Acetaminophen

Pharmacokinetic factors influence the correct time of sample collection after the last drug dose. These factors included dosage form, mode of administration, concomitant drug therapy, and biological variations affecting drug disposition.
Draw a sample at least 4 hours after drug ingestion to ensure that the plasma or serum concentrations have peaked. Ingestion of massive quantities of acetaminophen or of a modified-release preparation may result in delayed peak serum acetaminophen levels. In such cases, repeated serum concentrations should be obtained.
If the time of ingestion is not known, the acetaminophen half-life, an indicator of potential hepatotoxicity, may be estimated by drawing 2 or more blood samples at intervals of 2 to 3 hours.

Serum/plasma should be separated from the cells and tested immediately after collection.
Samples are considered stable up to 2 weeks when stored at 2 - 8^o C and stable 45 days when stored frozen at <= -20^o C.

Acetaminophen is a widely used analgesic and antipyretic found in a number of over-the-counter and prescription products. When consumed in overdose quantities, acetaminophen may cause severe liver and kidney damage, or death.
Patients may have few or no symptoms early after acute overdose of acetaminophen. The only reliable early diagnostic indicator is provided by a quantitative measurement of the serum drug level. Clinical evidence of liver and kidney damage is usually delayed for 24 hours or more after ingestion. This is well past the time when a prophylactic antidote, acetylcysteine, can be effectively administered. Acetylcysteine is highly effective in preventing liver damage, especially if administered within 8 to 10 hours after overdose, and improves survival in patients with hepatic failure when initiated 12 to 16 hours after overdose.
Measurement of serum acetaminophen may also be used to estimate the drug elimination half-life. Serum half-life is recommended when the time of the ingestion is not known. Acetaminophen half-life is used to judge toxicity and may be a better predictor of hepatotoxicity than a single serum measurement.