Therapeutic level: 10-25 mcg/mL
Toxic level: >200 mcg/mL
The Rumack-Matthew nomogram is used to interpret the acetaminophen level. Interpretation of a toxic level depends on the time since ingestion of acetaminophen.
Acetaminophen level is usually drawn after 4 hours of ingestion of acetaminophen.
Collection and Panels
Red top tube is preferred for collection of the blood sample. A 0.6-2-mL blood sample is collected and serum is separated from the blood. A repeat sample is obtained 3-4 hours after the first sample.
Acetaminophen toxicity is primary cause of drug overdose related–liver failure in the United States.  Acetaminophen is commonly used as an antipyretic and analgesic. The toxic dose is 150 mg/kg. Peak plasma concentrations are seen within 2 hours of ingestion. The half-life is usually 1-3 hours.  Acetaminophen is metabolized by several pathways—conjugated form, glucuronidation, sulfate, and other pathways.
Acetaminophen level is measured by an enzymatic method. In the presence of the enzyme arylacylamidase, acetaminophen is converted to p-aminophenol. In the presence of o -cresol and periodate, p-aminophenol is converted to a blue indophenol chromophore and the absorbance of light of the wavelength 615 nm is measured.
To diagnose acetaminophen overdose
To treat acetaminophen toxicity
To monitor patients with overdose
Gel tube should not be used to collect sample; gel leads to slow diffusion of the acetaminophen.
A half-life of more than 4 hours is suggestive of hepatic damage/necrosis. However, it has also been seen in some patients with minimal liver toxicity.