Antimicrobial Susceptibility Testing AST Purpose Offer guidance to

Antimicrobial Susceptibility Testing (AST)

Purpose Offer guidance to physician in selecting effective antibacterial therapy for a pathogen

Factors to Consider When Determining Whether Testing is Needed • Body site of infection Susceptibility not performed on bacteria isolated from body site where there are normal flora Ex. Susceptibility for E. coli is NOT performed when isolated from stool, but IS performed when isolated from blood

Factors to Consider When Determining Whether Testing is Needed • Presence of other bacteria in specimen • Ex. Two or more organisms grown in a urine specimen • Health status of patient • Immunocompromised patients • Allergies to usual antimicrobials

Selecting Antimicrobial Agents for Testing Primary objective • Use the least toxic • Refrain from more costly agents • Avoid broader-spectrum agents

Selecting Antimicrobial Agents for Testing • There approximately 50 antibacterial agents • Each laboratory should have a battery of antibiotics ordinarily used for testing • Drugs to use decided by medical staff, pharmacists, and medical technologists

Selecting Antimicrobial Agents for Testing • There approximately 50 antibacterial agents • Each laboratory should have a battery of antibiotics ordinarily used for testing • Drugs to use decided by medical staff, pharmacists, and medical technologists

Selecting Antimicrobial Agents for Testing • Generally, labs choose 10 -15 antibiotics to test susceptibility for Gram positive organisms and another 10 -15 for Gram negative organisms • Too many choices can confuse physicians and be too expensive

Drug Ampicillin Enterococcus Staphylococcus spp. X Cefazolin X Clindamycin Erythromycin Linezolid X X Oxacillin Penicillin G X X Rifampin X X Streptomycin-2000 X Tetracycline X Trimeth/ Sulfa Vancomycin X X X

Disk Diffusion/ Kirby- Bauer • Procedure • Use a well-isolated, 18 -24 hour old organism • Inoculate agar by swabbing in three different directions “Lawn of growth” • Place filter paper disks impregnated with antimicrobial agents on the agar • Invert and incubate for 16 -18 hours at 35 o. C in non-CO 2

Disk Diffusion/ Kirby-Bauer • During incubation, drug diffuses into agar • Depending on the organism and drug, areas of no growth form a zone of inhibition • Zones are measured to determine whether the organism is susceptible, intermediate, or resistant to the drug

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