Daptomycin In Vitro Susceptibility Methodology: a Review of Methods, Including Determination of Calcium in Testing Media

Daptomycin has been available as a gram-positive antimicrobial agent in the United States for over 6 years, and during this time there have been developments in daptomycin susceptibility testing. Daptomycin has a unique requirement for calcium; consequently, a standard calcium concentration is needed when performing in vitro testing. The calcium levels in broth and agar are presented, and with the exception of calcium-adjusted Mueller-Hinton broth, demonstrate variation between types and manufacturers. The reference methodology for daptomycin MIC testing is broth microdilution utilizing Mueller-Hinton broth with 50 μg/ml of calcium. Daptomycin is available in several commercial testing systems, which have been shown to be comparable to the reference method. Specific MICs from several Etest studies have been compared to broth microdilution MICs, and essential agreements were 79.1 to 100% (Staphylococcus aureus) and 66.7 to 100% (enterococci). The performance of routine quality control is especially important for in vitro testing of daptomycin, and validation of nonsusceptible daptomycin MICs is recommended.