Unit Code 8698:
Acetaminophen, Plasma
Useful For
Monitoring toxicity in overdose cases
Serum concentration and half-life are the only way to assess
degree of intoxication in early stages since other liver function
studies (e.g., bilirubin, liver function enzymes) will not show
clinically significant increases until after tissue damage has
occurred, at which point therapy is ineffective.
Clinical Information
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 <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.
Reference Values
Therapeutic concentration: <50 ug/mL
Toxic concentration: > or =120 ug/mL
Half-life: <4 hours
Toxic half-life: >4 hours
The toxic level is dependent on half-life. When the half-life is
4 hours, hepatotoxicity generally is not seen until the concentration
is > or = to 120 ug/mL. The level at which toxicity occurs
decreases with increasing half-lives until it is encountered at
values as low as 50 ug/mL when the half-life reaches 12 hours.
Interpretation
Therapeutic concentration: <50 ug/mL
Normal half-life: <4 hours
Toxic concentration: > or =120 ug/mL
Toxic half-life: >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
> or =120 ug/mL. The level at which toxicity occurs decreases with
increasing half-life until it is encountered at values as low as 50 ug/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
First specimen should be drawn no sooner than 2 hours
post-ingestion.
Clinical Reference
Rumack BH, Peterson RG: Acetaminophen overdose:
incidence, diagnosis, and management in 416 patients.
Pediatrics 1978;62:898-903
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