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Susceptibility tests determine a microbe's vulnerability to antimicrobial drugs by exposing a standardized concentration of organism to specific concentrations of antimicrobial drugs. Susceptibility testing can be performed on bacteria, fungi, and viruses. For some organisms, results obtained with one drug predict results with similar drugs. Thus, not all potential drugs are tested.
Susceptibility testing occurs in vitro and may not account for many in vivo factors (eg, pharmacodynamics and pharmacokinetics, site-specific drug concentrations, host immune status, site-specific host defenses) that influence treatment success. Thus, susceptibility test results do not always predict treatment outcome.
Susceptibility testing can be done qualitatively, semi-quantitatively, or using nucleic acid–based methods. Testing can also determine the effect of combining different antimicrobials (synergy testing).
Qualitative methods:
Qualitative methods are less precise than quantitative. Results are usually reported as susceptible (S), intermediate (I), or resistant (R). The commonly used disk diffusion method (also known as the Kirby-Bauer test) is appropriate for rapidly growing organisms. It places antibiotic-impregnated disks on agar plates inoculated with the test organism. After incubation (typically 16 to 18 h), the diameter of the zone of inhibition around each disk is measured. Each organism-antibiotic combination has different diameters signifying S, I, or R.
Other methods that require less rigid adherence to test methods can be used to rapidly screen for resistance of a single organism to a single drug or drug class or to specific antimicrobial combinations (eg, oxacillin resistance of methicillin-resistant Staphylococcus
aureus, β-lactamase production).
Semiquantitative
methods:
Semiquantitative methods determine the minimal concentration of a drug that inhibits growth of a particular organism in vitro. This minimum inhibitory concentration (MIC) is reported as a numerical value that may then be translated to 1 of 3 groupings: S (sensitive), I (intermediate), or R (resistant). MIC determination is used primarily for bacteria, including mycobacteria and anaerobes but is sometimes used for fungi. Minimal killing (bactericidal) concentration (MBC) can also be determined but is technically difficult, and standards for interpretation have not been agreed upon.
The antibiotic can be diluted in agar or broth, which is then inoculated with the organism. Broth dilution is the gold standard but is labor intensive because only one drug concentration can be tested per tube. A more efficient method uses a strip of polyester film impregnated with antibiotic in a concentration gradient along its length. The strip is laid on an agar plate containing the inoculum and the MIC determined by the location on the strip at which inhibition begins; multiple antibiotics can be tested on one plate.
The MIC allows correlation between drug susceptibility of the organism and the achievable tissue concentration of the drug. If the tissue concentration is higher than the MIC, successful treatment is likely. Similarly, reports of S, I, and R are correlated with MIC but generally are not tissue concentration specific. That is, they are usually based on achievable serum or plasma concentration of drugs.
Nucleic
acid–based methods:
These tests incorporate nucleic acid techniques similar to those used for organism identification (see Laboratory Diagnosis of Infectious Disease: Nucleic Acid–Based Identification Methods) but modified to detect known resistance genes or mutations. An example is mecA, a gene for oxacillin resistance in S. aureus; if this gene is present, the organism is considered resistant to all β-lactam drugs regardless of apparent susceptibility results. However, although a number of such genes are known, their presence does not uniformly confer in vivo resistance. And, because new mutations or other resistance genes may be present, their absence does not guarantee drug sensitivity. For these reasons and because the tests are limited in number, expensive, and not widely available, they have not replaced standard culture and sensitivity testing.
Last full review/revision November 2005
Content last modified November 2005
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