Infectious Diseases II

Automated systems (e.g., Vitek, Microscan, Sensititre, Phoenix, Accelerate Pheno test)

Uses computerized algorithms for interpreting MIC values

Can usually perform AST more quickly than traditional methods

More than 80% of all clinical microbiology laboratories report using an automated system as their

primary method of susceptibility testing.

Standard inoculums of pathogens are used. For infections in which the in vivo inoculum may be

higher, certain antimicrobials may have higher MIC values.

Specific confirmatory tests for antimicrobial resistance

Macrolide-lincosamide-streptogramin resistance

Strains of Staphylococcus spp. can have a transferable resistance mechanism called macrolide-

lincosamide-streptogramin, which is inducible by clindamycin and can lead to treatment

failure.

ii.

Inducible resistance is not detected by routine AST.

iii.

Detected using double-disk diffusion

ESBL

CLSI procedures exist for K. pneumoniae, Klebsiella oxytoca, E. coli, and Proteus mirabilis.

ii.

Initial screening with susceptibility testing for ceftazidime, aztreonam, cefotaxime, or

ceftriaxone

iii.

Confirmatory testing with broth microdilution, disk diffusion, or E-test strips. Presence of

ESBL is confirmed if adding clavulanic acid results in three 2-fold dilution decreases in the

paired-cephalosporin MIC.

iv.

Beginning in 2010, CLSI adopted lower MIC breakpoints for many cephalosporins for

Enterobacterales (see Table 7). The rationale for this change is that the lower MIC may more

readily detect the presence of ESBL, hence eliminating the need for the labor-intensive

confirmatory tests. Compliance with this new practice is variable among clinical microbiology

laboratories. Clinicians should inquire within their local clinical microbiology laboratory

regarding whether the new standards have been adopted. If a microbiology laboratory

has adopted the new CLSI recommendations for ESBL detection, confirmatory tests are

unnecessary.

Agent

Pre-2010 CLSI Breakpoints

2023 CLSI Breakpoints

S

I

R

S

I

R

Cefazolina

≤ 8

≥ 32

≤ 2

≥ 8

Cefazolinb

≤ 8

≥ 32

≤ 16

—

≥ 32

Cefotaxime

≤ 8

16–32

≥ 64

≤ 1

≥ 4

Ceftriaxone

≤ 8

16–32

≥ 64

≤ 1

≥ 4

Ceftazidime

≤ 8

≥ 32

≤ 4

≥ 16

Aztreonam

≤ 8

≥ 32

≤ 4

≥ 16

aCefazolin used as therapy for infections other than uncomplicated UTIs.

bCefazolin used as therapy for uncomplicated UTIs.

CLSI = Clinical & Laboratory Standards Institute; UTI = urinary tract infection.

AmpC β-Lactamase

Inhibitor-resistant β-lactamase (e.g., clavulanic acid, tazobactam, avibactam, vaborbactam,

durlobactam, and relebactam)