier is a group of stat toxicology assays that ideally provides results within an hour and includes tests such as:

• Acetaminophen—often ordered because overdoses are relatively common and can be missed. The affected person may initially have no, few, or nonspecific symptoms, even while significant liver damage is occurring. There is a therapy available for acetaminophen overdose, but it must be given within a few hours of ingestion.
• Salicylates—may be ordered if a person has symptoms consistent with an overdose, an acid base imbalance, or is known to have ingested a significant quantity.
• Ethyl alcohol (ethanol)—a breath test is usually performed in the field and occasionally at the point of care, such as an emergency room. An ethanol blood test is usually sent to a laboratory.
• Methyl alcohol (methanol)—tests are not readily available in most laboratories. When methanol poisoning is suspected, a serum osmolality test is ordered and the osmolar gap is calculated. The osmolar gap is the increase in the number of particles in the blood beyond what is considered normal and will be increased when ethanol, methanol, or ethylene glycol are present in the blood.
• Ethylene glycol (antifreeze)—also requires calculating the osmolar gap, which is also increased when ethylene glycol has been ingested.
• Iron tests—generally used only when iron overdose is suspected in a child or infant.

Not every test is ordered for every patient; the ER health practitioner's choice would be guided by the person's clinical findings.

Also included with first tier testing are several prescribed and monitored medications that can cause toxicity at levels only slightly higher than those needed to treat disease. They include:

• Carbamazepine, often prescribed for seizures and nerve pain
• Digoxin, a heart medication
• Lithium, a psychiatric drug
• Phenobarbital, prescribed for seizure control or anxiety relief
• Phenytoin, used to treat and prevent seizures
• Theophylline, used to prevent and treat wheezing, shortness of breath, and chest tightness caused by asthma, chronic bronchitis, emphysema, and other lung diseases
• Valproic acid, prescribed for seizures, bipolar disorder, and to prevent migraine headaches

Typically one of these tests would only be ordered if a person was regularly taking the drug or if someone was suspected of ingesting it.

A specific hospital might have additional tier one tests on its list, based upon the needs of its community and ER.

The tier two tests include assays for several classes of drugs of abuse, such as cocaine, opiates, amphetamines, barbiturates, and tricyclic antidepressants. The sensitivity and specificity of these tests have improved considerably, but results are still generally interpreted in conjunction with clinical findings.

Confirmatory testing is available but not generally ordered as it is not rapid enough to be useful for treatment decisions.

Other tests that may be ordered to evaluate the person's health status and distinguish between a drug overdose and another cause for the person's symptoms include:

• Electrolytes, lactate, and blood gases to evaluate the person's acid-base balance and oxygen level
• BUN and creatinine tests to look at kidney function
• Liver panel to evaluate liver function
• Glucose test to determine whether hyper- or hypoglycemia is present

Testing may be ordered when a person presents to the emergency room with acute overdose symptoms. Symptoms will vary depending upon what the person has taken, whether multiple drugs were involved, when the substances were ingested and how much, and may vary over time. Two people taking the same types and quantities of drug(s) may exhibit different symptoms. Signs and symptoms may include:

• Changes in consciousness and behavior, ranging from confusion to agitation, paranoia, panic, hallucinations, delirium, lethargy, seizures, unconsciousness, and coma
• Changes in body temperature: hyper- or hypothermia, with pale or reddened skin, dry or sweaty
• Changes in breathing, signs of acid-base imbalance, difficulty breathing, and respiratory failure
• Changes in heart rate and rhythm and in blood pressure, with hypertension or hypotension
• Nausea and vomiting
• Seizures
• Slurred speech and blurred vision
• Characteristic odors

When a person is known to have ingested a specific substance, that drug test will usually be ordered. If testing reveals an overdose of a therapeutic drug but the time of ingestion is unknown, then the test may be repeated several hours later to determine whether concentrations are still increasing. 

Serum osmolality and the osmolar gap may be ordered when methanol or ethylene glycol ingestion is suspected.

Supporting tests, such as electrolytes, BUN, creatinine, and liver tests, may be ordered initially and at intervals to evaluate imbalances and organ function and to verify that levels have returned to normal.

Results of tests must be carefully interpreted in conjunction with signs and symptoms. There are established therapeutic and potentially toxic levels for many drugs, but people may experience serious side effects or have symptoms of a drug overdose even when concentrations of drugs are at normal or therapeutic levels.

Symptoms may also be affected by other drugs that are present and by the person's age and state of health. In addition, a positive result for a particular substance does not necessarily mean that the symptoms are due to the substance detected.

Drug concentrations may correlate poorly with a person's overdose symptoms. In general, however, most people will not have symptoms when a drug is at therapeutic concentrations and many will have overdose symptoms as they approach toxic concentrations.

It should be noted that established levels and the units that are used to report them will differ slightly from source to source. It is important to evaluate concentrations in accordance with the testing laboratory's reference ranges. Examples of therapeutic and toxic levels for some drugs in adults include:

Sometimes, results from emergency or overdose tests may be used for legal (forensic) purposes in addition to being used to help treat a critically ill person. If such results are to be used in a court of law, then usually sample collection, handling, and testing processes as well as reporting of results are required to follow legal procedures. These may involve some of the following:

• Any samples collected, including blood and urine, are collected into containers that are sealed with a tamper-proof seal.
• Anyone who handles the samples as they are collected, transported, and tested must record their involvement in a document called a "chain-of-custody."
• Preliminary positive results may be reported quickly to facilitate treatment in emergency situations, but then may be later confirmed by a second testing method for legal cases.

Sometimes, medical conditions such as uncontrolled diabetes may produce some of the same symptoms as a drug overdose.

Most of the treatments and antidotes used for drug overdoses must be given within a few hours of the person ingesting the drug. Treatments may work either by preventing the formation of toxic metabolites, such as with acetaminophen, methanol, and ethylene glycol, or by binding to the drug, such as the treatment for a digoxin overdose.

People with drug overdoses must be monitored closely. Changes in consciousness and the ability to breathe unassisted can happen abruptly.

Other procedures may help evaluate a person with suspected overdose or poisoning symptoms. These may include an electrocardiogram (ECG, EKG) to evaluate the heart's rate and rhythm or imaging tests such as magnetic resonance imaging (MRI) or computed tomography (CT) scan.

It is important for the health practitioner to consider what drug interactions might be contributing to a person's symptoms.