How this instrument works
The Rumack-Matthew nomogram is the tool emergency clinicians use to decide whether a measured blood acetaminophen level, drawn at a known time after a single acute overdose, is high enough to warrant treatment with N-acetylcysteine. This calculator reproduces the treatment line clinicians plot against: it starts at 150 mcg/mL at 4 hours post-ingestion and falls by half roughly every 4 hours after that, following the drug's typical elimination rate. A measured level at or above that line at the corresponding time indicates possible toxicity requiring treatment.
This is a narrow, specific tool, and it only works under narrow, specific conditions. The nomogram applies ONLY to a single, acute ingestion of a known amount of immediate-release acetaminophen at a known, single point in time, with the level drawn no earlier than 4 hours after that ingestion. It does NOT apply — at all — to extended-release acetaminophen products, staggered or repeated supratherapeutic dosing over hours or days, chronic ingestion, or any situation where the ingestion time is unknown or uncertain. Using this calculator's line to reassure yourself in any of those situations is not just unhelpful, it's actively unsafe: real toxicity can be present even when a level would plot 'below the line' under those circumstances.
The 150 mcg/mL figure at 4 hours is the modern US clinical treatment line, deliberately set 25% below the original 1975 Rumack-Matthew study's 200 mcg/mL line, reflecting the lower threshold used in later FDA-required treatment studies for N-acetylcysteine. Both the original nomogram and this line are read on a semi-logarithmic scale in clinical practice; this calculator's formula is a direct closed-form re-derivation of that same line, validated in print by Rumack himself.
- Enter hours since ingestion (the nomogram only applies from 4 hours onward — earlier levels can't be plotted).
- Enter the measured acetaminophen level in mcg/mL from a blood draw at that time.
- Read the treatment line value at that time point, and whether the measured level falls at or above it.
- Treat this only as a reference for a single, known-time, immediate-release ingestion — get emergency evaluation regardless of what this shows.
Worked example — level drawn at 8 hours
At exactly 4 hours post-ingestion, the treatment line is 150 mcg/mL. A measured level of 200 mcg/mL at 4 hours sits above that line — a result indicating possible toxicity requiring treatment, which is why this scenario is used as this calculator's reference check.
By 8 hours — one 4-hour half-life past the 4-hour mark — the line has fallen to 75 mcg/mL (150 × 0.5). A measured level of 60 mcg/mL at 8 hours falls below that line. By 16 hours, three half-lives past 4 hours, the line has dropped further to about 18.75 mcg/mL, low enough that even a level of 25 mcg/mL — which would have been reassuring at an earlier time point — sits above the line at that hour. This is exactly why the same numeric level means different things depending on when it was drawn, and why the timing has to be known and accurate for the nomogram to mean anything at all.
Questions
What should I do right now if I think someone has overdosed on acetaminophen?
Call Poison Control immediately at 1-800-222-1222 (US) or call 911 / go to an emergency department. Do this before, not instead of, using any calculator — acetaminophen overdose can cause serious, delayed liver injury that isn't obvious from how someone feels in the first several hours, and treatment is time-sensitive.
Does this nomogram apply to extended-release acetaminophen?
No. The Rumack-Matthew nomogram was derived from and validated for immediate-release acetaminophen only. Extended-release formulations absorb differently over time, and a level that looks reassuring on this line can understate the real risk. Any extended-release ingestion needs direct guidance from Poison Control or a clinician, not this calculator.
What if the doses were staggered over several hours or days, or the ingestion time is unknown?
The nomogram does not apply, at all, to staggered or repeated supratherapeutic dosing, chronic ingestion, or ingestions where the timing is unknown or uncertain. All of these situations require a different clinical assessment than a single-ingestion, known-time nomogram can provide. Call Poison Control (1-800-222-1222, US) for guidance specific to the actual situation.
Why does the treatment line start at 4 hours instead of right away?
Acetaminophen absorption after an acute ingestion isn't complete immediately, so a level drawn earlier than 4 hours can still be rising and doesn't reliably reflect the peak. The nomogram is only validated starting at the 4-hour mark; a level drawn earlier needs to be repeated at or after 4 hours to be interpreted.
Why is this line lower than the original 1975 Rumack-Matthew nomogram (200 mcg/mL at 4 hours)?
The original 1975 study used a 200 mcg/mL line at 4 hours. Later FDA-required treatment studies for N-acetylcysteine adopted a line set 25% lower — 150 mcg/mL at 4 hours — as the 'possible toxicity' treatment threshold now used in US clinical practice, which is the line this calculator reproduces.
If my measured level is below the treatment line, does that mean it's safe to skip emergency care?
Not on your own judgment — no. A level below the line, in a case that actually meets all the nomogram's conditions (single acute ingestion, immediate-release product, known time, drawn at or after 4 hours), is one input a clinician uses alongside the full clinical picture. It is not a substitute for emergency evaluation, and it means nothing reliable at all if any of those conditions aren't met. Always confirm with Poison Control or emergency medical staff.
Is a single measured level below the line always the end of the story?
No — clinicians typically also consider the reported time and amount ingested, symptoms, and sometimes a repeat level, not a single point in isolation. Coingestants, liver disease, chronic alcohol use, and malnutrition can also affect risk in ways this line alone doesn't capture. This calculator reproduces one specific reference line; it is not a full toxicology assessment.
References
- White SJ, Rumack BH. "The acetaminophen toxicity equations: solutions for a complex problem." Ann Emerg Med. 2005;45(5):563-564
- Rumack BH, Matthew H. "Acetaminophen poisoning and toxicity." Pediatrics. 1975;55(6):871-876
- National Capital Poison Center — Use Acetaminophen Safely: Tips to Prevent Poisoning
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.