Agar dilution method for detection of inducible clindamycin resistance in Staphylococcus spp

We describe the development and validation of an agar dilution method for the detection of inducible clindamycin resistance by using 227 previously characterized erythromycin-resistant, clindamycin-susceptible Staphylococcus sp. isolates. Mueller-Hinton agar with defibrinated horse blood containing a range of erythromycin concentrations (1 to 8 mg/liter) combined with clindamycin at 0.5 mg/liter was used to determine the optimal concentration that produced growth of inducible isolates while inhibiting that of isolates without the inducible phenotype. A concentration of clindamycin of 0.5 mg/liter with erythromycin at 1 mg/liter was the optimal combination for detection of inducible resistance and resulted in a sensitivity of 100% (95% confidence interval [CI], 97.9 to 100) and a specificity of 100% (95% CI, 93.0 to 100). Attention must be paid to ensuring that a sufficient inoculum has been used, since an inoculum below the standard 10(7) bacteria/ml may result in false-negative results. This method has been incorporated into routine use in our laboratory.