Acetaminophen level

Acetaminophen level assay is a quantitative analysis of the drug acetaminophen and its metabolites in blood
Acetaminophen is primarily used as an antipyretic and analgesic medication
Acetaminophen is one of the most common medications involved in intentional and accidental toxicity/poisoning in all age groups
Potentially fatal hepatotoxicity may occur as a result of accumulation of reactive and toxic metabolites from either acute or chronic acetaminophen overdosage, particularly in the presence of liver disease or decreased drug metabolism and excretion
Ingestion of 150-200 mg/kg corresponds to a toxic dose
ALT and AST usually take 24-48 hours following ingestion to rise
Extended release (e.g. Tylenol 8 hour, Tylenol Arthritis 8 hour and generic equivalent preparations) has complicated the assessment of acetaminophen ingestions

Clinical ⬆ ⬇

To determine toxicity/overdose/poisoning
Aids in therapeutic monitoring of use of acetaminophen
To evaluate and monitor for undesirably high levels after use of standard doses in conditions such as hepatic or renal disease
To evaluate hepatotoxicity if ingested with alcohol or other drugs of abuse
Evaluation of undiagnosed coma or where history is unreliable

Patient age (Children seem to tolerate a somewhat higher level without hepatic damage in acute ingestions)
Patient weight compared to mg ingested
Interacting medications
Potential use of sustained release acetaminophen preparations
Electrolyte balance
Hepatic or renal disease
Protein levels
Water balance
Conditions that affect absorption and excretion
Use of foods, herbals, vitamins, and minerals that can potentiate or inhibit the intended target concentration

Acetaminophen intoxication may present in 4 clinical phases (single acute overdose):

Asymptomatic in the hours following ingestion in most cases
Presents with anorexia, nausea, vomiting, malaise, diaphoresis, lethargy, and pallor
May be asymptomatic in some cases

Presents with pain and tenderness in the right upper quadrant (hepatomegaly), tachycardia, and hypotension
Elevated levels of transaminases (ALT/AST), prothrombin time, INS, and bilirubin values

Presents with anorexia, nausea, vomiting, malaise, symptoms of hepatic failure with jaundice, hypoglycemia, coagulation disorders, or encephalopathy
May lead to renal failure, cardiomyopathy, cerebral edema, sepsis, or multiorgan failure
AST, ALT, bilirubin, and INR levels peak

Either fulminant hepatic failure continues and death ensues (or liver transplant obtained) OR
The physical findings resolve with partial or complete recovery of liver function

Chronic use of acetaminophen is usually associated with increased risk of end-stage renal disease and hepatotoxicity
Peak serum concentrations of acetaminophen should be interpreted with caution in delayed or extended release form of acetaminophen overdosage

Additional information

Healthy children of the age group 1-6 years are less susceptible to acute acetaminophen toxicity or poisoning than adults
Acetaminophen is available in more than 200 over the counter (OTC) and prescription medications as a single agent or combined with other drugs in various formulations (liquids, tablets, capsules, and suppositories)
This drug is usually absorbed from the upper gastrointestinal tract and reaches a peak concentration by 0.5-2 hours after a therapeutic dose or 2-4 hours after an overdose. More than 90% is metabolized in the liver and excreted in urine, and only 2% is excreted unchanged by the kidneys
Treatment with the antidote N-acetylcysteine should be initiated within 8 hours of overdose and should be based upon the Rumack-Matthew nomogram
High performance liquid chromatography (HPLC) method is preferred as it is less susceptible to interference than some enzyme-based assays
Factors interfering with test results include:

Nl Result ⬆ ⬇

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

High Result ⬆ ⬇

Conditions associated with elevated levels of acetaminophen include:

Low Result ⬆ ⬇

Low levels of acetaminophen can indicate noncompliance with the therapeutic regimen
A low result in suspected acetaminophen ingestion may indicate inaccurate assessment of timing or quantity of ingestion; or that the report of ingestion is erroneous

Drugs that may decrease acetaminophen levels include:

References ⬆

ARUP Laboratories®. Acetaminophen. [Homepage on the internet]©2007. Last accessed on May 29, 2007. Available at URL: http://www.aruplab.com/guides/ug/tests/0090001.jsp
BC Drug and Poison Information Centre. Acetaminophen Poisoning, Practice Update. [Homepage on the Internet] ©2002. Last accessed on May 29, 2007. Available at URL: http://www.bccdc.org/downloads/pdf/epid/reports/05_v1_APAP.pdf
eMedicine from WebMD®. Toxicity, Acetaminophen. [Homepage on the Internet] ©1996-2007. Last updated on May 15, 2007. Last accessed on May 29, 2007. Available at URL: http://www.emedicine.com/ped/topic7.htm
Laboratory Corporation of America®. Acetaminophen (Tylenol®), Serum. [Homepage on the internet]©2007. Last accessed on May 29, 2007. Available at URL:http://www.labcorp.com/datasets/labcorp/html/chapter/mono/td000100.htm
Mahadevan SB et al. Paracetamol induced hepatotoxicity. Arch Dis Child. 2006 Jul;91(7):598-603. Epub 2006 Mar 17.
Morgan OW et al. Interrupted time-series analysis of regulations to reduce paracetamol (acetaminophen) poisoning. PLoS Med. 2007 Apr;4(4):e105.
Parra D et al. The effect of acetaminophen on the international normalized ratio in patients stabilized on warfarin therapy. Pharmacotherapy. 2007 May;27(5):675-83.