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Zone diameter interpretive standards and equivalent Minimum Inhibitory Concentration (MIC) breakpoints
+
~1
-~1 i
-I
ii
30/~g 30/~g 30/~g 75#g 30p.g
30~g
Cefamandolei
CefazolinJ
Cefonicid i
CefoperazoneJ
CefotaximeJ
CefotetanJ
100#g
100/~g
Carbenicillin when testing the Enterobacteriaceaed
when testing Pseudomonas
30#g
10#g
when testing non-enterococcal streptococci and Listeria monocytogenes g.h
Aztreonam
10#g
when testing enterococci g'h
75#g
10/~g
when testing Haemophilus species e
Azlocillin when testing Pseudomonasd
10/zg
_<12
< 14
_<15
< 14
-< 14
14
_<13
< 17
< 15
< 14
<21
< 16
_< 19
< 28
_< 11
_<13
10/~g
_< 19
20110~g
-<14
20110/~g
when testing staphylococci d
Ampicillin f when testing gram-negative enteric organisms
when testing other organisms
30#g
--
--
--
15-17
15-17
15-17
14-16
18-22
--
15-17
--
--
--
--
12-13
14-17
--
15-16
13-15
15-22
16-20
--
--
--
--
--
16-21
--
22-29
> 17 h
--
--
--
--
--
--
>_16
>-23
>-21
> 18
>- 18
>_ 18
>_17
> 23
>22
> 16
>30
--
> 20
> 29
>_14
>__18
>-20
>-17
Zone Diameter, nearest whole mm Moderately Disk Content Resistant Intermediate b Susceptible b Susceptible
--
32#g/mL
32/~g/mL
>__
>
>
>
>-
>
>
>
>
>
>
>
>
64~glmL
64#g/mL
64#g/mL
32#g/mL
32#g/mL
32#g/mL
512#g/mL
32#g/mL
32/~g/mL
256p.g/mL
4/ig/mL
16/~g/mL
4/~g/mL
~-Iactamase d
>_
>-32116~glmL
>
16/~g/mL
8/~g/mL
2p.g/mL --
16#g/mL
8#g/mL
64#g/mL
_<
_<
_<
_<
-<
<
16~.glmL
8#g/mL
16/zg/mL
8#g/mL
8#g/mL
8#g/mL
_< 128#g/mL
<
<
_<
< 0.12#g/mL
<
< 0.25#g/mL
<
_<814~glmL
_< 412~glmL
-<
Equivalent MIC Breakpoints a Resistant Susceptible
Zone Diameter Interpretive Standards and Equivalent Minimum Inhibitory Concentration ( MIC) Breakpoints
Amoxicil|in/clavulanic acid when testing Haemophilus& staphylococci d,e
Amikacin c
Antimicrobial Agent
TABLE 2, M7- A
These values represent MC breakpoints used in determining approximate zone size interpretive criteria. They relate to MiCs determined by M7 methodology. Occasional discrepancies may exist between M2 and Y7 due to methodological limitations.
Resistant strains of S. aureus produce @-lactamase and the testing of the 10 unit penicillin G disk is preferred. Penicillin G should be used to test the susceptibility of all peniciilinasesensitive penicillins. such as ampicillin, amoxicillin alone. ariociilin. bacampiciliin. hetaciliin, carbeniciiiin. meziociilin. piperaciiiin, and ticarciiiin. Results may also be applied to phenoxymethyl penicillin or phenethiciiiin. For testing Haemophilus use Mueller-Hinton agar supplemented with 1% hemoglobin (2-3 % horse blood) and 1% IsoVitaleX (EBL). or 1% Supplement XV (Difco) or an equivalent synthetic supplement. Adjust pH to 7.2. Prepare the inoculum by suspending growth from a 24hour chocolate agar plate in Mueller-Hinton broth to the density of a turbidity standard (see 4.1.3). The vast majority of ampicillin-resistant strains of Haemophilus produce detectable alactamase. Class disk for ampicillin, amoxiciilin. bacampicillin. cyciacillin, and hetaciilin. For
enterococci. other Streptococcus spp. and non-penicillinase-producing peniciliinsensitive organisms, and Listeria monocy?ogenes. the former intermediate interpretation should be reported as “Moderately Susceptible.” Results in this category include enterococci and Listeria which for blood or serious invasive tissue infections require high dosage of penicillin or ampicillin, often combined with an aminoglycoside (gentamicin) for improved therapeutic response and bactericidal action. For streptococci. staphylococci, and other penicillin-sensitive organisms, “Susceptible” results should be regarded as “Very Susceptible.” Enterococci strains (S. faecalis. S. faecium, and S. durans) producing zones L3Omm for ampicillin or Z28mm for penicillin are quite unusual and the streptococcal speciation procedures should be reexamined. Aztreonam, cefotetan, ceftazidime. ceftriaxone, and imipenem are among the most recently studied beta-iactams having a separate diagnostic disk and a generally wider spectrum of antimicrobial activity, especially against gram-negative bacilli when compared to previously approved cephaiosporins such as cephalothin. Therefore, the 30 fig cephaiothin disk cannot be used as the cephaiosporin-class representative for these beta-lactams and other cephalosporins available since the mid-1970s. The cephaiothin disk should only be used for testing the susceptibility to cephalothin, cefacior, cefadroxii, cephaiexin. cephapirin, cefazolin, and cephradine. Cefazolin should not be used as a class representative for ceohalothin and other first-aeneration ceohalosoorins because of an unacceotabie rate of false susceptibilities, parh&farly with E:‘co/i. See Table 1 for other drug groupings with a similar spectrum of activity. S. aureus strains exhibiting resistance to one of thepenicitlinase-
d.
e.
f. g.
h.
j.
MRSA infection, the patient has responded poorly to the cephalosporin therapy or convincing clinical data has yet to be derived confirming clinical efficacy (ciavulanic acid combinations and imipenem). Methiciliin-resistant. coaguiase-negative Staphylococcus spp. also appear not to respond well to the above cited drugs.
resistant oenicillins IMRSA) must be reoorted as resistant to ceohalosoorins and other newer biia-/act&k such as am~xici//in/cla&/anic acid, imipenem and ti&rcillin/clavulanic acid, regardless of in vitro tests results. This is primarily because in most cases of documented
The zone sizes obtained with aminogiycosides, particularly when testing P. aeruginosa. are very medium dependent because of variations in divalent cation content. These interpretive standards are to be used only with Mueller-Hinton medium that has yielded zone diameters within the correct range shown in Table 3 when performance tests were done with P. aeruginosa ATCCe 27853. Organisms in the intermediate category may be either susceptible or resistant when tested by dilution methods and should therefore more properly be classified as “indeterminate” in their susceptibility.
c.
b. The category “intermediate” should be reported. It generally indicates that the test result is equivocal or indeterminate (see 2.3.2.3). When designated in this table, a “moderately susceptible” result should be reported to indicate susceptibility under certain conditions (see 2.3.2.2). Several other P-iactams are currently being considered for definition of a moderately susceptible category.
a.
~1
-~1 i
-I
ii
30/~g 30/~g 30/~g 75#g 30p.g
30~g
Cefamandolei
CefazolinJ
Cefonicid i
CefoperazoneJ
CefotaximeJ
CefotetanJ
100#g
100/~g
Carbenicillin when testing the Enterobacteriaceaed
when testing Pseudomonas
30#g
10#g
when testing non-enterococcal streptococci and Listeria monocytogenes g.h
Aztreonam
10#g
when testing enterococci g'h
75#g
10/~g
when testing Haemophilus species e
Azlocillin when testing Pseudomonasd
10/zg
_<12
< 14
_<15
< 14
-< 14
14
_<13
< 17
< 15
< 14
<21
< 16
_< 19
< 28
_< 11
_<13
10/~g
_< 19
20110~g
-<14
20110/~g
when testing staphylococci d
Ampicillin f when testing gram-negative enteric organisms
when testing other organisms
30#g
--
--
--
15-17
15-17
15-17
14-16
18-22
--
15-17
--
--
--
--
12-13
14-17
--
15-16
13-15
15-22
16-20
--
--
--
--
--
16-21
--
22-29
> 17 h
--
--
--
--
--
--
>_16
>-23
>-21
> 18
>- 18
>_ 18
>_17
> 23
>22
> 16
>30
--
> 20
> 29
>_14
>__18
>-20
>-17
Zone Diameter, nearest whole mm Moderately Disk Content Resistant Intermediate b Susceptible b Susceptible
--
32#g/mL
32/~g/mL
>__
>
>
>
>-
>
>
>
>
>
>
>
>
64~glmL
64#g/mL
64#g/mL
32#g/mL
32#g/mL
32#g/mL
512#g/mL
32#g/mL
32/~g/mL
256p.g/mL
4/ig/mL
16/~g/mL
4/~g/mL
~-Iactamase d
>_
>-32116~glmL
>
16/~g/mL
8/~g/mL
2p.g/mL --
16#g/mL
8#g/mL
64#g/mL
_<
_<
_<
_<
-<
<
16~.glmL
8#g/mL
16/zg/mL
8#g/mL
8#g/mL
8#g/mL
_< 128#g/mL
<
<
_<
< 0.12#g/mL
<
< 0.25#g/mL
<
_<814~glmL
_< 412~glmL
-<
Equivalent MIC Breakpoints a Resistant Susceptible
Zone Diameter Interpretive Standards and Equivalent Minimum Inhibitory Concentration ( MIC) Breakpoints
Amoxicil|in/clavulanic acid when testing Haemophilus& staphylococci d,e
Amikacin c
Antimicrobial Agent
TABLE 2, M7- A
These values represent MC breakpoints used in determining approximate zone size interpretive criteria. They relate to MiCs determined by M7 methodology. Occasional discrepancies may exist between M2 and Y7 due to methodological limitations.
Resistant strains of S. aureus produce @-lactamase and the testing of the 10 unit penicillin G disk is preferred. Penicillin G should be used to test the susceptibility of all peniciilinasesensitive penicillins. such as ampicillin, amoxicillin alone. ariociilin. bacampiciliin. hetaciliin, carbeniciiiin. meziociilin. piperaciiiin, and ticarciiiin. Results may also be applied to phenoxymethyl penicillin or phenethiciiiin. For testing Haemophilus use Mueller-Hinton agar supplemented with 1% hemoglobin (2-3 % horse blood) and 1% IsoVitaleX (EBL). or 1% Supplement XV (Difco) or an equivalent synthetic supplement. Adjust pH to 7.2. Prepare the inoculum by suspending growth from a 24hour chocolate agar plate in Mueller-Hinton broth to the density of a turbidity standard (see 4.1.3). The vast majority of ampicillin-resistant strains of Haemophilus produce detectable alactamase. Class disk for ampicillin, amoxiciilin. bacampicillin. cyciacillin, and hetaciilin. For
enterococci. other Streptococcus spp. and non-penicillinase-producing peniciliinsensitive organisms, and Listeria monocy?ogenes. the former intermediate interpretation should be reported as “Moderately Susceptible.” Results in this category include enterococci and Listeria which for blood or serious invasive tissue infections require high dosage of penicillin or ampicillin, often combined with an aminoglycoside (gentamicin) for improved therapeutic response and bactericidal action. For streptococci. staphylococci, and other penicillin-sensitive organisms, “Susceptible” results should be regarded as “Very Susceptible.” Enterococci strains (S. faecalis. S. faecium, and S. durans) producing zones L3Omm for ampicillin or Z28mm for penicillin are quite unusual and the streptococcal speciation procedures should be reexamined. Aztreonam, cefotetan, ceftazidime. ceftriaxone, and imipenem are among the most recently studied beta-iactams having a separate diagnostic disk and a generally wider spectrum of antimicrobial activity, especially against gram-negative bacilli when compared to previously approved cephaiosporins such as cephalothin. Therefore, the 30 fig cephaiothin disk cannot be used as the cephaiosporin-class representative for these beta-lactams and other cephalosporins available since the mid-1970s. The cephaiothin disk should only be used for testing the susceptibility to cephalothin, cefacior, cefadroxii, cephaiexin. cephapirin, cefazolin, and cephradine. Cefazolin should not be used as a class representative for ceohalothin and other first-aeneration ceohalosoorins because of an unacceotabie rate of false susceptibilities, parh&farly with E:‘co/i. See Table 1 for other drug groupings with a similar spectrum of activity. S. aureus strains exhibiting resistance to one of thepenicitlinase-
d.
e.
f. g.
h.
j.
MRSA infection, the patient has responded poorly to the cephalosporin therapy or convincing clinical data has yet to be derived confirming clinical efficacy (ciavulanic acid combinations and imipenem). Methiciliin-resistant. coaguiase-negative Staphylococcus spp. also appear not to respond well to the above cited drugs.
resistant oenicillins IMRSA) must be reoorted as resistant to ceohalosoorins and other newer biia-/act&k such as am~xici//in/cla&/anic acid, imipenem and ti&rcillin/clavulanic acid, regardless of in vitro tests results. This is primarily because in most cases of documented
The zone sizes obtained with aminogiycosides, particularly when testing P. aeruginosa. are very medium dependent because of variations in divalent cation content. These interpretive standards are to be used only with Mueller-Hinton medium that has yielded zone diameters within the correct range shown in Table 3 when performance tests were done with P. aeruginosa ATCCe 27853. Organisms in the intermediate category may be either susceptible or resistant when tested by dilution methods and should therefore more properly be classified as “indeterminate” in their susceptibility.
c.
b. The category “intermediate” should be reported. It generally indicates that the test result is equivocal or indeterminate (see 2.3.2.3). When designated in this table, a “moderately susceptible” result should be reported to indicate susceptibility under certain conditions (see 2.3.2.2). Several other P-iactams are currently being considered for definition of a moderately susceptible category.
a.