SULFATRIM (Page 2 of 6)

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1

These interpretative standards are applicable only to broth microdilution susceptibility tests with Haemophilus influenzae using Haemophilus Test Medium (HTM). 4

2

These interpretative standards are applicable only to broth microdilution susceptibility tests using cation-adjusted Mueller-Hinton broth with 2% to 5% lysed horse blood. 4

Quality Control

Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard sulfamethoxazole/trimethoprim powder should provide the following range of values:

Microorganism		MIC (µg/mL)
* This quality control range is applicable only to Haemophilus influenzae ATCC 49247 tested by broth microdilution procedure using Haemophilus Test Medium (HTM). 4 † This quality control range is applicable to tests performed by the broth microdilution method only using cation-adjusted Mueller-Hinton broth with 2% to 5% lysed horse blood. 4
Escherichia coli	ATCC 25922	≤ 0.5/9.5
Haemophilus influenzae *	ATCC 49247	0.03/0.59 – 0.25/4.75
Streptococcus pneumoniae †	ATCC 49619	0.12/2.4 – 1/19

Diffusion Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure ⁵ requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 1.25/23.75 µg of sulfamethoxazole/trimethoprim to test the susceptibility of microorganisms to sulfamethoxazole/trimethoprim.

Reports from the laboratory providing results of the standard single-disk susceptibility test with a 1.25/23.75 mcg of sulfamethoxazole/trimethoprim disk should be interpreted according to the following criteria:

For testing either Enterobacteriaceae or Haemophilus influenzae 3:

Zone Diameter (mm)	Interpretation
≥16	Susceptible (S)
11 – 15	Intermediate (I)
≤10	Resistant (R)

When testing Streptococcus pneumoniae 4:

Zone Diameter (mm)	Interpretation
≥19	Susceptible (S)
16 – 18	Intermediate (I)
≤15	Resistant (R)

Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for sulfamethoxazole/trimethoprim.

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3

These zone diameter standards are applicable only for disk diffusion testing with Haemophilus influenzae and Haemophilus Test Medium (HTM). 5

4

These zone diameter interpretative standards are applicable only to tests performed using Mueller-Hinton agar supplemented with 5% defibrinated sheep blood when incubated in 5% CO 2. 5

Quality Control

As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 1.25/23.75 µg sulfamethoxazole/ trimethoprim disk 5 should provide the following zone diameters in these laboratory test quality control strains:

Microorganism		Zone Diameter Range (mm)
* This quality control range is applicable only to Haemophilus influenzae ATCC 49247 tested by a disk diffusion procedure using Haemophilus Test Medium (HTM). 5 † This quality control range is applicable only to tests performed by disk diffusion using Mueller-Hinton agar supplemented with 5% defibrinated sheep blood when incubated in 5% CO 2. 5
Escherichia coli	ATCC 25922	23-29
Haemophilus influenzae *	ATCC 49247	24-32
Streptococcus pneumoniae †	ATCC 49619	20-28

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5

Mueller-Hinton agar should be checked for excessive levels of thymidine or thymine. To determine whether Mueller-Hinton medium has sufficiently low levels of thymidine and thymine, an Enterococcus faecalis (ATCC 29212 or ATCC 33186) may be tested with sulfamethoxazole/trimethoprim disks. A zone of inhibition ≥20 mm that is essentially free of fine colonies indicates a sufficiently low level of thymidine and thymine.

INDICATIONS AND USAGE

To reduce the development of drug-resistant bacteria and maintain the effectiveness of sulfamethoxazole and trimethoprim oral suspension and other antibacterial drugs, sulfamethoxazole and trimethoprim oral suspension should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to empiric selection of therapy.