Acetaminophen

Also Known As:
Paracetamol

This article was last modified on November 9, 2021

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Drug Testing

At a Glance

Why Get Tested?

To determine if an overdose has occurred and if an antidote is required; to determine risk of liver damage and to help monitor the outcome of antidote treatment

When To Get Tested?

When it is suspected that a person has ingested an overdose of acetaminophen or has signs and symptoms of toxicity, such as nausea, vomiting, and abdominal pain; when following a patient, every 4 to 6 hours after an overdose of the drug is ingested

Sample Required?

A blood sample drawn from a vein

Test Preparation Needed?

None

What is being tested?

Acetaminophen is one of the most common pain relievers (analgesics) and fever reducers (antipyretics) available over the counter. It is generally regarded as safe. However, it is also the most common cause of toxic hepatitis in North America and Europe and one of the most common poisonings from either accidental or intentional overdose.

Once entering your body, like many other medications, acetaminophen is processed in the liver into harmless substances and removed in your urine. However, about 5 to 10% of the absorbed acetaminophen is converted to a toxic, highly-reactive byproduct. Fortunately, your liver makes a antioxidant called glutathione that binds to the byproduct and minimizes its toxic effects. When a large dose of acetaminophen is ingested or that exceeds the recommended amount over a period of time, the liver is no longer able to produce enough protective glutathione; as a result, the toxic byproduct builds up in the liver and may cause severe liver damage if timely treatment is not given.

For this reason, acetaminophen can be harmful or even fatal if not taken correctly and children in particular are at risk if caregivers do not follow dosing instructions carefully. Often, people do not realize that acetaminophen is one of the ingredients in many combination medications such as cold and flu preparations. If two or more of these medications are taken together, levels of acetaminophen may exceed safe limits.

Acetaminophen preparations come in varying strengths and several different forms, including tablets, capsules and liquid.

For adults, the typical maximum daily limit for acetaminophen is 4000 milligrams (mg). Consuming more than 4000 mg in a 24-hour period is considered an overdose, while ingesting more than 7000 mg can lead to a severe overdose reaction unless treated promptly.
For children, the amount that is considered an overdose depends on their age and body weight. (For more on this, see the MayoClinic webpage Acetaminophen and children: Why dose matters.)

If it is known or suspected that someone has ingested an overdose of acetaminophen, it is recommended to take the person to the emergency room. If a health practitioner determines that an overdose has occurred, treatment may include an antidote, N-acetylcysteine (NAC), which can help minimize damage to the liver, especially if given within 8 to 12 hours after an overdose. Though NAC is ideally administered within this timeframe, people who seek treatment more than 12 hours after ingestion may still be given the antidote.

Until recently, NAC for people who visit health care providers later than 24 hours after acetaminophen ingestion was not the standard of care for acetaminophen overdose management in the United States. However, study data from England suggest that NAC may be beneficial for acetaminophen-induced liver failure more than 24 hours after ingestion.

Common Questions

How is the test used?

The test for acetaminophen is used to measure the level of drug in the blood in order to establish a diagnosis of overdose, to assess the risk of liver damage, and to help decide on the need for treatment. Prompt diagnosis and treatment are important for a positive outcome.

When is it ordered?

Health practitioners may order acetaminophen levels beginning at 4 hours after ingestion or possible ingestion and then every 4 to 6 hours to monitor whether the drug level is increasing or decreasing. Samples collected too soon after ingestion may not accurately reflect the amount absorbed from the stomach into the blood.

Testing may be ordered when a person has signs and symptoms of an overdose. These can appear as early as 2 to 3 hours after ingestion or may not occur for 12 or more hours. Some of these include:

Nausea, vomiting, diarrhea
Loss of appetite
Abdominal pain or cramping
Irritability
Sweating

If untreated, toxicity can progress within 3 to 4 days to include jaundice, liver and kidney failure, convulsions, coma and death. If treatment is received within 8 hours of the overdose, however, there is a very good chance of recovery.

For children who have taken acetaminophen in liquid form, a treatment decision may be made as soon as 2 hours after ingestion since the drug is absorbed more rapidly in this form.

What does the test result mean?

The table below summarizes some results that may be seen:

Acetaminophen Level	Result Interpretation
10-20 mcg/mL	Therapeutic levels
Less than 150 mcg/mL 4 hours after ingestion	Low risk of liver damage
Greater than 200 mcg/mL 4 hours after ingestion Or Greater than 50 mcg/mL 12 hours after ingestion	Associated with toxicity and liver damage

The levels discussed above typically apply to a single ingestion of a toxic amount of the drug. They do not necessarily apply to cases in which the recommended amount of acetaminophen has been exceeded over a period of time (chronic overdose ingestion). However, a healthcare practitioner may take acetaminophen levels into account along with clinical signs and symptoms and liver tests to determine the risk and/or presence of liver damage in chronic overdose cases.

Is acetaminophen dangerous if used appropriately?

Acetaminophen is one of the safest, most effective drugs known if used at the recommended dose and dose interval.

Children, particularly those under 6 years of age, are at risk of accidental overdose if caregivers do not follow dosing instructions carefully. However, the reported incidence for children are not as common as in adults. Deaths are typically only seen in cases involving multi-drug combinations.

How is an acetaminophen overdose treated?

Anyone who is showing signs of acetaminophen poisoning should be taken to the emergency room. If it is established that an overdose occurred, the person may be given an antidote (N-acetylcysteine or NAC), which is most effective if given within the first 8 hours following ingestion. The person may also receive other medicines to help treat symptoms.

If it is suspected that some of the drug is still in the stomach (usually within 4 hours after ingestion), the person may be given activated charcoal, which absorbs any residual drug and prevents it from being absorbed into the body. Within about 1 hour of ingestion, someone may receive gastric lavage. It involves pumping liquid, usually water or saline, into the stomach and suctioning the liquid and other stomach contents out through a tube.

Sometimes extensive liver damage occurs despite treatment with the antidote N-acetylcysteine. If liver failure results after an overdose, a liver transplant may be necessary.

In addition to acetaminophen levels, what other lab tests might be done?

Since high levels of acetaminophen can be toxic to the liver, healthcare practitioners may also order tests such as AST and ALT to detect liver damage. A PT may be used to detect impaired liver function. A healthcare practitioner might also order serum salicylate levels or a urine drug screen for unconscious patients or those for whom there is a suspicion of ingestion of other substances. Other possible tests include blood gases, lactate level, and metabolic panel with creatinine. These are used to monitor severity of liver failure and, in cases of severe overdose, determine whether an overdose patient would benefit from liver transplantation.

Women of childbearing age who have suspected acetaminophen overdose may get human chorionic gonadotropin (hCG) tests to determine if they are pregnant because the drug crosses the placenta and can harm the fetus.

Is there anything else I should know?

Acetaminophen is the most common drug ingredient for medicines used in the U.S., including over-the-counter medicines for cold and the flu, allergies and sleeplessness. Consumers can buy many products containing acetaminophen without a prescription. Accidental overdose may happen when multiple medications containing acetaminophen are used simultaneously, resulting in a total combination dose that exceeds the maximum safe limits, especially for those with liver diseases and excessive alcoholic consumption.

If you drink three or more alcoholic beverages each day, ask your health care provider if you should take acetaminophen. If you will be taking more than the occasional 1 or 2 doses of acetaminophen, do not drink alcohol as this may increase the chance of liver damage.

It is important for you to tell your physicians, dentists and other health care providers about all the medications you are taking so that accidental overdose due to combination use can be avoided.

View Sources

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2019 Review performed by Li-Sheng Chen, PhD, DABCC, FACB.

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