The D-test is routinely performed on staphylococcal isolates to detect CA-MRSA with inducible resistance. CA-MRSA may have constitutive or inducible resistance to clindamycin. Constitutive clindamycin resistance denotes innate resistance (listed in susceptibility reports as resistant), whereas inducible clindamycin resistance refers to drug resistance that is induced by exposure to another macrolide antibiotic (eg, erythromycin). This is important because CA-MRSA with inducible resistance initially may be reported as sensitive to clindamycin (as in this case), but resistance to clindamycin may develop quickly after treatment. To perform the D-test, the patients bacterial growth is measured on a plate with erythromycin and clindamycin discs placed in close proximity. Inducible resistance is indicated by a flattening of the zone of inhibition (D shape) as erythromycin diffuses into the same area where clindamycin is present. A positive D-test indicates that therapy with clindamycin should not be used due to the risk of clinical failure. The E-test is a method for obtaining antibiotic susceptibility information, which has already been established in this case. The Schlichter test, or serum bactericidal activity test, is used to monitor antibiotic therapy in patients with endocarditis, osteomyelitis, and other serious bacterial infections and is not indicated in this case.
ß-Lactam resistance is conferred to CA-MRSA by the mec-A gene. Testing for the mec-A gene is helpful prior to obtaining susceptibility results; thus, this test would not provide additional information in this case since susceptibility has already been established.
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Updated 5/23/08 nvf