Beckman Coulter MicroScan B1016-138 Manuel D'utilisation page 10

CEFOXITIN SCREEN INTERPRETIVE CRITERIA
Test
Cefoxitin Screen
The Cefoxitin Screen is intended to determine the susceptibility of staphylococci to the penicillinase-stable beta-lactams (e.g.,
oxacillin), using the Cefoxitin Screen Well (CfxS) and the oxacillin MIC result at 16-20 hours. The CfxS result and oxacillin MIC
are read independently at 16-20 hours and then processed through the LabPro software or interpreted manually to determine
the final interpretation to oxacillin. The interpretation rules are shown in the following table:
CfxS Result
≤4 μg/mL Negative
> 4 μg/mL Positive
Comment: CNS with CfxS ≤4 and oxacillin MICs of 1 or 2 give variable mecA results. Perform mecA testing if beta-lactam
therapy is critical for patient care.
Interpretations of S* or R* are used by the LabPro software when the Cefoxitin Screen result changes the interpretation of the
oxacillin MIC result. The LabPro software will flag CNS isolates when CfxS is negative and oxacillin MICs are 1 or 2 µg/mL.
The default interpretation will be R, however, mecA status is variable and must be checked before these isolates are reported
as oxacillin susceptible (mecA negative only). These criteria should also be followed when interpreting the results manually;
however, the asterisk is not required.
INDUCIBLE CLINDAMYCIN TEST
The Inducible Clindamycin test is intended to detect inducible clindamycin resistance in staphylococci resistant or intermediate
to erythromycin and susceptible or intermediate to clindamycin. Expression of resistance due to the erm gene may require
induction by erythromycin. Results of ICd are equivalent to the D-zone disk approximation test. The interpretive criteria are
shown in the following table:
Antimicrobial Test
Inducible Clindamycin Test -
Staphylococci
When erythromycin is I or R and clindamycin is S or I, and the ICd test is positive, clindamycin should be reported as resistant.
KANAMYCIN, STREPTOMYCIN AND GENTAMICIN SYNERGY SCREEN
In enterococcal endocarditis the use of penicillin or ampicillin alone results in frequent treatment failures. When enterococci
are susceptible in vitro to high levels of kanamycin, streptomycin or gentamicin, the addition of this antimicrobic to penicillin
or ampicillin is synergistic and correlates clinically with an improved cure rate.
recommended method for detection of high level aminoglycoside resistance (HLAR) for broth microdilution is as follows.
Antimicrobic
Gentamicin
Streptomycin
1. Comparable results have been shown in limited testing with dextrose phosphate broth.
The performance of Kanamycin, Gentamicin and Streptomycin Synergy Screens on the MicroScan Panels was compared
to the micro-broth reference methods recommended by the CLSI. Any evidence of turbidity should be considered growth or
reincubated to confirm results. The results obtained with Gentamicin Synergy after 18 hours incubation were comparable to
those obtained at 24 hours with the reference method. For best detection of resistance with Streptomycin Synergy Screen,
MicroScan Panels should be incubated for 24 to 48 hours. For best detection of resistance with Kanamycin Synergy Screen,
MicroScan Panels should be incubated for 24 hours.
THYMIDINE FREE GROWTH WELL
Some bacteria require thymidine for growth. These bacteria may exhibit false susceptibility to the sulfonamides due to the lack
of thymidine in the Mueller-Hinton broth. If an organism does not grow in the TFG well, an MIC should not be reported for T/S.
C29870–AD
Oxacillin MIC
≤0.25
0.5
1 or 2
>2
≤0.25
0.5
1 or 2
>2
≤4/0.5 µg/mL
Medium Incubation
BHI 24 hours
1
BHI 24-48 hours
1
Negative
≤4
Final Oxacillin Interpretation
S. aureus or
S. lugdunensis
S
S
S
R
R*
R*
R*
R
Negative
34,35 According to CLSI Document M07-A10, the
Concentration
500 μg/mL
1000 μg/mL
10 of 339
Positive
>4
Other CNS
S
S*
R check mecA (see
comment)
R
R*
R
R
R
Positive
>4/0.5 μg/mL