Carbamazepine Unit Converter
Carbamazepine Unit Converter
Convert carbamazepine between µg/mL, mg/L and µmol/L, with the autoinduction that makes an early level unreliable and the epoxide metabolite the routine assay does not see.
Carbamazepine converter
Mass ⇄ molarCarbamazepine 8.5 µg/mL, trough sample at 6 weeks
Formula and conversion factor
µg/mL = µmol/L ÷ 4.23245
- 4.23
- derived from the molecular mass of carbamazepine, 236.27 Da (1000 ÷ 236.27)
- mg/L
- numerically identical to µg/mL
- epoxide
- carbamazepine-10,11-epoxide is active but is not measured by the routine assay, so the reported number is not the whole exposure
Worked example
Carbamazepine 8.5 µg/mL, trough sample at 6 weeks
8.5 × 4.23245 = 36 µmol/L
= 8.5 mg/L
Within the 4 – 12 µg/mL trough range, and taken after autoinduction is complete
Conventional and SI thresholds
| µg/mL (= mg/L) | µmol/L | |
|---|---|---|
| Trough target, epilepsy | 4 – 12 | 17 – 51 |
| Below target | < 4 | < 17 |
| Toxicity increasingly likely | > 12 | > 51 |
Autoinduction: what a level means at each stage
| Time since starting or increasing the dose | What the level tells you |
|---|---|
| First few days | Reflects pre-induction clearance — will fall from here |
| 2 – 4 weeks | Clearance still rising; the level is still drifting down |
| After about 4 weeks | Induction complete; the level is now a stable steady-state figure |
Autoinduction, and the metabolite you cannot see
Carbamazepine is reported in µg/mL or mg/L, which are numerically identical, and in µmol/L. The molar factor is 4.23, derived from a molecular mass of 236.27, so a level of 8.5 µg/mL is 36 µmol/L. The usual trough target is 4 to 12 µg/mL, though carbamazepine is ultimately dosed to seizure control and tolerability rather than to a number.
Autoinduction is the single most important thing to know about monitoring carbamazepine. The drug induces the enzymes that clear it, so its own clearance rises over the first two to four weeks of treatment, and rises again after every dose increase. A level that was comfortably therapeutic on starting can drift down over that period with no change in dose at all, and seizure control can be lost with it. Levels taken during initiation should be repeated once induction is complete rather than treated as stable.
Timing matters as much here as with any narrow-index drug. Levels are taken at trough, immediately before the next dose, and a result without a recorded time relative to the dose cannot be interpreted. A second problem is invisible on the report: the active metabolite carbamazepine-10,11-epoxide is not measured by the routine assay, and it can cause toxicity — diplopia, ataxia, nausea — while the parent level looks entirely acceptable. This is particularly likely alongside valproate, which inhibits epoxide hydrolase and raises epoxide concentrations.
Carbamazepine is also a potent enzyme inducer in its own right, and lowers concentrations of a long list of other drugs: combined oral contraceptives, direct oral anticoagulants, warfarin, tacrolimus and ciclosporin among them. Starting carbamazepine is therefore a prescribing event for the whole regimen, not only for the epilepsy. It additionally carries a risk of severe cutaneous reaction associated with the HLA-B*15:02 allele, for which screening is recommended before starting in people of Han Chinese, Thai and other South-East Asian ancestry.
Frequently asked questions
How do I convert carbamazepine from µg/mL to µmol/L?
Multiply by 4.23, derived from carbamazepine’s molecular mass of 236.27 Da. A level of 8.5 µg/mL is 36 µmol/L. Note that µg/mL and mg/L are the same number.
What is carbamazepine autoinduction?
Carbamazepine induces the enzymes that clear it, so clearance rises over the first two to four weeks of treatment and again after each dose increase. A level that was therapeutic at the start can drift down without any change in dose.
When should a carbamazepine level be taken?
At trough, immediately before the next dose, and — for a level intended to represent steady state — after about four weeks, once autoinduction is complete. A level with no recorded timing relative to the dose is not interpretable.
Can carbamazepine cause toxicity with a normal level?
Yes. The active metabolite carbamazepine-10,11-epoxide is not measured by the routine assay, so diplopia, ataxia and nausea can occur while the parent level looks acceptable. Co-administered valproate raises epoxide concentrations and makes this more likely.
Which drugs does carbamazepine affect?
As a potent enzyme inducer it lowers concentrations of many other drugs, including combined oral contraceptives, direct oral anticoagulants, warfarin, tacrolimus and ciclosporin. The whole regimen should be reviewed when it is started or stopped.
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References
- Patsalos PN, Berry DJ, Bourgeois BFD, et al. Antiepileptic drugs — best practice guidelines for therapeutic drug monitoring: ILAE position paper. Epilepsia. 2008;49(7):1239–1276.
- Bertilsson L, Tomson T. Clinical pharmacokinetics and pharmacological effects of carbamazepine and carbamazepine-10,11-epoxide. Clin Pharmacokinet. 1986;11(3):177–198.
- Joint Formulary Committee. British National Formulary. London: BMJ Group and Pharmaceutical Press.