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Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

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ANTIMICROBIAL SUSCEPTIBILITY TESTING

Item Value
Available Stat? No
Test code MC
Performed by? Microbiology
Sendout? no
Price range $
In House Availability Susceptibility tests for blood and CSF isolates are setup and read 7 days a week. Isolates from other sites are set up Monday-Saturday; for tests set up on Saturday, results are available on Monday. By special request, susceptibilities on non-blood or non-CSF isolates may be read or set-up on Sundays.

Organisms tested: Aerobic bacterial isolates that are potentially pathogenic will be tested for antimicrobial susceptibility. For specific organisms, refer to additional information section.

Antimicrobial susceptibility tests available at SFGH include:

1. MIC (Minimum Inhibitory Concentration ) test by microdilution
2. Kirby Bauer disk diffusion test
3. Beta-lactamase test
4. MIC (Minimum Inhibitory Concentration) test by macrodilution, available upon request
5. MLC (Minimum Lethal Concentration) test by macrodilution, available upon request
6. Synergy tests: Combination MLC test or Time Kill Curve, available upon request


All cultures are saved for at least three days after the final report date, which permits requesting a susceptibility test if one has not been performed. Special susceptibility tests outside those routinely performed must be requested through the Microbiology Resident 206-5699 or pager (415) 443-1438.

MIC and Interpretative information for certain antimicrobial agents is suppressed. Suppressed information may be obtained through the Infectious Diseases Team in Medicine (pgr: 443-2847) or Pediatrics (pgr: 443-9503), or through the Microbiology Resident at 206-5699 or pager (415) 443-1438. A decision to suppress information on antimicrobial activity is made by the SFGH Antibiotic Advisory Subcommittee of the Pharmacy and Therapeutics Committee.

Principle Bacteria cultured from a patient's specimen are exposed to antimicrobial agents, either in broth or on the surface of an agar plate. What is most commonly measured is the ability of an antimicrobial agent to inhibit bacterial growth. Other susceptibility methods can measure the ability of an agent to kill bacteria. Antimicrobial susceptibility tests can yield quantitative or qualitative results.
Interpretation Antimicrobial susceptibility tests are used to predict the successful treatment of an infection with a particular antimicrobial agent.

Susceptible: Organism is inhibited by concentrations of antimicrobial achieved in blood with usual doses of drug.

Intermediate: Organism may be susceptible or resistant. This interpretive category has clinical applicability in body sites where the antimicrobial is physiologically concentrated, or when higher than usual doses of the antimicrobial can be used. Note: Some antibiotics, such as penicillins and cephalosporins, are concentrated in the urine at a very high level. If the object of therapy is only to clear the urine of bacteria, some antibiotics may be effective when the organism is reported Intermediate by the above definition.

Resistant: Organism is not inhibited by concentrations of antimicrobial achieved in serum with usual therapy.

Beta-lactamase positive: Indicates resistance to all penicillinase-labile penicillins, including ampicillin, amoxicillin, ticarcillin, and piperacillin.

Beta-lactamase negative: A negative beta-lactamase test result cannot predict susceptibility to the above antimicrobials, as other resistance mechanisms may be present.
Synonyms Beta-lactamase Test; Disk Diffusion Susceptibility Tests; Drug Sensitivity; Macro Tube Dilution Tests; MIC (Minimum Inhibitory Concentrations); Microdilution Susceptibility Tests; Minimum Inhibitory concentrations (MIC); Minimum Lethal Concentrations (MLC) and Synergy; MLC (Minimum Lethal Concentrations); Serum Antimicorbial Titer; Serum Cidal Test; Streptococcus pneumoniae Susceptibility to Penicillin; Susceptibility Testing for Anaerobic Bacteria; Susceptibility Testing for Bacteria; Susceptibility Testing for Fungi and Nocardia; Time Kill Curve/Killing Rate by an Antimicrobial;
Turn around times Results are generally available 24 hours after isolation of the organism.
Additional information Supplemental Susceptibility Information

Organism Comment
Enterococcus species MIC susceptibility test is performed when isolated from blood or CSF, and when in pure culture from other sterile sites.
All other Enterococcus isolates are batch tested for vancomycin resistance once a week. If vancomycin resistance is detected, it is reported and Infection Control is notified.
Escherichia coli, Klebsiella pneumoniae, and Klebsiella oxytoca When susceptibilities are performed, these isolates are evaluated for extended spectrum beta-lactamase production.
Haemophilus species All isolates are tested for beta-lactamase production.
Moraxella catarrhalis All isolates are tested for beta-lactamase production.
Neisseria gonorrhoeae All isolates are tested for beta-lactamase production.
Staphylococcus sp When susceptibilities are performed, the clindamycin result is based upon two tests, the MIC test and the D test (detects inducible clindamycin resistance).
Streptococcus pneumoniae MIC susceptibility test is performed when isolated from blood or CSF. All other isolates are tested against penicillin G, erythromycin and tetracycline; if isolate is not susceptible to any of the above, MIC tests for ceftriaxone and other antimicrobials are performed.
Streptococcus species not pneumoniae MIC susceptibility test is performed on isolates from blood, CSF or bone, and when in pure culture from other sterile sites. When susceptibilities are performed on beta-hemolytic Streptococci, the clindamycin result is based upon two tests, the MIC test and the D test (detects inducible clindamycin resistance).
Anaerobes A test for beta-lactamase is performed on anaerobic Gram negative rods. Other tests available upon request through the Microbiology Resident, 206-8576 or pager (415) 327-9695.
Mycobacterium tuberculosis Tests are performed by the S.F. Department of Public Health. For these results call TB registry, 206-8524.
Fastidious Organisms
(bacteria, fungi, viruses, Nocardia)
Tests are available upon request through the Microbiology Resident, 206-8576 or pager (415) 443-1438.



TABLE M-I: REPORTED ANTIMICROBIALS
CONCENTRATIONS TESTED BY MICRODILUTION (mcg/mL) (x=reported)
Gram Positive Cocci Staphylococci Staphylococci S. pneumoniae
Concentrations
Tested (mcg/mL)
Nonurine
Isolates
Urine
Isolates
Concentrations
Tested (mcg/mL)
Penicillin 0.03, 0.12-0.25,
2,8
X X 0.03-4
Nafcillin 0.25-2 X X -
Amoxicillin /
Clavulanate
4/2 X - -
Vancomycin 0.25-16 X - 0.12-1
Trimeth/Sulfa 0.5/9.5, 2/38 X X 0.25/4.75-2/38
Cefazolin 8-16 X X -
Cefotaxime - - - 0.25-2
Ceftriaxone 4-32 X X 0.25-2
Clindamycin 0.25-4 X - 0.06-0.5
Erythromycin 0.25-4 X - 0.06-0.5
Tetracycline 1-8 X X 0.5-4
Nitrofurantoin 32-64 - X -
Levofloxacin 0.5-4 X X -
Gentamicin 1-8 X X -



TABLE M-I: REPORTED ANTIMICROBIALS,
CONCENTRATIONS TESTED BY MICRODILUTION (mcg/mL)

Gram Negative Rods
Concentrations
Tested (mcg/mL)
Enterics Non-
Enterics
Salmonella/
Shigella
Ampicillin 8-16 X - X
Piperacillin/Tazobactam 8/4-16/4, 64/4 X * -
Trimeth / Sulfa 2/38 X X** X
Cefazolin 4-16 X - -
Ceftriaxone 8,32 X X X
Ceftazidime 2-16 X X -
Cefepime 2-16 X X -
Ciprofloxacin 1-2 X X X
Levofloxacin 2-4 X X X
Gentamicin 1-8 X X -
Tobramycin 1-8 X X -
Ertapenem 2-4 X - -

* Reported on Ps. aeruginosa
** Not reported on Ps. aeruginosa


For Antimicrobial Pharmacology, body fluid concentrations, see Table M-II Antimicrobial Pharmacology.
References 1. Clinical Laboratory Standards Institute (CLSI), Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard: 8th Edition, Document M7-A8 Vol. 29, No. 2, January 2009.

2. Clinical Laboratory Standards Institute (CLSI), Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard: 9th Edition, Document M2-A10, Vol. 29, No. 1, January 2009.

3. Clinical and Laboratory Standards Institute (CLSI), Performance Standards for Antimicrobial Susceptibility Testing; Twentieth Informational Supplement, M100-S20 (2010), Vol. 30, No. 1, January 2010.

Last Updated 11/10/2014 4:53:38 PM
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