Rumack-Matthew Acetaminophen Nomogram

Q: What is the Rumack-Matthew nomogram?

The Rumack-Matthew nomogram is a graphical tool that plots serum acetaminophen concentration against time since ingestion. A treatment line defines the threshold above which N-acetylcysteine (NAC) treatment is recommended to prevent hepatotoxicity.

Q: When is the nomogram applicable?

The nomogram is applicable ONLY for a single acute ingestion (one time ingestion) with a KNOWN time of ingestion. It is valid for serum levels obtained 4 to 24 hours after ingestion. It does NOT apply to repeated ingestions, chronic use, or staggered ingestions.

Q: What if the time of ingestion is unknown?

If the time of ingestion is unknown, the nomogram cannot be used. In this case, obtain acetaminophen level, hepatic function tests (AST, ALT, INR), and renal function. Treat with NAC if there is any concern for toxicity, and consult toxicology.

Q: What if the level is obtained before 4 hours?

Levels obtained before 4 hours post-ingestion may not have peaked and are unreliable for ruling out toxicity. Repeat the level at 4 hours or later. If there is clinical concern, start NAC and recheck.

Q: Does this nomogram apply to sustained-release acetaminophen?

The nomogram is not validated for sustained-release formulations. For sustained-release ingestion, consider prolonged monitoring and consult toxicology, as peak levels may be delayed.

Determines whether N-acetylcysteine (NAC) treatment is indicated following a single acute acetaminophen (paracetamol) ingestion, based on the serum acetaminophen concentration and time since ingestion.

Emergency Emergency MedicineEvidence-BasedUpdated 2026-08

Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.

Clinical Notes

The Rumack-Matthew nomogram was published in 1975 (Pediatrics 55:871-876) and is the standard tool for determining NAC treatment in acute acetaminophen ingestion in the United States. The nomogram uses a semi-log plot of serum acetaminophen concentration versus time since ingestion, with a treatment line derived from reported cases of hepatotoxicity. The treatment line represents the minimum acetaminophen level associated with potential hepatotoxicity at each time point.

Calculator Information

Specialty

Emergency Medicine

📋 Reference Ranges

Normal Range

Below treatment line

Reference ranges for this calculator
Classification	Range
Below treatment line	Level < threshold at given time
Above treatment line	Level ≥ threshold at given time

What This Calculator Measures

Determines whether N-acetylcysteine (NAC) treatment is indicated following a single acute acetaminophen ingestion, based on the serum acetaminophen concentration and time since ingestion.

How to Use

Confirm the ingestion was a single acute event with a known time of ingestion.
Obtain a serum acetaminophen level at 4 or more hours after ingestion.
Enter the time since ingestion (hours) and the serum level (mcg/mL).
The calculator computes the treatment threshold at that time point.
If the level is at or above the threshold, initiate NAC per institutional protocol.

How the Calculation Works

The Rumack-Matthew nomogram compares the serum acetaminophen level to a time-dependent treatment threshold, threshold (mcg/mL) = exp(5.33252 - 0.08047 x hours since ingestion). If the level is at or above the threshold between 4 and 24 hours after ingestion, N-acetylcysteine treatment is indicated; the nomogram is not valid before 4 hours or after 24 hours.

Variables Explained

Time Since Ingestion

Hours since the acute ingestion, used to compute the treatment threshold. Values under 4 h or over 24 h fall outside the valid nomogram window.

Serum Acetaminophen Level

Serum acetaminophen concentration in mcg/mL, compared with the time-based threshold.

Interpretation Guide

Level at or above treatment threshold: Initiate NAC therapy. Level below treatment threshold: NAC not indicated for single acute ingestion with known timing. The nomogram is valid only for 4–24 hours post-ingestion.

Clinical Significance

The Rumack-Matthew nomogram is the standard tool for determining NAC treatment in acute acetaminophen overdose in the United States. Early NAC treatment is highly effective at preventing hepatotoxicity when initiated within 8 hours of ingestion.

When It Is Useful

Single acute acetaminophen ingestion with known time of ingestion
Serum acetaminophen level obtained 4–24 hours post-ingestion
To determine need for N-acetylcysteine (NAC) treatment

When It Is Not Useful

Repeated supratherapeutic ingestion or chronic use
Staggered ingestion (multiple doses over time)
Unknown time of ingestion
Sustained-release formulations (different pharmacokinetics)
Patients presenting with hepatotoxicity (treat with NAC regardless of level)

Common Mistakes

Applying the nomogram to a level drawn before 4 hours after ingestion, when it is not valid.
Using the nomogram for repeated supratherapeutic or staggered ingestions instead of a single acute ingestion with known timing.
Withholding N-acetylcysteine from a patient with hepatotoxicity because the level falls below the treatment line.

Limitations

Valid only for single acute ingestion with known timing.
Valid only for levels obtained 4–24 hours post-ingestion.
Does not apply to repeated, staggered, or chronic ingestion.
Does not account for co-ingestants or individual patient factors.
Clinical toxicology consultation supersedes this calculator when uncertainty exists.

Worked Example

A patient ingested a single acute dose of acetaminophen. At 6 hours post-ingestion, the serum level is 120 mcg/mL.

Inputs:

time-since-ingestion: 6

acetaminophen-level: 120

Expected Result: Treatment threshold at 6h ≈ 130 mcg/mL. Level 120 mcg/mL is BELOW the threshold — NAC not indicated for this single acute ingestion.

Evidence & References

Source:: Emergency Medicine
Content updated:: 2026-08

References

Rumack BH, Matthew H. Pediatrics. 1975;55(6):871-876. (Original description)
Smilkstein MJ, et al. N Engl J Med. 1988;319(24):1557-1562. (Validation study)

Frequently Asked Questions

What is the Rumack-Matthew nomogram?

The Rumack-Matthew nomogram is a graphical tool that plots serum acetaminophen concentration against time since ingestion. A treatment line defines the threshold above which N-acetylcysteine (NAC) treatment is recommended to prevent hepatotoxicity.

When is the nomogram applicable?

The nomogram is applicable ONLY for a single acute ingestion (one time ingestion) with a KNOWN time of ingestion. It is valid for serum levels obtained 4 to 24 hours after ingestion. It does NOT apply to repeated ingestions, chronic use, or staggered ingestions.

What if the time of ingestion is unknown?

If the time of ingestion is unknown, the nomogram cannot be used. In this case, obtain acetaminophen level, hepatic function tests (AST, ALT, INR), and renal function. Treat with NAC if there is any concern for toxicity, and consult toxicology.

What if the level is obtained before 4 hours?

Levels obtained before 4 hours post-ingestion may not have peaked and are unreliable for ruling out toxicity. Repeat the level at 4 hours or later. If there is clinical concern, start NAC and recheck.

Does this nomogram apply to sustained-release acetaminophen?

The nomogram is not validated for sustained-release formulations. For sustained-release ingestion, consider prolonged monitoring and consult toxicology, as peak levels may be delayed.

Disclaimer: This calculator is intended for educational and clinical decision support purposes only. For acetaminophen overdose, always consult clinical toxicology and follow institutional protocols. This calculator does not replace clinical judgment.