Acetaminophen Level 

Updated: May 07, 2013
  • Author: Setu K Patolia, MD, MPH; Chief Editor: Eric B Staros, MD  more...
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Reference Range

Therapeutic level: 10-25 mcg/mL

Toxic level: >200 mcg/mL

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Interpretation

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.

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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.

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Background

Acetaminophen toxicity is primary cause of drug overdose related–liver failure in the United States. [1] 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. [2] 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.

Indications/Applications

To diagnose acetaminophen overdose

To treat acetaminophen toxicity

To monitor patients with overdose

Considerations

Gel tube should not be used to collect sample; gel leads to slow diffusion of the acetaminophen.

Liver toxicity usually peaks at 72 hours, but the serum level of liver enzymes has been described to start rising as early as 24 hours after the ingestion. [3, 4, 5]

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. [6]

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