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Trimethoprim Concentration in Serum/Plasma (Quantitative)

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

This test measures the amount of the antibiotic trimethoprim in your blood. It helps assess whether your medication levels are within the recommended range for effective treatment.

Quick summary

["This test helps ensure your antibiotic therapy is working properly by checking your blood levels of trimethoprim."]

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

Key facts at a glance

LOINC
23614-1
Short code
Trimethoprim Islt SlowMyco
Synonyms
Trimethoprim Islt SlowMyco
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
method-for-slow-growing-mycobacteria
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 this test if you’re taking trimethoprim (often combined with sulfamethoxazole) to treat infections like urinary tract infections, pneumonia, or certain parasitic diseases. It may also be used to monitor long-term therapy or if you have kidney issues, which can affect how the drug is processed in your body.

How the Test Works

A blood sample is taken from a vein in your arm. The sample is then analyzed using a sensitive laboratory method to measure the precise amount of trimethoprim present in your serum or plasma.

Sample Collection

You’ll have a small needle inserted into your arm to draw blood. This is usually done in a doctor’s office or lab. The process is quick and typically causes only minor discomfort. If you’re on long-term trimethoprim, your blood may be tested at regular intervals to track your levels.

Preparation

No special preparation is needed for this test. However, if you’re on other medications or have recent infections, your doctor may advise adjustments to timing or fasting rules.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Low levels of trimethoprim may indicate insufficient medication absorption or kidney problems, which could reduce its effectiveness. High levels might suggest overdose or improper drug metabolism. Monitoring helps prevent side effects and ensures the infection is properly treated.

Short name

Trimethoprim Level

Frequently asked questions

What Does a Low Level Mean?
A low trimethoprim level could mean your body isn’t absorbing enough of the drug or that your kidneys aren’t filtering it properly. This might require your doctor to adjust your dose or consider alternative treatments.
What Does a High Level Mean?
High levels may indicate you’re taking too much trimethoprim or that your kidneys aren’t working well to remove it. This could lead to side effects like nausea, rash, or kidney damage. Your doctor will review your results and treatment plan together.
How Often Should I Get This Test?
The frequency depends on your treatment plan. For short-term infections, tests may be done once or twice. For long-term use or kidney issues, your doctor might schedule regular checks every few weeks or months.
  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