Acetaminophen, Plasma

CPT CODE:

USEFUL FOR:

Monitoring toxicity in overdose cases
Serum concentration and half-life are the only way to assessdegree of intoxication in early stages since other liver functionstudies (e.g., bilirubin, liver function enzymes) will not showclinically significant increases until after tissue damage hasoccurred, at which point therapy is ineffective.

SPECIMEN REQUIRED:

Draw blood in a lavender-top (EDTA) tube(s). Spin down and send 1 mL of EDTA plasma.

TRANSPORT TEMPERATURE:

Ambient\Refrig OK\Frozen OK

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 normalelimination 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.

CLINICAL 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 withincreasing half-life until it is encountered at values as low as 50 ug/mLwhen the half-life reaches 12 hours.
For half-life determination, draw 2 specimens at least 4 hours apartand note the exact time of each draw. Half-life can be calculated from the concentrations and the time interval.

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.