Trimethoprim Concentration in Serum/Plasma
This test measures the amount of the antibiotic trimethoprim in your blood, typically taken from serum or plasma. It helps assess drug levels to ensure proper dosing and monitor for effectiveness or toxicity.
["This test helps doctors check if your trimethoprim dose is working as intended."]
Key facts at a glance
- LOINC
- 72938-4
- Short code
- Trimethoprim (neut) IgG SerPl Ql FC
- Synonyms
- Trimethoprim (neut) IgG SerPl Ql FC
- Category
- chemistry
- Sample
- serum
- Container
- Red-top tube (SST)
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- flow-cytometry
- CPT
- 82784
- 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
Doctors may order a trimethoprim level test if you’re taking this antibiotic for infections like urinary tract infections, pneumonia, or other bacterial infections. It helps ensure the right amount of medication is in your bloodstream to fight the infection without causing harmful side effects. This is especially important for people with kidney problems or those taking other medications that interact with trimethoprim.
What to Expect Before the Test
No special preparation is usually needed. You may be asked to fast before the blood draw, but this depends on the clinic or lab. Bring a list of all medications, including over-the-counter drugs and supplements, to share with your healthcare provider.
A healthcare professional will draw blood from a vein in your arm. The process is quick and typically causes only minor discomfort. The blood sample is then sent to a lab for analysis.
The blood sample is tested using an immunoassay method to measure the precise amount of trimethoprim in your serum or plasma.
What the Results Mean
Results are usually reported as a concentration value (e.g., micrograms per milliliter). Your doctor will compare these results to your prescribed dose and clinical picture to adjust treatment if needed. Abnormally low or high levels may indicate issues with absorption, kidney function, or drug interactions.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Trimethoprim is a folate pathway inhibitor used to treat bacterial infections. Monitoring its levels helps prevent underdosing (which may lead to infection recurrence) or overdosing (which can cause side effects like nausea, rash, or blood disorders). In patients with kidney disease, lower kidney function may reduce trimethoprim clearance, requiring dose adjustments to avoid toxicity. This test is also critical when trimethoprim is combined with sulfamethoxazole (co-trimoxazole) to ensure both drugs reach therapeutic levels.
Short name
Trimethoprim Level
Frequently asked questions
Why might my doctor order this test?
What are the typical reference ranges for trimethoprim levels?
Can I eat or drink 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