Sulfamethoxazole, Serum (SFZ)

Test Code: SULFASO

Overview
Ordering
Specimen
Performing
Clinical/Interpretive
Contacts
Coding

Synonyms/Keywords

Bactrim (order both tests), Bactrim (Sulfamethoxazole and trimethoprim), Gantanol (Sulfamethoxazole), SEPTRA (Sulfamethoxazole and Trimethoprim), Sulfamethoxazole, Sulfamethoxazole and Trimethoprim (Bactrim, SEPTRA), Trimethoprim and Sulfamethoxazole (Bactrim, SEPTRA)

Useful For

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

Specimen Requirements

Specimen Type	Preferred Container/Tube	Acceptable Container/Tube	Specimen Volume	Specimen Minimum Volume (allows for 1 repeat)	Pediatric Minimum Volume (no repeat)
Serum	Red Top Tube (RTT)		1 mL	0.5 mL

Collection Processing Instructions

1. Serum for a peak level should be collected 60 minutes after dose.

2. Within 2 hours of collection, centrifuge and aliquot serum into a plastic vial.

Specimen Stability Information

Specimen Type	Temperature	Time
Serum	Refrigerated (preferred)	28 days
	Ambient	28 days
	Frozen	28 days

Rejection Criteria

Specimens collected in gel tubes/ Serum Separator tubes (SST)

Interference

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

Performing Laboratory Information

Performing Location	Day(s) Test Performed	Report Available	Methodology/Instrumentation
Mayo Clinic Laboratories	Monday, Thursday	2-5 days	Liquid-Chromatography Tandem Mass Spectrometry (LC-MS/MS)

Reference Lab

Mayo Clinic Laboratories

Test Information

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 individuals who are HIV-positive.

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 1 to 4 hours after an oral dose. Its average elimination half-life is approximately 10 hours. Toxicity includes crystalluria with resultant calculi and kidney 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 Range Information

Performing Location	Reference Range
Mayo Clinic Laboratories	>50 mcg/mL (Peak)

Interpretation

Peak concentrations of sulfamethoxazole should be obtained 1 hour after the end of an intravenous dose or 2 to 3 hours after an oral dose, while peak concentrations of trimethoprim can be collected at least 1 hour after an oral dose. Serum drug concentrations should be interpreted with respect to the minimal inhibitory concentration 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. Target 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.

Trimethoprim: Most patients will display peak steady-state serum concentrations of 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.

Outreach CPTs

CPT	Modifier (if needed)	Quantity	Description	Comments
80299		1

Synonyms/Keywords

Ordering Applications

Ordering Application	Description
Cerner	Sulfamethoxazole, Serum (SFZ)

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