Ciprofloxacin Hydrochloride (Page 3 of 14)

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drug products used in resident hospitals 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 product 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 method (broth and/or agar).1,3,4 The MIC values should be interpreted according to criteria provided in Table 1.

• 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.2,3,4 This procedure uses paper disks impregnated with 5 mcg ciprofloxacin to test the susceptibility of bacteria to ciprofloxacin. The disc diffusion interpretive criteria are provided in Table 1.

Table 1: Susceptibility Test Interpretive Criteria for Ciprofloxacin

MIC (mcg/mL)

Zone Diameter (mm)

Bacteria

S

I

R

S

I

R

Enterobacteriaceae

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Enterococcus faecalis

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Staphylococcus aureus

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Staphylococcus epidermidis

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Staphylococcus saprophyticus

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Pseudomonas aeruginosa

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Haemophilus influenzae a

≤ 1

≥ 21

Haemophilus parainfluenzae a

≤ 1

≥ 21

Salmonella typhi

≤ 0.06

0.12 to 0.5

≥ 1

≥ 31

21 to 30

≤ 20

Streptococcus pneumoniae

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Streptococcus pyogenes

≤ 1

2

≥ 4

≥ 21

16 to 20

≤ 15

Neisseria gonorrhoeae

≤ 0.06

0.12 to 0.5

≥ 1

≥ 41

28 to 40

≤ 27

Bacillus anthracis a

≤ 0.25

S = Susceptible, I = Intermediate, and R = Resistant.

a The current absence of data on resistant isolates precludes defining any results other than “Susceptible”. If isolates yield MIC results other than susceptible, they should be submitted to a reference laboratory for further testing.

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 the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test.1,2,3,4 Standard ciprofloxacin powder should provide the following range of MIC values noted in Table 2. For the diffusion technique using the ciprofloxacin 5 mcg disk the criteria in Table 2 should be achieved.

Table 2: Acceptable Quality Control Ranges for Ciprofloxacin

Bacteria

MIC range (mcg/mL)

Zone Diameter (mm)

Enterococcus faecalis ATCC 29212

0.25 to 2

Escherichia coli ATCC 25922

0.004 to 0.015

30 to 40

Haemophilus influenzae ATCC 49247

0.004 to 0.03

34 to 42

Pseudomonas aeruginosa ATCC 27853

0.25 to 1

25 to 33

Staphylococcus aureus ATCC 29213

0.12 to 0.5

Staphylococcus aureus ATCC 25923

22 to 30

Neisseria gonorrhoeae ATCC 49226

0.001 to 0.008

48 to 58

Campylobacter jejuni ATCC 33560

0.06 to 0.25 and 0.03 to 0.12

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