infectious diseases
Susceptibility Testing
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 4 groupings: S (sensitive), I (intermediate), R (resistant), or sometimes nonsusceptible. MIC determination is used primarily for isolates of bacteria, including mycobacteria and anaerobes, and sometimes for fungi, especiallyCandidaspecies. Minimal killing (bactericidal) concentration (MBC) can also be determined but is technically difficult, and standards for interpretation have not been agreed on. The value of MBC testing is that it indicates whether a drug may be bacteriostatic or bactericidal. 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 is determined by the location on the strip where 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 free drug (ie, drug not bound to protein). If the tissue concentration of free drug is higher than the MIC, successful treatment is likely. Designations of S, I, and R derived from the MIC study usually correlate with achievable serum, plasma, or urine concentrations of free drug. 