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High Level Aminoglycoside Resistance (HLAR) Screening

HLAR is another significant acquired resistance factor. Since the standard approach for treating systemic infections is a combination of a cell wall targeted antibiotic with an aminoglycoside, assessment of resistance to both classes of antibiotics is important. High level resistance to aminoglycosides will negate the synergistic effect of combined therapy with either penicillin or vancomycin.
Standard susceptibility methods (either disk diffusion or broth dilution) will not detect HLAR patterns, unless the protocol incorporates testing at increased concentrations of gentamicin and/or streptomycin. CLSI document M100-S23 outlines recommended protocols for screening for HLAR.
Gentamicin HLAR
Disk diffusion
MHA agar; 120 µg gentamicin disk; standard inoculum; standard incubation temperature (35 +/- 2° C; ambient air), 16-18 hours incubation duration
Interpretation
  • Resistant = 6 mm
  • Inconclusive = 7-9 mm
  • Susceptible > 10 mm
Broth microdilution
BHI broth; 500 µg/mL gentamicin; standard inoculum; standard incubation conditions, incubation duration 24 hours.
Interpretation
Any growth equates to resistant.
Streptomycin HLAR
Disk diffusion
MHA agar; 300 µg streptomycin disk; standard inoculum; standard incubation conditions, incubation duration16 - 18 hours.
Interpretation
  • Resistant = 6 mm
  • Inconclusive = 7-9 mm
  • Susceptible > 10 mm
Broth microdilution
BHI broth; 1000 µg/mL streptomycin; standard inoculum; standard incubation conditions; incubation duration 24-48 hours. If susceptible at 24 hours, reincubate and re-read at 48 hours.
Interpretation
Any growth equates to resistant.
Clinical correlation
A resistant result indicates that synergistic effects will not be achieved between the indicated aminoglycoside and the cell wall active agent (eg, ampicillin, penicillin, or vancomycin).
A susceptible result indicates that synergistic effects are possible.