What this test measures
Quantitative serum carbamazepine (CBZ). Time to steady state 2–4 weeks for chronic dosing (auto-induction of CYP3A4 metabolism). Trough level (just before next dose) is the standard. Total CBZ is usually adequate; free CBZ rarely needed unless severe hypoalbuminaemia. CBZ-10,11-epoxide is the active metabolite — measured separately when toxicity is suspected with normal parent levels.
Why it matters
CBZ is widely used in India for focal epilepsy, trigeminal neuralgia, and bipolar disorder. Therapeutic range 4–12 µg/mL. Levels above 12 µg/mL are associated with diplopia, nystagmus, ataxia, and drowsiness. Severe toxicity (> 20 µg/mL) can cause coma, seizures, cardiac conduction abnormalities. CBZ has a major Indian-relevant safety concern — HLA-B*1502 allele (prevalent in Han Chinese, parts of South-East Asia, some Indian populations) carries up to 100× higher risk of Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis with CBZ. Pre-treatment HLA-B*1502 screening is FDA-recommended for at-risk populations.
How to prepare
Trough sample — just before the next dose, after at least 2–4 weeks at the current dose (auto-induction completed). Disclose other medications (CBZ has many CYP3A4 interactions — phenytoin, phenobarbital, warfarin, OCPs, antifungals, macrolides). Note clinical status (seizure frequency, side effects).
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Serum Carbamazepine (µg/mL (mg/L))[1][2] | Therapeutic 4 – 12 µg/mL | < 4 µg/mL: sub-therapeutic — review adherence, drug interactions inducing metabolism, or dose escalation needed. | 12 – 15: above range — diplopia, nystagmus, ataxia, drowsiness common. Reduce dose. 15 – 20: significant toxicity. > 20: severe — coma, seizures, cardiac arrhythmias possible. Manage in monitored setting; activated charcoal if recent ingestion; supportive care; haemodialysis can help in severe cases. |
Carbamazepine therapeutic and toxic ranges
| CBZ (µg/mL) | Status | Action |
|---|---|---|
| < 4 | Sub-therapeutic | Review adherence, interactions, dose |
| 4 – 12 | Therapeutic | Maintain |
| 12 – 15 | Above range | Reduce dose; diplopia, ataxia common |
| 15 – 20 | Toxic | Hold dose; monitor symptoms |
| > 20 | Severe toxicity | Hospitalise; supportive care; consider haemodialysis |
Frequently asked questions
When should I check my carbamazepine level?
2–4 weeks after starting or changing dose (auto-induction completes), then annually or as clinically needed. Sample as a trough (just before next dose).
Why does carbamazepine cause double vision?
CBZ blocks sodium channels in the cerebellum and brainstem; supratherapeutic levels affect eye movement control — diplopia, nystagmus, and ataxia are the classic toxicity triad.
Should I be tested for HLA-B*1502 before starting CBZ?
Yes if you have ancestry from areas with high HLA-B*1502 prevalence (Han Chinese, Thai, Malay, Filipino, some Tamil and other South Indian populations). Carriers have ~100× higher risk of Stevens-Johnson Syndrome / TEN — alternative anticonvulsants are preferred.
Will it interact with my contraceptive pill?
Yes — CBZ is a strong CYP3A4 inducer and reduces oral contraceptive efficacy. Use a non-hormonal method or higher-dose hormonal preparation; discuss with your doctor.
What does the active metabolite do?
Carbamazepine-10,11-epoxide is pharmacologically active and contributes to both efficacy and side effects. In overdose or unusual toxicity with normal parent levels, the epoxide can be measured separately.
How long is the wash-out period?
CBZ half-life shortens with chronic use (auto-induction) — typically 8–24 hours. Levels fall to negligible within 3–5 days after stopping.
Related Drugs / Therapeutic Monitoring tests
Tests commonly ordered alongside CARBAMAZEPINE / TEGRETOL, or that help interpret an unexpected result.
Sources & references
- Mayo Clinic Labs — Carbamazepine, Serum · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Carbamazepine Test · accessed 2026-05-30T00:00:00.000Z
- StatPearls — Carbamazepine · accessed 2026-05-30T00:00:00.000Z
- ILAE — AED Therapeutic Monitoring · accessed 2026-05-30T00:00:00.000Z
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