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Blood Test Tracker

Carbamazepine (Total)

Measures how much carbamazepine (Tegretol), a seizure and nerve-pain medicine, is in your blood.

AI-verified against cited sources. Awaiting expert reviewAlso called CBZLOINC 3432-2

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What it is

Carbamazepine is a medicine used for epilepsy and for trigeminal neuralgia, a type of facial nerve pain. Some forms are also used for bipolar disorder. This test shows the total amount of the drug in your blood. It helps your care team check that you are taking it, that your dose is reaching you, and that it is not too high. Levels can differ a lot between people on the same dose.

Why it matters

The usual range is a guide, not a goal by itself. The right level for you is the one where symptoms are controlled and side effects are manageable. Too much can cause dizziness, double vision, unsteadiness and drowsiness. Levels also shift when other medicines are started or stopped, because carbamazepine interacts with many drugs.

If it’s low

A level below 4 mcg/mL means less drug in your blood than the usual range. That can be fine if your symptoms are well controlled, but it can also mean doses are being missed or the drug is being cleared faster than expected.

Common causes

  • Missed or skipped doses
  • Carbamazepine speeding up its own breakdown over the first 3 to 5 weeks of treatment
  • Medicines that speed up breakdown, such as phenytoin, phenobarbital, primidone and rifampin
  • A sample drawn at the lowest point of the dosing cycle, which is expected for a trough

What can help

  • Tell your prescriber about missed doses, without worrying about blame
  • Use reminders or a pill organizer
  • Ask your prescriber before changing or stopping carbamazepine, since stopping suddenly can increase seizures

If it’s high

Levels above about 12 mcg/mL are more likely to come with side effects such as dizziness, double vision, unsteady walking, nausea and drowsiness. Some people tolerate higher levels, so symptoms matter as much as the number.

Common causes

  • A sample drawn a few hours after a dose instead of just before the next one
  • A recent dose increase
  • Medicines or foods that slow carbamazepine breakdown, such as erythromycin, clarithromycin, fluoxetine, some antifungals, verapamil and grapefruit juice
  • Reduced liver function, since the liver breaks the drug down
  • Extra doses taken by mistake

What can help

  • Tell your prescriber about every medicine, supplement and any grapefruit juice habit, since interactions are common
  • Ask when the level should be drawn in relation to your dose
  • Report dizziness, double vision or unsteadiness to your prescriber instead of changing the dose yourself

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 4–12 mcg/mL. Source: U.S. FDA label via NLM DailyMed (Bionpharma), ARUP Laboratories, Journal of Clinical Medicine.

What can affect your result

  • Time since your last dose, with the trough just before the next dose as the usual draw time
  • How long you have been on the drug: levels often fall over the first 3 to 5 weeks as it speeds up its own breakdown
  • Formulation: suspension, conventional tablets and extended-release tablets peak at different times
  • Other medicines that raise or lower levels
  • Liver function and age
  • Protein binding: about 76% of carbamazepine is bound to blood proteins, so low protein levels can change how a total level should be read
  • Immunoassays can cross-react with the active epoxide metabolite to different degrees, so results can differ from LC-MS/MS.
  • A sample drawn a few hours after a dose will read higher than a trough sample. Peaks come about 4 to 5 hours after conventional tablets and 3 to 12 hours after extended-release tablets.
  • Many medicines raise or lower carbamazepine levels by changing how it is broken down.

Usually just before the next dose (trough), at steady state. Carbamazepine speeds up its own breakdown (autoinduction), which is complete 3 to 5 weeks after starting a fixed dose. Once that has settled, a new steady level takes a few days after a dose change (half-life 12 to 17 hours on repeated dosing).

When to talk to a clinician

Contact your prescriber promptly for new severe dizziness, double vision, unsteady walking or confusion, or for a new rash, fever, sore throat or mouth sores. Call 911 or your local emergency number for a seizure that does not stop, fainting, trouble breathing, or if you think you took too much.

Questions you could ask

  • When should my level be drawn in relation to my dose?
  • How often should I have blood counts, sodium and liver tests while on this medicine?
  • Could one of my other medicines, supplements or foods be raising my carbamazepine level?
  • Should we check my sodium and liver tests along with this?
  • Could a missed dose or another medicine explain this result?
  • Is this level a concern given how well my symptoms are controlled?

Questions people ask

Does being in the usual range mean my dose is right?

Not by itself. The 4 to 12 mcg/mL range describes where most people respond. Some people have good control at lower levels, and some need or tolerate higher ones. Your symptoms, side effects and prescriber's judgment decide.

Why did my level drop after a few weeks on the same dose?

Carbamazepine speeds up its own breakdown in the liver. This effect builds over the first 3 to 5 weeks, so levels often fall even though the dose has not changed. Your prescriber may recheck the level during this period.

Why is the lab asking for blood before my morning dose?

That timing gives the trough, the lowest and most consistent level in the day. A sample taken a few hours after a dose, near the peak, reads higher and is harder to compare with earlier results.

Show the science and sources

Units

  • 1 mcg/mL = 1 mcg/mL
  • 1 mcg/mL = 1 mg/L
  • 1 mcg/mL = 4.232 µmol/L

Open the Carbamazepine unit converter

For clinicians and researchers

Carbamazepine is about 76% protein bound, so total concentrations can misrepresent free drug when binding changes. CYP3A4 forms the pharmacologically active carbamazepine-10,11-epoxide, which is cleared by microsomal epoxide hydrolase; inhibitors of that enzyme (valproate, loxapine, quetiapine, brivaracetam per the FDA label) raise epoxide levels, and some labs report the epoxide separately. Carbamazepine is also a CYP3A4 inducer and induces its own metabolism: the half-life falls from 25 to 65 hours initially to 12 to 17 hours on repeated dosing, with autoinduction complete after 3 to 5 weeks of a fixed regimen. The FDA label gives the generally accepted therapeutic range of total carbamazepine in plasma as 4 to 12 mcg/mL for adults and children and notes wide interindividual variability (0.5 to 25 mcg/mL) with no apparent relationship to daily intake. ILAE therapeutic drug monitoring guidance advises interpreting concentrations in the whole clinical context and establishing an individual therapeutic concentration once a person is doing well. ARUP prints total carbamazepine 4.0 to 12.0 mcg/mL (pre-dose, steady state) with a toxic range above 15 mcg/mL, and notes that metabolite cross-reactivity can explain differences between methods. Immunoassays vary in epoxide cross-reactivity; LC-MS/MS measures the parent drug and epoxide separately. Hyponatremia from SIADH is a dose-related risk, and the label recommends baseline and periodic blood counts, liver function and urinalysis. Conversion: 1 mcg/mL = 1 mg/L = 4.23 µmol/L.

LOINC 3432-2 (primary), 14639-9, 74973-9. Specimen: serum-or-plasma.

Sources

  1. Carbamazepine prescribing information (Clinical Pharmacology, Precautions, Warnings), U.S. FDA label via NLM DailyMed (Bionpharma) (2026)
  2. Therapeutic Drug Monitoring of Carbamazepine: A 20-Year Observational Study, Journal of Clinical Medicine (2021)
  3. Antiepileptic drugs - best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies, Epilepsia (2008)
  4. Carbamazepine, Free and Total, Serum or Plasma (ARUP test 2011763), ARUP Laboratories
  5. Carbamazepine (PubChem CID 2554), PubChem, National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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