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.

Paracetamol Toxicity Nomogram (Rumack-Matthew)

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.

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

That gives 100 mg/L at 4 h, 50 mg/L at 8 h, 25 mg/L at 12 h, and about 12.5 mg/L at 16 h. A measured level on or above the line at its time point means treat with NAC; below the line generally means NAC is not indicated for a clear single ingestion.

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

Notes and limits

Before 4 hours a level is unreliable because absorption is incomplete — the tool tells you to repeat the sample rather than plotting it. Beyond 15 hours, or for staggered ingestions and unknown timing, the nomogram does not apply and UK guidance is to treat if paracetamol is detectable or ALT is raised. This is an educational aid only; follow local toxicology advice (e.g. TOXBASE or a poisons centre) and clinical judgement.