FACTIVE (Page 3 of 12)

Susceptibility Tests

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 gemifloxacin powder. The MICs should be interpreted according to the following criteria:

For testing Klebsiella pneumoniae:

MIC (μg/mL)

Interpretation

≤0.25

Susceptible (S)

0.5

Intermediate (I)

≥1.0

Resistant (R)

For testing Haemophilus influenzae and Haemophilus parainfluenzae a:

MIC (μg/mL)

Interpretation

≤0.12

Susceptible (S)

a This interpretive standard is applicable only to broth microdilution susceptibility testing with Haemophilus influenzae and Haemophilus parainfluenzae using Haemophilus Test Medium (HTM)1.

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

For testing Streptococcus pneumoniae b:

MIC (μg/mL)

Interpretation

≤0.12

Susceptible (S)

0.25

Intermediate (I)

≥0.5

Resistant (R)

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

A report of “Susceptible” indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentration 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 concentration 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 gemifloxacin powder should provide the following MIC values:

Microorganism

MIC Range (μg/mL)

Escherichia coli

ATCC 25922

0.004-0.016

Haemophilus influenzae

ATCC 49247

0.002-0.008c

Streptococcus pneumoniae

ATCC 49619

0.008-0.03d

c This quality control range is applicable to only H. influenzae ATCC 49247 tested by a broth microdilution procedure using Haemophilus Test Medium (HTM)1.

d This quality control range is applicable to only S. pneumoniae ATCC 49619 tested by a broth microdilution procedure using cation-adjusted Mueller-Hinton broth with 2-5% lysed horse blood.

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 5 μg gemifloxacin to test the susceptibility of microorganisms to gemifloxacin.

Reports from the laboratory providing results of the standard single-disk susceptibility test with a 5 μg gemifloxacin disk should be interpreted according to the following criteria:

For testing Klebsiella pneumoniae:

Zone Diameter (mm)

Interpretation

≥20

Susceptible (S)

16-19

Intermediate (I)

≤15

Resistant (R)

For testing Haemophilus influenzae and Haemophilus parainfluenzae e:

Zone Diameter (mm)

Interpretation

≥18

Susceptible (S)

e This interpretive standard is applicable only to disk diffusion susceptibility testing with Haemophilus influenzae and Haemophilus parainfluenzae using Haemophilus Test Medium (HTM).2

The current absence of data on resistant strains precludes defining any results other than “Susceptible”. Strains yielding zone diameter results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.

For testing Streptococcus pneumoniae f:

Zone Diameter (mm)

Interpretation

≥23

Susceptible (S)

20-22

Intermediate (I)

≤19

Resistant (R)

f These zone diameter standards apply only to tests performed using Mueller-Hinton agar supplemented with 5% defibrinated sheep blood incubated in 5% CO2 .

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 gemifloxacin.

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 5 μg gemifloxacin disk should provide the following zone diameters in these laboratory quality control strains:

Microorganism

Zone Diameter (mm)

Escherichia coli

ATCC 25922

29-36

Haemophilus influenzae

ATCC 49247

30-37g

Streptococcus pneumoniae

ATCC 49619

28-34h

g This quality control range is applicable to only H. influenzae ATCC 49247 tested by a disk diffusion procedure using Haemophilus Test Medium (HTM)2.

h This quality control range is applicable to only S. pneumoniae ATCC 49619 tested by a disk diffusion procedure using Mueller-Hinton agar supplemented with 5% defibrinated sheep blood and incubated in 5% CO2 .

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