Test Catalog

Test ID: ACMA    
Acetaminophen, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Monitoring toxicity in overdose cases

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Acetaminophen (found in Anacin-3, Comtrex, Contac, Datril, Dristan, Excedrin, Nyquil, Sinutab, Tempera, Tylenol, Vanquish, and many others) is an analgesic, antipyretic drug lacking significant anti-inflammatory activity. It is metabolized by the liver with a normal elimination half-life of less than 4 hours. In normal therapeutic doses, a minor metabolite, possessing electrophilic alkylating activity, readily reacts with glutathione in the liver to yield a detoxified product. In overdose situations, liver glutathione is consumed and the toxic metabolite (postulated metabolite: benzoquinone) reacts with cellular proteins resulting in hepatotoxicity, characterized by centrilobular necrosis and possible death if untreated. N-acetylcysteine can substitute for glutathione and serves as an antidote.


Serum concentration and half-life are the only way to assess degree of intoxication in early stages since other liver function studies (eg, bilirubin, liver function enzymes) will not show clinically significant increases until after tissue damage has occurred, at which point therapy is ineffective.

Interpretation Provides information to assist in interpretation of the test results

The normal half-life is less than 4 hours, while the toxic half-life is greater than 4 hours.


The toxic level is dependent on half-life. When the half-life is 4 hours, hepatotoxicity generally will not occur unless the concentration is above 150 mcg/mL. The level at which toxicity occurs decreases with increasing half-life until it is encountered at values as low as 50 mcg/mL when the half-life reaches 12 hours.


For half-life determination, draw 2 specimens at least 4 hours apart and note the exact time of each draw. Half-life can be calculated from the concentrations and the time interval.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

First specimen should be drawn no sooner than 2 hours postingestion.

Clinical Reference Recommendations for in-depth reading of a clinical nature

Rumack BH, Peterson RG: Acetaminophen overdose: incidence, diagnosis, and management in 416 patients. Pediatrics Nov 1978;62:898-903