Carbamazepine Free, Serum/Plasma (Trough Level)
This test measures the amount of the active, unbound (free) form of carbamazepine in your blood at its lowest point (trough) after dosing. It helps doctors assess whether your medication levels are within the safe therapeutic range.
["This test helps ensure your carbamazepine dose is effective and safe for seizure control."]
Key facts at a glance
- LOINC
- 74974-7
- Short code
- CARBFREETR
- Synonyms
- carBAMazepine free Trough SerPl-mCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top serum collection tube (no anticoagulant) or serum separator tube (SST); for free fraction analysis, avoid gel separator when possible. Use plastic transfer tubes for storage.
- 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., enzyme-multiplied immunoassay technique, chemiluminescent immunoassay, or homogeneous enzyme immunoassay); confirmation by liquid chromatography-tandem mass spectrometry (LC-MS/MS) for free fraction
- CPT
- 80157 (Carbamazepine; free) or 80345 (Drug confirmation, each procedure) for LC-MS/MS confirmation
- SNOMED
- 395182000 |Carbamazepine measurement| with free fraction qualifier
- Specimen volume
- 1.0–2.0 mL of serum (minimum 0.5 mL)
- Stability
- Serum stable at room temperature for 24 hours; refrigerated (2–8°C) for 7 days; frozen (-20°C or lower) for up to 30 days. Avoid repeated freeze-thaw cycles.
- Hemolysis max
- 100
- Lipemia max
- 500
- Icterus max
- 25
- TAT
- 4–24 hours
- Order
- routine
Why This Test Matters
Carbamazepine is a medication used to treat certain types of seizures. While the total blood level of carbamazepine is important, doctors often focus on the free (unbound) portion because it’s the part that actually reaches your brain and works to control seizures. Your doctor may order this test to check if your dose is working properly and to avoid potential side effects from too much or too little medication.
What to Expect Before the Test
No special preparation is needed for this test. You may have a small needle inserted into a vein (usually in your arm) to draw a blood sample. The process is quick and typically causes only minor discomfort. If you’re on a regular medication schedule, this test is usually done just before your next dose (trough level).
How the Results Are Used
Your doctor will compare your free carbamazepine level to guidelines for the medication’s therapeutic range. If your level is too high, it may increase your risk of side effects like dizziness, drowsiness, or skin rash. If it’s too low, your seizures may not be well controlled. Adjustments to your dose may be needed based on these results.
Possible Side Effects or Considerations
Carbamazepine can cause side effects like dizziness, fatigue, or nausea. Free drug levels are monitored to balance effectiveness and safety. If you experience unusual symptoms (e.g., rash, confusion, or severe dizziness), contact your doctor immediately.
Reference range
Therapeutic reference range for free carbamazepine (trough): 1.0–3.0 mg/L (free fraction typically 20–30% of total). Total therapeutic range: 4.0–12.0 mg/L. Apply institution-specific ranges if available.N/A
Trough specimen collection: Draw immediately before the next scheduled dose (typically within 30 minutes prior to dosing) and at least 8–12 hours after the last dose. No fasting required. Consistent sampling time is recommended for therapeutic drug monitoring. Document date/time of last dose on requisition.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Clinical Significance
Short name
Free Carbamazepine Level
Frequently asked questions
What does a high or low free carbamazepine level mean?
Why is this test done at the trough level?
How often will I need this test?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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 14, 2026. · Read our editorial policy