Delayed Toxic Acetaminophen Level after Initial Four Hour Nontoxic Level

Antidotal therapy for acetaminophen poisoning is routinely based on a single acetaminophen level obtained four or more hours after ingestion. Some experts recommend additional acetaminophen levels if there are coingestants. This case report describes a 20-year-old woman who ingested acetaminophen 13 g, propoxyphene napsylate 2 g and naproxen sodium 3.75 g. A 4.5 h acetaminophen level was 83.5 mg/L (nontoxic). A 6.75 h acetaminophen level was 124.6 mg/L (toxic). The patient was treated with N-acetylcysteine and recovered without sequelae. This is the first published report of a delayed toxic acetaminophen level occurring after an initial nontoxic level. Although rare, the possibility of a delayed peak acetaminophen level merits consideration, particularly with coingestions that delay gastric emptying.