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Sulfamethoxazole trough concentration (serum/plasma)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of sulfamethoxazole, a medication used to treat bacterial infections, at its lowest point in your bloodstream before the next dose.

Quick summary

["Understanding your medication levels helps ensure the drug is working safely and effectively."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
72674-5
Short code
Sulfamethoxazole Trough SerPl-mCnc
Synonyms
Sulfamethoxazole Trough SerPl-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 healthcare provider may order this test if you’re taking sulfamethoxazole (often combined with trimethoprim, such as in Bactrim®) to monitor your response to treatment. It helps prevent dangerous levels of the drug that could cause side effects like kidney problems or blood disorders.

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 on a regular dosing schedule, this test is typically done at the lowest point (trough) of your medication cycle, often before the next dose.

What the Results Mean

Results are used to adjust your medication dose if needed. While specific reference ranges vary by lab, lower-than-expected levels might indicate the drug isn’t working well, while very high levels could signal toxicity. Your provider will interpret results in the context of your overall health and treatment plan.

Common Side Effects to Watch For

While this test focuses on medication levels, sulfamethoxazole can cause side effects like nausea, rash, or allergic reactions. If you experience severe symptoms (e.g., difficulty breathing, swelling, or skin blisters), seek medical attention immediately.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Sulfamethoxazole Level (Trough)

Frequently asked questions

How often should I get this test?
Your provider will determine the frequency based on your treatment plan, medication dose, and health status. Follow-up tests may be scheduled every few weeks or months to monitor your response.
Can I eat or drink before the test?
Yes, you can eat and drink normally before the test, as it’s a blood draw and not affected by food or fluids.
What if I miss my scheduled dose before the test?
If you miss your dose, contact your provider immediately. They may adjust your timing or schedule the test differently to ensure accurate results.
  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 16, 2026. · Read our editorial policy