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Lab Test: Acetaminophen (Tylenol) Level

    Lab Test

    Acetaminophen (Tylenol) Level

    • Other names:  Paracetamol blood level, paracetamol blood measurement
    • Note:  Paracetamol is another name for acetaminophen, which is used outside of the United States.
    Description
    • Measurement of acetaminophen level in plasma or serum for the evaluation and management of potential toxicity or suspected/known suicide attempt.
    Reference Range
    • Adults and children:  10-30 mcg/mL (66-199 micromole/L)

      Toxic Values:

    • 4 hours after ingestion = > 140 - 150 mcg/mL
    • 12 hours after ingestion = > 50 mcg/mL
    • Note:  A failure of the level to decrease by more than 50% after a repeat check in 4 hours from baseline would still be concerning for toxic metabolite (NAPQI) formation that might warrant treatment. 
    Indications & Uses
    • Suspected acute acetaminophen (paracetamol) poisoning or overdose.
    • In children with a known single acetaminophen ingestion of less than 140 to 200 mg/kg, and adults with less than 7.5 g, and the presence of reliable information, generally does not require assessment for toxicity.
    • Acetaminophen levels obtained 4-15 hours after a single ingestion - most predictive of potential hepatotoxicity when applied to the Rumack-Matthew nomogram.
    • Acetaminophen levels above the 'possible risk' line (i.e., the line that extends from 140 mg/L at 4 hours to 50 mg/L at 10 hours) on a modified Rumack-Matthew nomogram is predictive of potential hepatotoxicity and need for n-acetylcysteine.
    Clinical Application
    • Blood levels of acetaminophen peak usually between 30 minutes to 2 hours of ingestion of a therapeutic dose.  However, acetaminophen absorption may be slower in neonates possible secondary to prolonged gastric emptying and prolonged in coingestion of agents known to slow gastric motility (opioid analgesics, diphenhydramine.
    • When the phase 2 pathways of acetaminophen toxicity get saturated, acetaminophen will then get metabolized via the phase 1 pathway, CYP2E1, in the liver and a small amount in the kidneys to generate the toxic metabolite "NAPQI" (N-acetyl-p-benzoquinonimine).  NAPQI is usually neutralized by glutathione, but once the utilization of glutathione has exceeded its production NAPQI will then bind to cellular components in the liver and kidney leading to their destruction after 24-48 hours from ingestion.
    • Do not delay N-acetylcysteine (NAC) therapy for lack of an acetaminophen level, especially if the time of consumption is not known, if the patient is unreliable, and/or altered mental status (late in acetaminophen toxicity some patients can develop encephalopathy). 
    • Administer loading dose, then discontinue NAC if the level comes back below the treatment line on the Rumack-Matthew nomogram.  Do not discontinue NAC therapy if the initial acetaminophen level is above the treatment line and subsequent levels fall below the treatment line. 
    • Standard protocol - oral NAC started within 8 hours of ingestion and dosed every 4 hours for 72 hours or over 21 hours if given by IV. 
    • Alterative protocol includes stopping NAC therapy if serial serum acetaminophen levels become undetectable and there is no evidence of hepatotoxicity (ie, elevated AST or ALT level) at 36 hours following ingestion. 
    • Consider evaluating for other co-ingestions (e.g., salicylates, tricyclic antidepressants, ethanol, ethylene glycol, methanol) especially in patients with known or suspected suicide attempt. 
    • Since patients with acetaminophen toxicity can develop not only liver failure, but also renal failure, coagulopathy due to liver failure, and acid base disturbances, consider monitoring for the following additional labs (serum creatinine, urinalysis, lactic acid level, arterial blood gas, venous blood gas).  Serial labs will likely be needed to determine the clinical course of the patient.
    Related Tests
    • Arterial blood gas
    • Lactic acid
    • Liver enzymes (AST and ALT)
    • Partial Thromboplastin Time (PTT)
    • Prothrombin time (PT)
    • Salicylate level
    • Serum creatinine
    • Venous blood gas
    Drug-Lab Interactions
    • Unknown
    Test Tube Needed
    • 7 mL of whole blood in solid red top tube
    • Some labs may only need as little as 1 mL (depends on assay)
    • Do not use a serum separator tube
    Procedure
    • Regular or standard venipuncture
    • If time of ingestion is known, draw lab 4 hours after so that it can be plotted on the nomogram
    Storage and Handling
    • Pre-lab setting:  nothing specific. Send to lab as soon as possible after obtaining
    • In the lab:  Once the blood has clotted at room temperature, the red top tube is then centrifuged within 2 hours of receipt and the serum or plasma is transferred into vials for testing or freezing.
    What To Tell Patient Before & After
    • No preparation needed by the patient.
    References
      1. Halcomb SE et al.  Pharmacokinetics effects of diphendyramine or oxycodone in stimulated acetaminophen overdose. Acad Emerg Med 2005;12(2):169-172.  PMID: 15692141
      2. LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
      3. Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.
      4. Rumack BH et al. Acetaminophen poisoning and toxicity.  Pediatrics 1975;55(6):871-876.  PMID 22320209 
      5. Rumack BH et al.  Acetaminophen toxicity: the first 35 years. J Toxicol Clin Toxicol 2002;40(1):3-20.  PMID:  11990202

    MESH Terms & Keywords

    • Acetaminophen Level, Tylenol Level, Acetaminophen Lab Test