Clindamycin Palmitate Hydrochloride (Page 2 of 5)

Susceptibility Testing Methods

When available, the clinical microbiology laboratory should provide cumulative in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug for treatment.

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 test method2,3 (broth and/or agar). The MIC values should be interpreted according to the criteria provided in Table 1.

Diffusion Techniques

Quantitative methods that require the measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized method2,5. This procedure uses paper disks impregnated with 2 mcg of clindamycin to test the susceptibility of bacteria to clindamycin. The disk diffusion breakpoints are provided in Table 1.

Anaerobic Techniques

For anaerobic bacteria, the susceptibility to clindamycin can be determined by a standardized test method2,4. The MIC values obtained should be interpreted according to the criteria provided in Table 1.

Table 1. Susceptibility Test Interpretive Criteria for Clindamycin
NA = not applicable

Pathogen

Susceptibility Interpretive Criteria

Minimal

Inhibitory

Concentrations

(MIC in mcg/mL)

Disk

Diffusion

(Zone

Diameters in mm)

S

I

R

S

I

R

Staphylococcus spp.

≤ 0.5

1 to 2

≥ 4

≥ 21

15 to 20

≤ 14

Streptococcus pneumoniae and other Streptococcus spp.

≤ 0.25

0.5

≥ 1

≥ 19

16 to 18

≤ 15

Anaerobic Bacteria

≤ 2

4

≥ 8

NA

NA

NA

A report of Susceptible (S) indicates that the antimicrobial drug is likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration usually achievable at the site of infection. A report of Intermediate (I) 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 (R) indicates that the antimicrobial drug is not likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration 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 the supplies and reagents used in the assay, and the techniques of the individuals performing the test.2,3,4,5 Standard clindamycin powder should provide the MIC ranges in Table 2. For the disk diffusion technique using the 2 mcg clindamycin disk the criteria provided in Table 2 should be achieved.

Table 2. Acceptable Quality Control Ranges for Clindamycin
NA = Not applicableATCC® is a registered trademark of the American Type Culture Collection
*
Enterococcus faecalis has been included in this table for quality control purposes only
Quality control for C. difficile is performed using the agar dilution method only, all other obligate anaerobes may be tested by either broth microdilution or agar dilution methods.

QC Strain

Acceptable Quality Control Ranges

Minimum Inhibitory

Concentration Range

(mcg/mL)

Disk Diffusion Range

(Zone Diameters

in mm)

Enterococcus faecalis *

ATCC 29212

4 to 16

NA

Staphylococcus aureus

ATCC 29213

0.06 to 0.25

NA

Staphylococcus aureus

ATCC 25923

NA

24 to 30

Streptococcus pneumoniae

ATCC 49619

0.03 to 0.12

19 to 25

Bacteroides fragilis

ATCC 25285

0.5 to 2

NA

Bacteroides thetaiotaomicron

ATCC 29741

2 to 8

NA

Clostridium difficile

ATCC 700057

2 to 8

NA

Eggerthella lenta

ATCC 43055

0.06 to 0.25

NA

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