HSC 308 Exam 5- Susceptibility Testing ch 13

Know when is testing performed, factors to consider, primary objectives in reporting:

Perform testing when susc. pattern is unknown; appropriate for body site and normal flora; required for host (allergies, immunocomp)

What are the primary objectives of susc. testing?

MIC

MBC

SIC

SBC

breakpoints

susceptible

resistant

What is the CLSI and why are they important?

Clinical Laboratory Standards Institute – panels of experts that set the breakpoints.

What is the purpose of a McFarland standard and why should inoculums be standardized?

Turbidity standard used to prepare standardized and consistent inoculums, (usually 0.5 or half McFarland or 1.5X108CFU/ml), to reduce sources of error.

Broth Microdilution tests

Advantages – can use several conc. of many drugs; uses less media and drug; can set up quickly; can become automated

Disadvantages – Drugs and dilutions are set; Expensive

Understand how to read wells and how to determine the MIC

Disk diffusion/Kirby-Bauer: Gold standard for Susc. testing

Describe the procedure to set up a Kirby-Bauer susceptibility plate.

Discuss the advantages and disadvantages of this test.

What is the purpose of a purity plate?

Need purity plate to insure pure inoculum and that only 1 organism is being tested

What gene is associated with MRSA? mecA; with VRSA? vanA, vanB;

What is Inducible Resistance? What problems does it cause the lab? Why do we test for it?

What is the D-test and what is its purpose?

What is a VRE?

Vancomycin Resistant Enterococcus – use vancomycin screen plate to find. E. faecium is most common VRE, followed by E. faecalis.

What is ESBL, what problems does it cause for the lab, why and how do we test for it?

What is CRE and why is it important? What test is used to detect CRE?

E-test

Procedure – streak plate like K-B (3 directions) then drop E-test strip. Look for elliptical shape – MIC = where ellipse crosses strip.

Advantages: tests a gradient of antibiotic; can use with fastidious organisms on variety of media and in CO2 environment.

Disadvantages: Expensive

Tests for beta-lactamase presence

Cefinase disk – turns color with penicilloic acid

Special Susceptibility Test

MBC test: uses broth macrodilution with a serial dilution of antibiotic and organism to find MIC and then plate MIC tube and higher conc to find MBC

SBC/Schlichter test: Serial dilution of pt serum with organism to find SIC, then plate SIC tube and higher conc to find SBC. Both are used when pt. immune system needs more help than MIC level to eradicate organism; also with peak and trough dosages.