Cefizox (Page 2 of 6)

Dilution techniques:

Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure. Standardized procedures are based on a dilution method¹ (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of ceftizoxime powder. The MIC values should be interpreted according to the following criteria:

For testing non-fastidious aerobic microorganisms other than Haemophilus spp., Neisseria gonorrhoeae:

MIC (μg/mL)	Interpretation
≤8	Susceptible (S)
16-32	Intermediate (I)
≥64	Resistant (R)

For testing Haemophilus spp.*

MIC (μg/mL)	Interpretation †
* These interpretative standards are applicable only to broth microdilution susceptibility testing with Haemophilus spp. using Haemophilus Test Medium.2 † The current absence of data on resistant strains precludes defining any category other than “susceptible”. Strains yielding MIC results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.
≤2	Susceptible (S)

For testing Neisseria gonorrhoeae *

MIC (μg/mL)	Interpretation †
* These interpretative standards are applicable only to agar dilution susceptibility testing using GC agar base and 1% defined growth supplements. † The current absence of data on resistant strains precludes defining any category other than “susceptible”. Strains yielding MIC results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.
≤0.5	Susceptible (S)

A report of “Susceptible” indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable. 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, which prevents small-uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable, other therapy should be selected.

Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard ceftizoxime powder should provide the following MIC values:

Microorganism	MIC(μg/mL)
Escherichia coli ATCC 25922	0.03­0.12
Haemophilus influenzae ATCC 49247	0.06-0.5
Neisseria gonorrhoeae ATCC 49226	0.008-0.03
Pseudomonas aeruginosa ATCC 27853	16-64
Staphylococcus aureus ATCC 29213	2­8

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 procedure² requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 30-μg ceftizoxime to test the susceptibility of microorganisms to ceftizoxime.

Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30-μg ceftizoxime disk should be interpreted according to the following criteria:

Zone diameter interpretative standard for testing non-fastidious aerobic microorganisms other than Haemophilus spp. and Neisseria gonorrhoeae:

Zone Diameter (mm)	Interpretation
≥ 20	Susceptible (S)
15-19	Intermediate (I)
≤ 14	Resistant (R)

Zone diameter interpretative standard for Haemophilus spp.*

Zone Diameter (mm)	Interpretation †
* These zone diameter standards are applicable only to susceptibility testing with Haemophilus spp. using Haemophilus Test Medium.3 † The current absence of data on resistant strains precludes defining any category other than “susceptible”. Strains yielding MIC results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.
≥ 26	Susceptible (S)

Zone diameter interpretative standard for testing Neisseria gonorrhoeae.*

Zone Diameter (mm)	Interpretation †
* These interpretative standards are applicable only to disk diffusion testing using GC agar base and 1% defined growth supplements incubated at 5% CO2 . † The current absence of data on resistant strains precludes defining any category other than “susceptible”. Strains yielding MIC results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.
≥ 38	Susceptible (S)

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 ceftizoxime.

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 30-μg ceftizoxime disk should provide the following zone diameters in these laboratory test quality control strains:

Microorganism	Zone Diameter (mm)
Escherichia coli ATCC 25922	30-36
Haemophilus influenzae ATCC 49247	29-39
Neisseria gonorrhoeae ATCC 49226	42-51
Pseudomonas aeruginosa ATCC 27853	12-17
Staphylococcus aureus ATCC 25923	27-35