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Carbamazepine (Serum/Plasma Free Concentration)

Published: August 9, 2026Updated: August 15, 2026

This test measures the amount of carbamazepine, a medication used to control certain types of seizures, in your blood when it is not bound to proteins. Free concentration helps assess how effectively the drug is working and whether it is at a safe level.

Quick summary

["Your doctor may check free carbamazepine levels to monitor your treatment and avoid potential side effects."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 15, 2026Editorial policy

Key facts at a glance

LOINC
3433-0
Short code
CBZ-FREE
Synonyms
carBAMazepine free SerPl-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (serum separator tube acceptable); avoid gel separator tubes for some immunoassays
Units
N/A
Ref low
1
Ref high
3
Critical range
— – 4 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
Immunoassay (e.g., chemiluminescent immunoassay or fluorescence polarization immunoassay)
CPT
80157
SNOMED
395199006
Specimen volume
2-5 mL of whole blood (1-2 mL serum)
Stability
Serum stable for 24 hours at room temperature, 7 days at 2-8°C, and 30 days at -20°C
Hemolysis max
100
Lipemia max
500
Icterus max
20
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your healthcare provider may order this test if you are taking carbamazepine to manage epilepsy or nerve-related conditions. Free drug levels are particularly important because they reflect the portion of the medication that actually reaches your brain and other tissues, where it works to prevent seizures or reduce symptoms. This helps ensure the medication is effective without causing harmful side effects.

What to Expect Before the Test

No special preparation is needed for this test. You may have a small needle inserted into a vein in your arm, or a blood sample may be collected from a finger prick. The process usually takes a few minutes. If you are on blood thinners or have a history of bleeding issues, let your provider know before the test.

What the Results Mean

Results are not provided with specific numeric ranges in this summary. Your doctor will interpret the results in relation to your medical history, current symptoms, and other medications you are taking. Free carbamazepine levels help determine if your dose is too high or too low, which could affect your seizure control or risk of side effects like dizziness, drowsiness, or skin rashes.

Possible Side Effects or Considerations

Carbamazepine can cause side effects such as dizziness, fatigue, or gastrointestinal upset. Rarely, it may lead to more serious issues like liver problems or blood disorders. If you experience unusual symptoms like jaundice, severe nausea, or signs of infection (e.g., fever, sore throat), contact your healthcare provider immediately.

Reference range

Therapeutic range: 1.0-3.0 mcg/mL (free fraction). Free carbamazepine typically represents 20-30% of total carbamazepine concentration.N/A

Specimen: serum

No fasting required. Specimen should be collected at steady state, typically just before the next dose (trough level) for therapeutic drug monitoring. Patient should be on a stable dose for at least 3-5 days prior to testing.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Free Carbamazepine Level

Frequently asked questions

What questions might you have about this test?
This test helps monitor how well your carbamazepine dose is working by measuring the free (unbound) portion of the drug in your blood. Free levels are critical because they indicate the active concentration of the medication that reaches your brain and other tissues. Your doctor may adjust your dose based on these results to balance effectiveness and safety.
Why is it important to take this test regularly?
Regular testing is important because carbamazepine can be metabolized differently in each person. Your body may produce enzymes that break down the drug faster, reducing its effectiveness, or slower, increasing the risk of toxicity. Free levels help ensure your dose stays within a safe and therapeutic range.
Can diet or other medications affect this test?
Yes, certain foods, vitamins, or other medications (such as grapefruit juice or certain antibiotics) can affect how carbamazepine is processed in your body. Always inform your healthcare provider about all medications, supplements, and dietary changes before your test.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 15, 2026. · Read our editorial policy