Value of serum tests in combined drug therapy of endocarditis

Two in vitro tests, the serum killing level and the serum bactericidal rate assays, were evaluated for correlation with therapeutic efficacy in the rabbit model of Staphylococcus aureus endocarditis. Animals were treated with nafcillin alone and in combination with tobramycin or gentamicin. Both were effective therapies, but rapidity of vegetation sterilization by the single and combined regimens was shown by the serum bactericidal rate assay but not the serum killing level assay. As a direct measure of bactericidal activity in serum during therapy, the serum bactericidal rate assay may be a clinically useful supplemental test for providing information that the serum killing level assay cannot.