Theophylline Unit Converter
Theophylline Unit Converter
Convert theophylline between µg/mL, mg/L and µmol/L, with a narrow window that overlaps toxicity and the clearance changes — smoking cessation above all — that move a stable patient into it.
Theophylline converter
Mass ⇄ molarTheophylline 13 µg/mL, trough sample
Formula and conversion factor
µg/mL = µmol/L ÷ 5.55062
- 5.55
- derived from the molecular mass of theophylline, 180.16 Da (1000 ÷ 180.16)
- mg/L
- numerically identical to µg/mL
- aminophylline
- roughly 80% theophylline by weight — a dose written as one and given as the other is a 20% error
Worked example
Theophylline 13 µg/mL, trough sample
13 × 5.55062 = 72 µmol/L
= 13 mg/L
Within the 10 – 20 µg/mL adult trough range
Conventional and SI thresholds
| µg/mL (= mg/L) | µmol/L | |
|---|---|---|
| Adult trough target | 10 – 20 | 56 – 111 |
| Below target | < 10 | < 56 |
| Seizures and arrhythmias increasingly likely | > 20 | > 111 |
What moves theophylline clearance
| Factor | Effect on level |
|---|---|
| Smoking | Lowers — induces clearance; stopping raises the level within days |
| Heart failure, liver disease, older age | Raises — reduced clearance |
| Viral illness | Raises — reduced clearance |
| Ciprofloxacin, macrolides, cimetidine, allopurinol | Raise, sometimes substantially |
A narrow window that toxicity overlaps
Theophylline is reported in µg/mL or mg/L — identical numbers — and in µmol/L, with a molar factor of 5.55 derived from a molecular mass of 180.16. A level of 13 µg/mL is 72 µmol/L. The usual adult trough target is 10 to 20 µg/mL, and the drug is monitored precisely because that window is unusually narrow.
Toxicity overlaps the therapeutic range rather than beginning above it. Nausea, tremor and tachycardia occur inside the quoted range, while seizures and arrhythmias appear above it, and in chronic toxicity serious events can occur at lower concentrations than in acute overdose — an older patient accumulating theophylline slowly can seize at a level a healthy adult would tolerate after a single large ingestion. Interpretation depends entirely on knowing when the sample was taken relative to the dose: a peak and a trough are different tests, and a number with no timing attached cannot be read at all.
Clearance moves in both directions and does so quickly. It falls with heart failure, liver disease, older age and intercurrent viral illness, and rises with cigarette smoking. The practical consequence is that stopping smoking pushes a previously stable patient into toxicity within days — and admission to a smoke-free hospital does exactly that, so a patient admitted on a stable outpatient dose can become toxic on the ward with no change in prescription.
Drug interactions act the same way. Ciprofloxacin, macrolide antibiotics, cimetidine and allopurinol all reduce theophylline clearance and raise levels, sometimes substantially, so starting any of them should prompt a dose review and an early level rather than a wait for the next routine one. A separate and common error is dosing arithmetic: aminophylline is roughly 80% theophylline by weight, so a dose written in one and administered as the other is a 20% error in a drug with very little room for one.
Frequently asked questions
How do I convert theophylline from µg/mL to µmol/L?
Multiply by 5.55, derived from theophylline’s molecular mass of 180.16 Da. A level of 13 µg/mL is 72 µmol/L. Note that µg/mL and mg/L are the same number.
When should a theophylline level be taken?
The sampling time relative to the dose must be recorded, because a peak and a trough are different tests and a level with no timing cannot be interpreted. Trough sampling immediately before a dose is the usual basis for the 10 – 20 µg/mL range.
Can theophylline cause toxicity within the therapeutic range?
Yes. Nausea, tremor and tachycardia occur inside the range, and in chronic toxicity seizures and arrhythmias can occur at lower levels than in acute overdose. The window is narrow and toxicity overlaps it rather than starting cleanly above it.
Why does stopping smoking raise theophylline levels?
Cigarette smoke induces the enzymes that clear theophylline, so smokers need higher doses. When smoking stops — including on admission to a smoke-free hospital — that induction reverses within days and a stable patient can become toxic without any dose change.
How does aminophylline relate to theophylline?
Aminophylline is roughly 80% theophylline by weight. A dose written as aminophylline and administered as theophylline, or the reverse, is a 20% error, which is a common and avoidable mistake in a drug with a narrow window.
Related calculators
References
- Hendeles L, Weinberger M. Theophylline: a state of the art review. Pharmacotherapy. 1983;3(1):2–44.
- Shannon M. Predictors of major toxicity after theophylline overdose. Ann Intern Med. 1993;119(12):1161–1167.
- Joint Formulary Committee. British National Formulary. London: BMJ Group and Pharmaceutical Press.