Ceftriaxone (Page 2 of 7)

Susceptibility Tests:

Dilution Techniques:

Quantitative methods are used to determine antimicrobial minimal inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure.1 Standardized procedures are based on a dilution method (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of ceftriaxone powder. The MIC values should be interpreted according to the following criteria2 for aerobic organisms other than Haemophilus spp, Neisseria gonorrhoeae , and Streptococcus spp, including Streptococcus pneumoniae:

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

The following interpretive criteria2 should be used when testing Haemophilus species using Haemophilus Test Media (HTM).

MIC (mcg/mL) Interpretation
≤ 2 (S) Susceptible

The absence of resistant strains precludes defining any categories other than “Susceptible”. Strains yielding results suggestive of a “Nonsusceptible” category should be submitted to a reference laboratory for further testing.

The following interpretive criteria2 should be used when testing Neisseria gonorrhoeae when using GC agar base and 1% defined growth supplement.

MIC (mcg/mL) Interpretation
≤ 0.25 (S) Susceptible

The absence of resistant strains precludes defining any categories other than “Susceptible”. Strains yielding results suggestive of a “Nonsusceptible” category should be submitted to a reference laboratory for further testing.

The following interpretive criteria2 should be used when testing Streptococcus spp including Streptococcus pneumoniae using cation-adjusted Mueller-Hinton broth with 2 to 5% lysed horse blood.

MIC (mcg/mL) Interpretation
≤ 0.5 (S) Susceptible
1 (I) Intermediate
≥ 2 (R) Resistant

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 results 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 the 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. Standardized ceftriaxone powder should provide the following MIC values:2

Microorganism ATCC ® # MIC ( mcg /mL)
Escherichia coli 25922 0.03-0.12
Staphylococcus aureus 29213 1-8*
Pseudomonas aeruginosa 27853 8-32
Haemophilus influenzae 49247 0.06-0.25
Neisseria gonorrhoeae 49226 0.004-0.015
Streptococcus pneumoniae 49619 0.03-0.12

* A bimodal distribution of MICs results at the extremes of the acceptable range should be suspect and control validity should be verified with data from other control strains. ATCC® is a registered trademark of the American Type Culture Collection.

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 procedure3 requires the use of standardized inoculum concentrations. This procedure uses paper discs impregnated with 30 mcg of ceftriaxone to test the susceptibility of microorganisms to ceftriaxone.

Reports from the laboratory providing results of the standard single-disc susceptibility test with a 30 mcg ceftriaxone disc should be interpreted according to the following criteria for aerobic organisms other than Haemophilus spp, Neisseria gonorrhoeae, and Streptococcus spp:

Zone Diameter (mm) Interpretation
≥ 21 (S) Susceptible
14-20 (I) Intermediate
≤ 13 (R) Resistant

The following interpretive criteria3 should be used when testing Haemophilus species when using Haemophilus Test Media (HTM).

Zone Diameter (mm) Interpretation
≥ 26 (S) Susceptible

The absence of resistant strains precludes defining any categories other than “Susceptible”. Strains yielding results suggestive of a “Nonsusceptible” category should be submitted to a reference laboratory for further testing.

The following interpretive criteria3 should be used when testing Neisseria gonorrhoeae when using GC agar base and 1% defined growth supplement.

Zone Diameter (mm) Interpretation
≥ 35 (S) Susceptible

The absence of resistant strains precludes defining any categories other than “Susceptible”. Strains yielding results suggestive of a “Nonsusceptible” category should be submitted to a reference laboratory for further testing.

The following interpretive criteria3 should be used when testing Streptococcus spp other than Streptococcus pneumoniae when using Mueller-Hinton agar supplemented with 5% sheep blood incubated in 5% CO2 .

Zone Diameter (mm) Interpretation
≥ 27 (S) Susceptible
25-26 (I) Intermediate
≤ 24 (R) Resistant

Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disc test with the MIC for ceftriaxone.

Disc diffusion interpretive criteria for ceftriaxone discs against Streptococcus pneumoniae are not available, however, isolates of pneumococci with oxacillin zone diameters of > 20mm are susceptible (MIC ≤ 0.06 mcg/mL) to penicillin and can be considered susceptible to ceftriaxone. Streptococcus pneumoniae isolates should not be reported as penicillin (ceftriaxone) resistant or intermediate based solely on an oxacillin zone diameter of ≤ 19mm. The ceftriaxone MIC should be determined for those isolates with oxacillin zone diameters ≤ 19mm.

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 mcg ceftriaxone disc should provide the following zone diameters in these laboratory test quality control strains:3

Microorganism ATCC ® # Zone Diameter Ranges (mm)
Escherichia coli 25922 29-35
Staphylococcus aureus 25923 22-28
Pseudomonas aeruginosa 27853 17-23
Haemophilus influenzae 49247 31-39
Neisseria gonorrhoeae 49226 39-51
Streptococcus pneumoniae 49619 30-35

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