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Trimethoprim-free mass concentration in serum/plasma

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

This test measures the amount of free (unbound) trimethoprim, a medication used to treat bacterial infections, in your blood.

Quick summary

["Understanding your free trimethoprim levels helps assess how much of the drug is available to fight infections."]

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

Key facts at a glance

LOINC
80973-1
Short code
Trimethoprim free SerPl-mCnc
Synonyms
Trimethoprim free 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

What Is Free Trimethoprim?

Trimethoprim is often bound to proteins in the blood, but some remains in a free (unbound) form. This free trimethoprim is biologically active and can cross cell membranes to exert its antibacterial effects. The test specifically measures this free portion to evaluate drug exposure more accurately.

Why Is This Test Done?

Your healthcare provider may order this test if you’re taking trimethoprim to check if the drug is working effectively or if you’re experiencing side effects, such as kidney problems or allergic reactions. It helps ensure the correct dose is maintained for optimal treatment.

How Is the Test Performed?

A blood sample is drawn from a vein, usually in your arm. The sample is processed to separate serum or plasma, where the free trimethoprim concentration is measured using a specialized laboratory technique.

What Does the Result Mean?

Results are not provided with specific reference ranges, as they depend on your individual dose, metabolism, and health status. Your provider will interpret the result in relation to your clinical situation and treatment plan.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Trimethoprim Levels (Free)

Frequently asked questions

How does free trimethoprim relate to my treatment?
Free trimethoprim levels help determine if your dose is therapeutic (effective) or potentially toxic. High levels may indicate overmedication or impaired kidney function, while low levels may suggest insufficient drug exposure. Your provider will adjust your treatment based on these findings.
What if my levels are abnormal?
Abnormal results may prompt changes in your medication, such as a dose adjustment, temporary discontinuation, or monitoring for side effects like kidney damage or allergic reactions. Always discuss results with your healthcare team.
Can I eat or drink before this test?
Typically, fasting is not required for this test, but follow your provider’s instructions regarding food, drink, or other medications taken before the draw. Avoid caffeine or alcohol in the 24 hours before testing if advised.
  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