[Antibiotic combinations and serum concentrations: a study using a computer program]

An original computer program developed to study antibiotic combinations is presented. In vitro combination effects were determined by broth microtiter checkerboard method. Inhibitory and bactericidal concentrations for each antibiotic alone and in combination were registered on a microcomputer (Apple Lisa). The program (Pascal UCSD) allowed to determine FIC and FBC index and a therapeutical index (IFT) based on reduction of concentrations under mean serum levels for the two antibiotics. An IFT less than 1 correlated to a combination of antibiotics with achievable therapeutic levels. More the IFT was low more the combination was effective. This therapeutical index was calculated for various combinations of antibiotic concentrations.