The Rumack-Matthew nomogram data, which this calculation tool is based upon, applies ONLY to a single acute ingestion of acetaminophen (paracetamol) that is not a sustained release formulation (e.g. 8 hour Tylenol or similar). 

It is also necessary to know the time since the acute ingestion.  Levels obtained <4 hours after ingestion are not useful (these are usually higher then 4 hour levels) and the nomogram tracks levels from 4 hours post ingestion to 24 hours post ingestion.

There are 3 groups present on this nomogram:

• No hepatic toxicity (levels below those known to cause hepatic toxicity)
• Possible hepatic toxicity
• Probable hepatic toxicity

Approximately 60% of patients in the probable hepatic toxicity group will develop at least some degree of hepatic toxicity.  The possible hepatic toxicity group allows for some margin of error as the timing of ingestion may not precisely be known and there may be variations in laboratory methods that introduce at least some minor error.

Conventional Units micrograms/mL

SI Units µmol/L

Notes

• Consult your regional poison center
• Levels earlier than 4 hours post ingestion, typically not useful
• In patients with a good history of toxic ingestion, presenting >4 hours from ingestion; often recommend giving initial dose of antidote (N-acetylcysteine) while awaiting level
• These levels are based upon a single ingestion and one knowing when that ingestion occurred
• These levels do not apply to sustained release formulations of acetaminophen or paracetamol
• In patients taking acetaminophen/paracetamol in usual therapeutic doses; levels are 10-30 mcg/mL (66-200 µmol/L)

Reference:

Rumack BH, Matthew H. Acetaminophen poisoning and toxicity. Pediatrics. 1975;55(6):871-6.