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Co-trimoxazole (Trimethoprim-sulphamethoxazole) Levels

This is a reference laboratory investigation – tests performed in Southmead Hospital (NBT) Microbiology Department. Antimicrobial Reference Laboratory. We recommend monitoring co-trimoxazole levels in patients with renal failure and/or high dose co-trimoxazole therapy. They can also be useful for monitoring absorption in neutropenic patients.

Sample requirements and clinical information

  • Patient name, sex and age
  • Dosage, frequency and timing of samples
  • Time of dose in relation to the samples
  • Clinical Summary
  • Other medications (essential)
  • Appropriate hazard warnings – eg known positive BBV status

Storage/transport

  • Do not store. Send at ambient temperature to the laboratory as soon as possible.

Advance warning

We recommend that you take co-trimoxazole levels on weekdays – this is for logistical reasons.

Please contact the serology laboratory (extension 5072) on a weekday, in advance, to discuss the possibility of the sample being taken and the test being performed on Saturdays, Sundays and bank holidays.

Please note that co-trimoxazole levels are a reference laboratory investigation with sample transportation from GRH Microbiology Lab being arranged to the testing laboratory at Southmead Hospital.

Sample required

Container: Serum (Gold top)

Full size

For adults

  • 5.0 mL of blood taken into a gold topped gel tube

The testing lab need approximately 1-2 mL of separated serum (the minimum acceptable is 100 µL).

Leaking samples may not be processed

Timing of samples

When levels need to be monitored, we recommend doing the first pair of levels on the third or fourth day of treatment (third day if hepatic impairment).

We recommend taking pre-dose and post-dose samples.

Post-dose samples taken 1 hour after the end of iv administration or 2 hours after oral administration.

Normal (target) levels - sulphamethoxazole

Pre dose <100 mg/L

Post dose 120-150 mg/L ideally. But can be up to 200 mg/L

Normal (target) levels - trimethoprim

Pre dose 5-7 mg/L

Post dose 5-10 mg/L ideally. But can be up to 20 mg/L

Result interpretation

If there is any doubt regarding interpretation, or a dose change needs to be discussed, then please contact the Consultant Microbiologist.

Re-assay interval

6 to 8 days, assuming initial results are within expected range.

Turnaround time

This is a reference laboratory investigation - the turnaround time is affected by time taken to transport the sample to the testing lab (from the clinical area) and the turnaround time of the reference laboratory. The reference lab normally only routinely performs co-trimoxazole levels on weekdays. Southmead’s turnaround time, once the sample is in their laboratory, is < 3 days.

Published: 25th January 2024

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