Educational reference only · Last reviewed: March 2026
This tool plots a serum acetaminophen level against the Rumack–Matthew nomogram to estimate whether N-acetylcysteine (NAC) treatment may be indicated.
The nomogram is only valid for: single, acute overdoses at a known time of ingestion, with a level drawn between 4 and 24 hours post-ingestion.
It is not valid for staggered/repeated ingestions, unknown ingestion time, or chronic excess ingestion.
Always contact your regional poison centre or clinical toxicology service and follow local guidelines before making treatment decisions. This tool must not be used as the sole basis for clinical care.
The Rumack–Matthew nomogram is the most widely used clinical decision tool for determining whether a patient who has taken an acetaminophen overdose requires treatment with N-acetylcysteine (NAC) — the antidote that prevents hepatotoxicity when given early enough.
The nomogram plots serum acetaminophen concentration (in µg/mL or µmol/L) against time since ingestion (in hours). A 'treatment line' defines the boundary: patients whose levels fall above the line require NAC treatment; those below do not.
The calculator implements the treatment line using the semi-logarithmic interpolation of the two reference points: 150 µg/mL at 4 hours and 4.69 µg/mL at 24 hours. It also automatically converts between µg/mL and µmol/L (using the molecular weight of acetaminophen: 151.16 g/mol).
Enter the time since ingestion (in hours) and the serum acetaminophen concentration in either µg/mL or µmol/L. The calculator computes the nomogram threshold at that time point and compares it to the entered concentration.
The result is displayed with clear colour-coded guidance: green (below treatment line — NAC not indicated by nomogram alone) or red (above treatment line — NAC treatment indicated). The nomogram threshold at the entered time is displayed alongside the patient's level for reference.
An SVG nomogram chart renders the treatment line graphically and plots the patient's data point directly on the curve, providing a visual representation that matches the printed nomogram used at the bedside.
Critical warnings appear automatically for: time since ingestion under 4 hours (levels drawn before peak absorption — nomogram not yet applicable); time over 24 hours (nomogram no longer valid — clinical and biochemical assessment required); and staggered ingestions (nomogram not validated for repeated doses over time).
Unit conversion is automatic. Entering a value in µmol/L converts it to µg/mL using the formula: µg/mL = µmol/L × 151.16 × 0.001. Entering µg/mL converts to µmol/L for reporting both values.
The treatment line is calculated by semi-logarithmic interpolation between two anchor points, reflecting the exponential decline in the threshold as time passes and the drug is metabolised.
The nomogram was originally developed in the 1970s for single acute ingestions in otherwise healthy patients. It was designed with a built-in safety margin — the original 'possible hepatotoxicity' line at 200 µg/mL at 4 hours was lowered by 25% to create the treatment line at 150 µg/mL.
High-risk patients may require treatment at lower thresholds even if their level falls below the standard treatment line. Risk factors include: malnutrition or low body weight, chronic alcohol use, pre-existing liver disease, enzyme-inducing medications (e.g., rifampicin, carbamazepine, phenytoin), and fasting.
Staggered ingestions — multiple doses taken over hours or days, as opposed to a single large dose at a known time — cannot be assessed using the nomogram. These patients should be risk-assessed based on total estimated ingested dose relative to body weight.
N-acetylcysteine (NAC) is most effective when started within 8 hours of ingestion. Efficacy declines progressively after this but treatment is still warranted up to and beyond 24 hours in many high-risk scenarios. Never delay treatment while awaiting confirmation of exact ingestion time.
A Poison Control Centre or clinical toxicologist should be contacted for every acetaminophen overdose case — including those where the nomogram suggests treatment is not required — as clinical context may change the risk assessment.
Contact is especially urgent for: staggered ingestions, unknown ingestion time, patients with pre-existing liver disease, co-ingestion of other substances, paediatric patients, and patients who present more than 8 hours after ingestion (when the treatment window is narrowing).
Acetaminophen concentration thresholds at which NAC treatment is indicated, based on the standard treatment line. Concentrations above these values indicate treatment is required.
| Hours post-ingestion | Threshold (µg/mL) | Threshold (µmol/L) |
|---|---|---|
| 4 h | 150 µg/mL | 993 µmol/L |
| 5 h | 119.7 µg/mL | 792 µmol/L |
| 6 h | 95.5 µg/mL | 632 µmol/L |
| 7 h | 76.2 µg/mL | 504 µmol/L |
| 8 h | 60.8 µg/mL | 402 µmol/L |
| 9 h | 48.5 µg/mL | 321 µmol/L |
| 10 h | 38.7 µg/mL | 256 µmol/L |
| 12 h | 24.7 µg/mL | 163 µmol/L |
| 15 h | 13.3 µg/mL | 88 µmol/L |
| 18 h | 7.1 µg/mL | 47 µmol/L |
| 20 h | 5.3 µg/mL | 35 µmol/L |
| 24 h | 4.7 µg/mL | 31 µmol/L |
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Usman Shaoukat is the founder and editor of BestMedCal. He is a computer and web specialist who researches every BestMedCal tool from published standards and medication-safety references, including NIST, the U.S. FDA, NCBI and the American Academy of Pediatrics. His focus is dose-to-volume calculations and unit conversions. Every formula and worked example is checked against the sources listed on each page. BestMedCal has no clinicians on staff: our tools check arithmetic and explain methods; they do not recommend doses. If you spot an error, contact us and we will review it.