Carbamazepine, Free (Serum/Plasma)
This test measures the amount of carbamazepine, a medication used to treat certain types of seizures or nerve pain, that is not bound to proteins in your blood.
["Understanding your free drug levels helps doctors monitor how well your body is processing this medication."]
Key facts at a glance
- LOINC
- 14057-4
- Short code
- CBZ-SAL
- Synonyms
- carBAMazepine free Sal-mCnc
- Category
- chemistry
- Sample
- serum
- Container
- Saliva collection device (Salivette with polyester swab or specialized oral fluid collection tube)
- Units
- N/A
- Ref low
- 1
- Ref high
- 3
- Critical range
- 0.5 N/A – 4 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)
- CPT
- 80335
- SNOMED
- 395088002
- Specimen volume
- 1-2 mL of saliva (minimum 0.5 mL)
- Stability
- Room temperature: 8 hours. Refrigerated (2-8°C): 7 days. Frozen (-20°C): 30 days. Centrifuge collection device before transport if applicable.
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 24–72 hours
- Order
- routine
Why This Test Is Done
Your healthcare provider may order this test if you’re taking carbamazepine to check if your blood levels are within the safe range. Free drug levels are important because they reflect the portion of the medication that actually reaches your body’s tissues, rather than being trapped in proteins like albumin.
What to Expect Before the Test
No special preparation is needed. You may have a small needle inserted into your arm or vein to draw blood. If you’re taking this test for monitoring, your provider will explain how often you’ll need follow-ups.
What the Results Mean
Results are typically reported in micrograms per milliliter (µg/mL). While exact ranges vary by patient and condition, lower levels may indicate insufficient medication, while dangerously high levels could signal toxicity. Your provider will interpret results in the context of your overall health and treatment plan.
Common Causes of Abnormal Results
Low levels might occur if your body metabolizes carbamazepine too quickly, or if you’re taking medications that interfere with its processing. High levels could result from too much medication, poor kidney function, or interactions with other drugs.
Reference range
Therapeutic reference range: 1.0-3.0 mg/L (saliva). Free carbamazepine represents the unbound pharmacologically active fraction, typically 20-30% of total carbamazepine concentration.N/A
No fasting required. Patient should rinse mouth with water 10-15 minutes prior to collection. Avoid eating, drinking, chewing gum, or using oral hygiene products for at least 30 minutes before saliva collection. Collect specimen approximately 30 minutes before next scheduled dose (trough level) for therapeutic drug monitoring. Document time of last dose.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Free carbamazepine levels are critical for managing side effects and effectiveness. Too little may mean seizures or pain aren’t controlled, while too much can cause serious side effects like dizziness, drowsiness, or even life-threatening conditions. Regular monitoring helps balance safety and treatment goals.
Short name
Free Carbamazepine Level
Frequently asked questions
Why do I need this test if I’m already taking carbamazepine?
What are the most common side effects of carbamazepine?
Can I drink alcohol before 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 16, 2026. · Read our editorial policy