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 having taken an overdose
• 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 follow the effectiveness of treatment and/or the level of the drug or substance in the blood or body.

Results from emergency and overdose testing are mainly used for treatment. If results are needed for legal cases, for example, finding the cause of death or if drugs had contributed to the cause of an accident, then special legal (forensic) procedures for the collection, handling, testing of samples, and reporting of results must be followed. A variety of different approaches may be used to investigate someone who is thought to have taken an overdose or ingested a toxic substance. The tests requested in individual cases will depend on what will be useful to the A&E doctor in guiding appropriate treatment. These tests must be available rapidly. In many cases, treatment is directed to specific clinical problems, and knowledge of the actual toxin present does not influence the treatment. Selection of appropriate tests may be influenced by how quickly they can be done, the test’s sensitivity and specificity, the ability to provide quantitative results, the availability of treatment, and the potential for treating the patient's symptoms in the A&E department.

One approach, recommended by the UK National Poisons Information Service (NPIS) and the Association for Clinical Biochemistry and Laboratory Medicine (ACB) uses two tiers of testing. The first tier is a group of toxicology tests that should be available,24 hours a day, in all hospitals that admit patients with acute poisoning. These tests are:

• Paracetamol - a blood test is often requested because overdoses are common and can be missed, since the affected person may have little or no symptoms. There is a specific treatment for paracetamol overdose, but it must be given within a few hours of taking the overdose to be effective.
• Salicylate (Aspirin) - this test may be used if a person has symptoms of an overdose, an acid base imbalance, or is believed to have taken a lot of the drug.
• Ethanol (ethyl alcohol) - a breath test is usually done at the point of care such as in the A&E department; a blood test might be sent to a laboratory.
• Carboxyhaemoglobin – measured urgently in all patients with suspected carbon monoxide poisoning (from car exhausts, smoke inhalation or defective gas heaters).
• Iron tests - usually only used when overdose of iron tablets is suspected in a child or infant.

The A&E doctor's choice of tests will be guided by the person's symptoms and the clinical history. Other first tier tests are some drugs that can cause toxicity at levels only slightly higher than those needed to treat disease. They include:

• Carbamazepine
• Digoxin
• Lithium
• Theophylline

Typically these tests would only be requested if a person was regularly taking the drug or was known to have ingested the drug e.g. by taking someone else’s tablets.

The second tier of toxicology tests includes a range of specialist or infrequently required tests, which are normally not required with the same degree of urgency and may be referred to regional specialist toxicology centres. Such tests include methanol and ethylene glycol (see below), poisonous metals such as arsenic, lead, and mercury, acetylcholinesterase(in cases of organophosphate insecticide/herbicide poisoning), paraquat, phenobarbital, and therapeutic drugs like methotrexate and thyroxine.

• Methyl alcohol (Methanol) - a test for methanol is not often available in most laboratories, so if methanol poisoning is suspected, a serum osmolality test may be done. An unexpectedly high serum osmolality may indicate that ethanol, methanol, or ethylene glycol are present in the blood.
• Ethylene glycol (antifreeze) - a serum osmolality test may be done as an indirect check as above.

Second tier testing may also include testing for several groups of commonly abused drugs, such as cocaine, opiates, amphetamines, barbiturates, and tricyclic antidepressants. These tests are not for specific drugs but for drug classes and must be interpreted carefully in conjunction with clinical findings. In rare and very severe cases of poisoning or in critically ill patients where no diagnosis has been made, a comprehensive screen for a wide range of drugs may be requested, but this is a complex and time-consuming procedure which rarely contributes to treatment.

Other tests may be requested along with the drug test to help make a diagnosis and distinguish between a drug overdose and other causes of the person's symptoms, or to guide appropriate treatment. These tests may include:

• Electrolytes and blood gases  for acid-base or oxygen investigations
• Urea and creatinine tests to determine kidney function
• Liver function tests to check at the liver function
• Glucose test to see if the blood glucose concentration is too high or too low

One or more emergency drug overdose tests may be requested when a person appears in the A&E department with a history or symptoms of acute overdose. Symptoms will vary depending upon what the person has taken, whether lots of drugs are involved, when and how much of the substances were taken, and over what period of time. Two people who have taken the same types and quantities of drug(s) may have different symptoms.  Signs and symptoms may include:

• Changes in consciousness and behaviour, ranging from confusion to agitation, paranoia, panic, hallucinations, hallucination, tiredness, seizures, unconsciousness, and coma
• Changes in body temperature: high or low temperature, with pale or reddened skin, dry or sweaty
• Changes in breathing, difficulty breathing, signs of acid base imbalance
• Changes in heart rate and rhythm and in blood pressure, with high (hypertension) or low (hypotension) blood pressure
• Feeling sick and vomiting

When a person is known to have taken a particular substance, then a test for that drug may be sometimes requested if it will help with treatment. For an overdose of a therapeutic drug in cases where the time it was taken is unknown, then the test may be repeated several hours later to see whether its concentration is still increasing. In most cases, drug concentrations are not measured to check whether treatment is working.

Serum osmolality  may be used as an indirect indicator whenever methanol or ethylene glycol ingestion is suspected.

Supporting tests, such as electrolytes, urea, creatinine, and liver test may be used initially and at intervals to check that levels have returned to normal.

The results of tests must be interpreted carefully in conjunction with the patient's 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 usual therapeutic levels. Symptoms may also be affected by other drugs that are present, by the person's age and state of health and whether the patient is on regular therapy or has never taken the drug before. 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 present in therapeutic concentrations and many will have overdose symptoms as they approach toxic concentrations. Suggested therapeutic and toxic levels and the units used to report drug tests may differ slightly from laboratory to laboratory. Drug concentrations must be interpreted using the testing laboratory's ranges.

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

• Any samples collected, including blood and urine, are clearly identified and 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 form."
• Preliminary positive results may be reported quickly to facilitate treatment in emergency situations, but then may later need to be confirmed by a second testing method for legal cases.

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

Many of the treatments and antidotes that are used must be given within a few hours of the person ingesting the drug. They may work by either preventing the formation of toxic metabolites, such as with paracetamol, methanol, and ethylene glycol, or by inactivating the drug, such as the treatment for a digoxin overdose.

Patients with drug overdoses must be monitored closely. Changes in consciousness levels and/or breathing and heart rate can happen abruptly.

Other procedures may help assess a person with suspected overdose or poisoning symptoms. These may include an electrocardiogram (ECG) to evaluate the heart's rate and rhythm or imaging tests such as resonance imaging (MRI) or computed tomography (CT) to look for head injuries which may have caused loss of consciousness.

It is important for the doctor to consider what drug interactions might be contributing to a person’s symptoms. Also, knowing that a specific drug has been taken does not exclude the possibility that other substances have also been ingested.