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What is being tested?

Emergency and overdose drug testing is requested for single drugs or groups of drugs by an A&E or Intensive Care doctor to detect, assess, and monitor someone who has taken a drug overdose. A drug overdose may be caused by a variety of prescription and over-the-counter (OTC) medicines, illegal drugs, and household substances. Once inside the body, these substances are often broken down by the liver and removed in the urine.

Some substances only cause symptoms if they are present at high levels or are above the preferred levels used for treatment. Some common examples of these include:

  • Paracetamol - an ingredient in many over-the-counter preparations. It can cause serious liver damage if recommended doses are exceeded.
  • Aspirin (also known as salicylate) - at higher levels it can cause an acid-base imbalance
  • Other therapeutic drugs – for example those used to treat conditions such as heart failure and epilepsy

Some substances can cause symptoms at both low and high levels depending on the user. Long term users of drugs such as alcohol and illegal drugs (drugs of abuse) may be able to tolerate more drug than someone who has taken it for the first time.

Other substances are toxic at any concentration and some have toxic breakdown products (metabolites). Examples of these toxins include:

  • Methanol
  • Ethylene glycol (antifreeze)

Ingesting a variety of other drugs and chemicals may cause rapid toxicity. This article only describes a small number of drugs and other substances which are more commonly tested for in the clinical laboratory. Some of the groups of substances that may be tested are listed below:

Prescription and Over-the-counter (OTC) Medications

Overdoses caused by prescription and OTC drugs may be due to:

  • Taking too much of a medicine
  • Interaction of several drugs which have been taken
  • A reduction in the body's ability to remove a drug and/or its breakdown products. For many drugs, the liver changes the drug into a different form (metabolite), which is then removed by the kidneys through the intestines from the body into the urine or faeces. If the liver or kidneys are not working properly, then the drug and/or its metabolites may build up in the body.

A good example of a drug which has a toxic breakdown product is paracetamol, a common pain reliever that is also present in a variety of other prescription medicines. One of the breakdown products of paracetamol is toxic to the liver, but the liver can detoxify the small amounts formed after normal doses of paracetamol have been taken. However, if someone takes more paracetamol than the liver can process, then the toxic product builds up, damaging the liver and, in some cases, causing liver failure.

Illegal Drugs
Overdoses of illegal drugs can also occur. The particular illicit drugs seen in the A&E department will depend on the extent of their use in the local community and on their ability to cause acute symptoms alone or in combination with other substances. Certain drugs, such as cannabis/marijuana, can linger in the body for days to weeks but rarely cause overdose symptoms. Other substances, such as gamma-hydroxybutyrate (GHB), can cause acute symptoms such as loss of consciousness but are removed so rapidly that testing for them is rarely useful. For more information on these, see the article on Drugs of Abuse Testing.

Household Substances
There are a wide variety of household substances that may be abused or accidentally ingested. Those commonly seen include methanol, isopropyl alcohol, and ethylene glycol (antifreeze), which may be used by some people as substitutes for ethyl alcohol. Other poisons, such as rodenticides, aerosol and cleaning products, insecticides, and metals such as lead and mercury can also have toxic effects.  For more information in emergency, contact the Poisons Information Centre.

How is it used?

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 something toxic. The tests requested in individual cases will depend on what will be useful to the A&E doctor in guiding appropriate treatment, and must be available rapidly. In many cases, treatment is directed to particular clinical problems, and the particular toxin present does not influence the treatment. Selection of tests may be influenced by how quickly they can be done, test 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.

Typically two tiers of testing are available 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 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 iron overdose 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 and the osmolar gap calculated from this is increased when ethanol, methanol, or ethylene glycol are present in the blood.
  • Ethylene glycol (antifreeze) - a serum osmolality test may be done as an indirect check on whether alcohols such as ethanol, methanol, or ethylene glycol are present in the blood.

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 particular drugs but for drug classes and must be interpreted carefully in conjunction with clinical findings. In rare and very severe cases of poisoning, a comprehensive screen of a range of drugs may be requested, but this is a complex and time-consuming procedure which rarely contributes to helping 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. 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

When is it requested?

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, signs of acid base imbalance, difficulty breathing
  • 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 test may be requested to 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 measure 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.

What does the result mean?

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 normal 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. 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.

Is there anything else I should know?

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 usually rigorous 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 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 breathing 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.

Common questions

  • Why would an A&E doctor want to know about all other medications and supplements that a person has taken when it is known that they have taken a large quantity of a specific drug?

It is important for the doctor to consider what drug interactions might be contributing to a person’s symptoms.

  • How would an A&E doctor evaluate an unconscious person or a child who is unable to tell them what has been taken?

The doctor would base their selection of tests based upon the person’s clinical findings and information from ambulance personnel or paramedics. They would consider an overdose but also consider other medical conditions. If the results of initial testing (such as tier one tests) do not show the cause of the person’s symptoms and their status is unchanged, then the doctor may request additional tests to further investigate the cause.

  • Why might an A&E doctor test a person for paracetamol if they say that they have not taken it?

The person might not realise that one or more of the drugs that they are taking have paracetamol in them and the person may not initially have symptoms. However, it is important to detect the presence of an overdose of paracetamol so that the proper treatment can be given as soon as possible after ingestion

Last Updated: Thursday, 1st June 2023

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