Sulfamethoxazole and Trimethoprim (Page 2 of 7)

Diffusion Techniques

Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized test method ^{14, 15}. This procedure uses paper disks impregnated with 1.25/23.75 mcg of trimethoprim and sulfamethoxazole to test the susceptibility of microorganisms to trimethoprim and sulfamethoxazole. The disc diffusion interpretive criteria are provided in Table 1.

Table 1: Susceptibility Test Interpretive Criteria for Trimethoprim and Sulfamethoxazole
Bacteria	Minimal Inhibitory Concentration (mcg/mL)			Zone Diameter (mm)
	S	I	R	S	I	R
Enterobacteriaceae	≤ 2/38	—	≥ 4/76	≥ 16	11 – 15	≤ 10
Haemophilus influenzae	≤ 0.5/9.5	1/19 – 2/38	≥ 4/76	≥ 16	11 – 15	≤ 10
Streptococcus pneumoniae	≤ 0.5/9.5	1/19 – 2/38	≥ 4/76	≥ 19	16 – 18	≤ 15

A report of Susceptible indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations at the site of infection necessary to inhibit growth of the pathogen. A report of Intermediate indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of Resistant indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations usually achievable at the infection site; other therapy should be selected.

Quality Control

Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay and the techniques of the individuals performing the test ^{4, 14, 15}. Standard trimethoprim and sulfamethoxazole powder should provide the following range of MIC values noted in Table 2. For the diffusion technique using the 1.25/23.75 mcg trimethoprim and sulfamethoxazole disk the criteria in Table 2 should be achieved.

Table 2: Acceptable Quality Control Ranges for Susceptibility Testing for Trimethoprim and Sulfamethoxazole
QC Strain	Minimal Inhibitory Concentration (mcg/mL)	Zone Diameter (mm)
Escherichia coli ATCC 25922	≤ 0.5/9.5	23–29
Haemophilus influenzae ATCC 49247	0.03/0.59 – 0.25/4.75	24–32
Streptococcus pneumoniae ATCC 49619	0.12/2.4 – 1/19	20–28

INDICATIONS AND USAGE

To reduce the development of drug-resistant bacteria and maintain the effectiveness of sulfamethoxazole and trimethoprim tablets, USP and other antibacterial drugs, sulfamethoxazole and trimethoprim tablets, USP 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.

Urinary Tract Infections

For the treatment of urinary tract infections due to susceptible strains of the following organisms: Escherichia coli, Klebsiella species, Enterobacter species, Morganella morganii, Proteus mirabilis and Proteus vulgaris. It is recommended that initial episodes of uncomplicated urinary tract infections be treated with a single effective antibacterial agent rather than the combination.

Acute Otitis Media

For the treatment of acute otitis media in pediatric patients due to susceptible strains of Streptococcus pneumoniae or Haemophilus influenzae when in the judgment of the physician sulfamethoxazole and trimethoprim tablets offer some advantage over the use of other antimicrobial agents. To date, there are limited data on the safety of repeated use of sulfamethoxazole and trimethoprim tablets, USP in pediatric patients under two years of age. Sulfamethoxazole and trimethoprim tablets, USP are not indicated for prophylactic or prolonged administration in otitis media at any age.

Acute Exacerbations of Chronic Bronchitis in Adults

For the treatment of acute exacerbations of chronic bronchitis due to susceptible strains of Streptococcus pneumoniae or Haemophilus influenzae when a physician deems that sulfamethoxazole and trimethoprim tablets, USP could offer some advantage over the use of a single antimicrobial agent.

Shigellosis

For the treatment of enteritis caused by susceptible strains of Shigella flexneri and Shigella sonnei when antibacterial therapy is indicated.

Pneumocystis jiroveci Pneumonia
For the treatment of documented Pneumocystis jiroveci pneumonia and for prophylaxis against P. jiroveci pneumonia in individuals who are immunosuppressed and considered to be at an increased risk of developing P. jiroveci pneumonia.

Traveler’s Diarrhea in Adults

For the treatment of traveler’s diarrhea due to susceptible strains of enterotoxigenic E. coli.

CONTRAINDICATIONS

Sulfamethoxazole and trimethoprim tablets are contraindicated in patients with a known hypersensitivity to trimethoprim or sulfonamides, in patients with a history of drug-induced immune thrombocytopenia with use of trimethoprim and/or sulfonamides, and in patients with documented megaloblastic anemia due to folate deficiency.
Sulfamethoxazole and trimethoprim tablets are contraindicated in pediatric patients less than 2 months of age. Sulfamethoxazole and trimethoprim tablets are also contraindicated in patients with marked hepatic damage or with severe renal insufficiency when renal function status cannot be monitored.