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Sulfamethoxazole in Serum/Plasma (Substance Concentration)

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

This test measures the amount of the antibiotic sulfamethoxazole in your blood. It helps assess how your body is responding to treatment with this medication.

Quick summary

["This test helps monitor your sulfamethoxazole levels to ensure proper dosing and avoid potential side effects."]

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

Key facts at a glance

LOINC
468-9
Short code
Sulfamethoxazole Islt MIC
Synonyms
Sulfamethoxazole Islt MIC
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
mic
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) to treat infections like urinary tract infections, respiratory infections, or certain types of pneumonia. It helps ensure the medication is working effectively and at safe levels in your bloodstream.

What to Expect Before the Test

No special preparation is needed. You may have blood drawn from a vein in your arm, similar to other blood tests. If you’re on a blood thinner, let your provider know before the test.

What the Results Mean

Results are typically reported as a concentration value (e.g., micrograms per milliliter). While reference ranges vary by lab, low levels may indicate insufficient medication, while very high levels could signal toxicity or improper absorption.

Common Uses and Considerations

This test is often used in patients with kidney issues, as sulfamethoxazole is processed by the kidneys. It may also be checked in pregnant individuals or those with allergies to sulfonamide medications.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Sulfamethoxazole Level

Frequently asked questions

Why might my doctor want to check this level?
Your doctor may order this test to confirm you’re taking the right dose of sulfamethoxazole, to check for side effects like kidney problems or allergic reactions, or to adjust your treatment if needed.
What happens if my levels are too high or too low?
High levels could cause nausea, rash, or kidney issues, while low levels might mean the infection isn’t fully treated. Your doctor will discuss these findings with you and adjust your plan if necessary.
Can I eat or drink before this test?
You can eat and drink normally unless instructed otherwise. Avoid smoking or caffeine before the blood draw, as it may affect some lab 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