Influence of technical factor variations on serum inhibition and bactericidal titers

The influence of technical factor variations on serum bactericidal and serum inhibitory titers was studied by using Staphylococcus aureus clinical isolates versus oxacillin-spiked human serum. Parallel tests, both with and without the use of beta-lactamase in count plates to inactivate oxacillin carryover, were performed with a conventional macrodilution approach, a carefully controlled macrodilution procedure, and a standard microdilution method. Careful control of technical factor variations diminished the incidence of low serum bactericidal titers and decreased the dispersion of results, a finding corollary to the known influence of technical factor variations on the measurement of MBCs. The incorporation of beta-lactamase into count plates resulted in a shift of serum bactericidal titers to lower values. The microdilution method appeared to be least influenced by technical variations and, with the addition of beta-lactamase to count plates, provided the best results.