Sulfamethoxazole in Plasma or Serum (Mass Concentration)
This test measures the amount of the antibiotic sulfamethoxazole in your blood, helping your healthcare provider assess your drug levels and adjust treatment if needed.
["This test helps monitor how well your body is processing sulfamethoxazole, especially for infections treated with this medication."]
Key facts at a glance
- LOINC
- 25271-8
- Short code
- Sulfamethoxazole Spec-mCnc
- Synonyms
- Sulfamethoxazole Spec-mCnc
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
Why This Test Is Done
Your doctor may order this test if you’re taking sulfamethoxazole (often combined with trimethoprim, such as in Bactrim®) to treat or prevent certain bacterial infections. It helps ensure you’re receiving the right dose and that the medication is working effectively in your bloodstream.
What to Expect Before the Test
No special preparation is needed. You’ll likely have a small needle inserted into your arm or vein to draw blood. If you’re on a long-term medication like sulfamethoxazole, your doctor may schedule this test periodically to check levels.
What the Results Mean
Results provide a quantitative measure of sulfamethoxazole concentration in your blood. While reference ranges vary by lab and patient factors, they help determine if your dose is therapeutic or if adjustments are needed (e.g., too low or too high).
Common Uses and Considerations
This test is most relevant for patients on long-term sulfamethoxazole therapy (e.g., for urinary tract infections, Pneumocystis jirovecii pneumonia, or prophylaxis). It’s not typically done for short-term use unless directed by your doctor.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Abnormally low levels may indicate poor absorption, drug interactions, or kidney issues affecting metabolism. High levels could signal overdose, kidney problems, or incorrect dosing. Always discuss results with your healthcare provider, as they’ll interpret them in the context of your overall health and treatment plan.
Short name
Sulfamethoxazole Level
Frequently asked questions
When might I need this test?
What does a ‘therapeutic’ level mean?
Can food or other medications affect 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