Comparison of direct and standardized antimicrobial susceptibility testing of positive blood cultures

In an effort to determine the feasibility and accuracy of performing direct susceptibility tests from positive blood cultures, minimal inhibitory concentrations (MICs), determined by the agar dilution method, of direct and standardized tests with seven antibiotics were compared. Results were analyzed as to the number of very major (change in MIC from susceptible in preliminary direct testing to resistant in final standardized testing), major (change in MIC from resistant to susceptible), and minor (change in MIC without change in interpretation) discrepancies. The results for gram-positive cocci and for gram-negative bacilli were 5, 6, and 82 of 162 strains tested and 3, 12, and 79 of 90 strains tested, respectively. Of the total number of susceptibility tests compared, major and very major discrepancies occurred in only 1 and 2.4% of instances with gram-positive and -negative isolates, respectively. The majority of discrepancies were noted with Staphylococcus epidermidis (four very major, five major), Klebsiella (two very major, four major), and Alcaligenes (five major). The antibiotics most often exhibiting discrepancies were penicillin, ampicillin, and cephalothin. The results indicate that preliminary susceptibility testing directly from positive blood culture bottles is generally both feasible and accurate.