Which treatment line does this use?

It uses the single UK/MHRA 100 line introduced in 2012, anchored at 100 mg/L at 4 hours post-ingestion. The older US nomogram had a separate higher 150/200 line; UK practice abolished it so anyone on or above the 100 line is treated.

Why must the level be taken at 4 hours or later?

Absorption is incomplete before 4 hours, so an early level under-represents the peak and cannot be plotted reliably. Unless the overdose is staggered or massive, repeat the level at 4 hours from ingestion before deciding.

How is the treatment line value calculated?

The line decays with a 4-hour half-life: threshold equals 100 times 2 to the power of (4 minus hours) divided by 4, in mg/L. So it reads 100 at 4 h, 50 at 8 h, 25 at 12 h, and about 12.5 at 16 h.

What happens beyond 15 hours or with staggered overdose?

The nomogram no longer applies. Current UK guidance is to treat with N-acetylcysteine if paracetamol is detectable or ALT is raised. This tool flags that case rather than plotting against the line.

Is this a substitute for clinical advice?

No. It is an educational reference for a single acute ingestion with known timing. Always follow local toxicology guidance such as TOXBASE or your poisons information centre and use clinical judgement.

What is the Paracetamol Toxicity Nomogram (Rumack-Matthew)?

Plot a serum paracetamol (acetaminophen) level against time post-ingestion on the Rumack-Matthew treatment line and get an N-acetylcysteine recommendation using the UK/MHRA single 100 mg/L line, with mg/L and micromol/L conversion. It runs free in your browser on Gera Tools, with nothing uploaded.

Paracetamol Toxicity Nomogram (Rumack-Matthew)

Name: Paracetamol Toxicity Nomogram (Rumack-Matthew)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

Paracetamol (acetaminophen) is one of the most common overdose agents, and the Rumack-Matthew nomogram turns a single blood level plus the time since ingestion into a clear decision about whether to start the antidote, N-acetylcysteine (NAC). This tool implements the UK/MHRA single treatment line so you can plot a level and read the recommendation instantly.

Background: the Rumack-Matthew nomogram

The nomogram was originally developed in the 1970s by Barry Rumack and Howard Matthew and published with two treatment lines — a higher “probable hepatotoxicity” line (150 mg/L at 4 h in the US version) and a lower “possible hepatotoxicity” line (100 mg/L at 4 h). In 2012 the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) simplified practice to a single line at 100 mg/L at 4 hours, removing the stratification by risk factor. This tool uses that single UK/MHRA line.

How it works

For a single acute ingestion with a known time, the treatment line is anchored at 100 mg/L at 4 hours post-ingestion and falls with a half-life of 4 hours:

threshold(t) = 100 × 2^((4 − t) / 4)   mg/L,   valid for t in 4–15 h

Key threshold values at common time points:

Hours post-ingestion	Treatment line (mg/L)	Treatment line (micromol/L)
4 h	100	662
6 h	70.7	468
8 h	50	331
10 h	35.4	234
12 h	25	166
15 h	14.9	99

A measured level on or above the line at the time of sampling means treat with NAC; a level below the line for a clear single acute ingestion generally means NAC is not indicated.

Unit conversion uses the molecular weight of paracetamol (151.16 g/mol): 1 mg/L equals approximately 6.62 micromol/L. The tool accepts either unit and shows both.

When the nomogram does not apply

The nomogram is valid only within specific conditions. It does not apply when:

The blood sample was taken before 4 hours post-ingestion (absorption may be incomplete; repeat the test at 4 hours)
The time of ingestion is unknown or uncertain
The overdose was staggered (taken over more than one hour)
The patient has taken modified-release paracetamol

In any of these situations, current UK guidance is to start NAC if paracetamol is detectable or if the ALT is elevated, without waiting for a nomogram plot.

Important disclaimer

This is an educational reference tool only. It implements the mathematical treatment line for illustrative purposes. Clinical decisions must follow local toxicology guidance — in the UK, TOXBASE via the National Poisons Information Service. Always involve a senior clinician and contact your regional poisons centre when managing a paracetamol overdose.