Test Catalog

Test ID: SFZ    
Sulfamethoxazole, Serum

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

Monitoring therapy to ensure drug absorption, clearance, or compliance

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

Sulfamethoxazole is a sulfonamide antibiotic that is administered in conjunction with another antibacterial, trimethoprim. 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. Therapeutic drug monitoring is not commonly performed unless there are concerns about adequate absorption, clearance, or compliance. Monitoring of sulfamethoxazole is indicated only when prolonged (>3 months) therapy is required.


Sulfamethoxazole is absorbed readily after oral administration, with peak serum concentration occurring 2 to 3 hours after an oral dose. Its average elimination half-life is 6 to 10 hours. Toxicity includes crystalluria with resultant calculi and renal disease. Toxicity is due to a high concentration of acetylated, relatively insoluble forms of the drug. Excess fluid should be taken with sulfamethoxazole to avoid formation of urine sulfonamide crystals.

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.

>50 mcg/mL

Interpretation Provides information to assist in interpretation of the test results

Serum drug concentrations should be interpreted with respect to the minimum inhibitory concentration (MIC) of targeted organisms. Most patients will display peak steady-state serum concentrations greater than 50 mcg/mL when collected at least 1 hour after an oral dose. Targets concentrations may be higher, depending on the intent of therapy.


For Pneumocystis carinii pneumonia (PCP pneumonia), peak concentrations: 100-150 mcg/mL

Toxicity: >200 mcg/mL


Toxicity (formation of urinary crystals) associated with sulfamethoxazole occurs with prolonged exposure to serum concentrations greater than 125 mcg/mL.

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. Hughes WT, Feldman S, Chaudhary SC, Ossi MJ, et al: Comparison of pentamidine isethionate and trimethoprim-sulfamethoxazole in the treatment of Pneumocystis carinii pneumonia. J Pediatr 1978;92(2):285-291

2. Dao BD, Barreto JN, Wolf RC, Dierkhising RA, et al: Serum peak sulfamethoxazole concentrations demonstrate difficulty in achieving a target range: a retrospective cohort study. Curr Ther Res Clin Exp 2014;76:104-109

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

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

5. 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

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