Patient Test Information


  • 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?


  • 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 healthcare 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.

  • 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?

    Healthcare 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


    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 healthcare 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 healthcare providers about all the medications you are taking so that accidental overdose due to combination use can be avoided.

  • View Sources

    Sources Used in Current Review

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

    (Last updated 1/31/2017) MedlinePlus Medical Encyclopedia. Acetaminophen overdose. Available online at Accessed January 2019.

    (Last Revised 04/15/2017) MedlinePlus Drug Information. Acetaminophen. Available online at Accessed January 2019.

    (Last updated 3/29/2017) MayoClinic. Acetaminophen and children: Why dosage matters. Available online at Accessed January 2019.

    "Get Dental Pain Relief Safely: Tell Your Dentist About the Medications You are Taking." ADA patient education brochure (released July 2017).

    2016 Annual Report of the American Association of Poison Control Centers' National Poison Data System (NPDS): 34th Annual Report Clin Toxicol (Phila). 2017 55(10):1072-1252.

    The Toxicology Investigators Consortium Case Registry, J Med Toxicol. 2018 Sep;14(3):182-211.

    PharmGKB summary: Pathways of acetaminophen metabolism at the therapeutic versus toxic doses. Pharmacogenet Genomics. 2015 25(8): 416–426.

    Sources Used in Previous Reviews

    MedlinePlus Medical Encyclopedia. Acetaminophen overdose. Available online at Accessed August 2010.

    MedlinePlus Drug Information. Acetaminopen. Available online at Accessed August 2010. Acetaminophen. Available online at Accessed August 2010.

    ARUP Lab Tests. Acetaminophen. Available online at Accessed August 2010.

    Dart, R. et. al. (© 2006). Acetaminophen Poisoning: an Evidence-Based Consensus. Guideline for Out-of-Hospital Management. Clinical Toxicology 44:1-18, 2006 [On-line information]. PDF available for download at

    Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1238-1239.

    Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 470-471.

    Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. St. Louis: Elsevier Saunders; 2006, Pp 1807, 1304-1305.

    Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, P. 316.

    Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL eds, (2005) Harrison's Principles of Internal Medicine, 16th Edition, McGraw Hill, P. 1840-1841.

    Wu, A. and McKay, C., Editors (© 2003). Recommendations For The Use Of Laboratory Tests To Support Poisoned Patients Who Present To The Emergency Department. The National Academy of Clinical Biochemistry Laboratory Medicine Practice Guidelines, SECTION IV. Recommendations on Laboratory Assays for Other Toxicants as Causes of Poisonings: A. "Universal" acetaminophen and salicylate screening [On-line information]. Available online at Accessed August 2010.

    Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 361-364, 684-685.

    (Sept 23 2009) Farrell S. Toxicity, Acetaminophen: Treatment and Medication. Medscape article. Available online at Accessed August 2010.

    U.S. Food and Drug Administration. Press release: FDA recommends health care professionals discontinue prescribing and dispensing prescription combination drug products with more than 325 mg of acetaminophen to protect consumers. Available online at Released January 14, 2014. Accessed Aug 4, 2014.

    MedlinePlus. National Institutes of Health. Acetaminophen. Available online at Last reviewed January 1, 2014. Accessed August 3, 2014.

    Mayo Clinic. Acetaminophen and children: Why dose matters. Available at Last updated April 13, 2014. Accessed August 4, 2014.

    Susan E Farrell and Germaine L Defendi. Acetaminophen Toxicity Workup. Medscape. Available online at Updated June 24, 2014. Accessed August 4, 2014.

    Beauchamp G, Performance of a Multi-disciplinary Emergency Department Observation Protocol for Acetaminophen Overdose. J Med Toxicol. Sep 2013; 9(3): 235–241. Available online at Accessed September 2014.

    (May 7, 2013) Patolia S. Acetaminophen Level. Medscape. Available online at Accessed September 2014.