Test Catalog

Test ID: TMP    
Trimethoprim, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Monitoring trimethoprim therapy to ensure drug absorption, clearance, or compliance

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Trimethoprim is coadministered with sulfamethoxazole for prophylaxis or treatment of bacterial infections. These agents are used to treat a variety of infections including methicillin-resistant Staphylococcus aureus, and for prophylaxis in immunosuppressed patients such as HIV-positive individuals.


Trimethoprim has a wide therapeutic index and dose-dependent toxicity. Trimethoprim accumulates in patients with renal failure.


Therapeutic drug monitoring is not commonly performed unless there are concerns about adequate absorption, clearance, or compliance. Accordingly, routine drug monitoring is not indicated in all patients.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

>2.0 mcg/mL

Interpretation Provides information to assist in interpretation of the test results

Most patients will display peak steady state serum concentrations more than 2.0 mcg/mL when the specimen is collected at least 1 hour after an oral dose. Target concentrations may be higher depending on the intent of therapy.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Specimens collected in serum gel tubes are not acceptable, as the drug can absorb on the gel and lead to falsely decreased concentrations.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Kamme C, Melander A, Nilsson N: Serum and saliva concentrations of sulfamethoxazole and trimethoprim in adults in children: relation between saliva concentrations and in vitro activity against nasopharyngeal pathogens. Scand J Infect Dis 1983;15:107-113

2. Young T, Oliphant C, Araoyinbo I, Volmink J: Co-trimoxazole prophylaxis in HIV: the evidence. S Afr Med J 2008 April;98(4):258-259

3. Avdic E, Cosgrove S: Management and control strategies for community-associated methicillin-resistant Staphylococcus aureus. Expert Opin Pharmacother 2008 June;9(9):1463-1479

4. Goodman and Gilman's: The Pharmacological Basis of Therapeutics. 13th edition. Edited by LL Brunton, R Hilal-Dandan, BC Knollmann. McGraw-Hill Publishing, 2018