Rapid Method for Determining the Minimum Inhibitory Concentration of Ampicillin for Haemophilus influenzae

A rapid method for determining the minimal inhibition concentrations (MICs) of ampicillin using Haemophilus influenzae as the test organism is described. This semiautomated technique using the Autobac system was compared with the agar dilution MIC method. Fifty-seven isolates of both susceptible and resistant H. influenzae were tested. There was exact quantitative agreement with 65% of the isolates, and 91% of the isolates showed no more than a one-dilution-interval difference when MICs were determined by both methods. Total testing time was 3 to 4 h.