scope:

Foreword

All of the susceptibility test methods commonly performed by clinical microbiology laboratories (e.g., disk diffusion, broth dilution, and agar dilution) measure the inhibitory activity (MIC) of an antimicrobial agent.^1,2,3 In most clinical situations, this is sufficient as the role of the antibiotic is to prevent the spread of bacteria from the focus of infection by preventing microbial replication at new sites; the active participation of the host's defense mechanisms finally achieves bacterial eradication and clinical cure.⁴ Antimicrobial assays can provide additional valuable information on the pharmacokinetics of the agent(s) being used and, when combined with the MICs, can allow bacterial eradication to be predicted.

On occasion, it may be necessary to achieve bactericidal activity with an antimicrobial agent. This need has been well documented for endocarditis⁵ and has been suggested by some for meningitis,⁶ for osteomyelitis,⁷ as well as for infections in immunocompromised patients.⁸ The clinical occurrence of tolerance⁹ may on rare occasion necessitate bactericidal testing.

When assessment of bactericidal activity is deemed appropriate, an in vitro test method such as the MBC determination or the use of time-kill kinetic methodology may be useful. Bactericidal activity against the patient's isolate by the antibiotic tested allows eradication to be predicted based upon the usual dosing of this antibiotic or based upon the results of an antimicrobial assay. When clinical experience is lacking and assay methods are not readily available, the serum bactericidal test which integrates both pharmacodynamic and pharmacokinetic properties may be more useful. Depending on certain modifications to the serum bactericidal test, the test can provide a quantitative assessment of bactericidal activity relative to the MBC (the serum bactericidal titer), a dynamic assessment of rapidity of killing over time (the serum bactericidal rate), or both the magnitude of serum bactericidal activity and its duration (the area-under-the-bactericidal-titer-curve). In addition, methods using serum from persons (e.g., volunteers) receiving antibiotics (ex vivo) can be used to assess antimicrobial bactericidal activity across drug classes or between members of a class against a wide variety of microorganisms.

Because of the complexity involved with the serum bactericidal test (including the particular method used, the proper collection of timed serum specimens, and the interpretation of results), and the lack of clinical data clearly documenting the usefulness of this test for most infections, it is recommended that consultation with the microbiology laboratory be obtained as a prerequisite for this test. The assistance of the laboratory's director is useful in (1) determining if such a test is needed; (2) selecting NCCLS recommended methodology for testing; and (3) interpreting the results. Techniques for the conduct of the serum bactericidal test may be found in the most current edition of NCCLS document M21-Methodology for the Serum Bactericidal Test.

This document describes the details of bactericidal testing and, in particular, the effects of variations in methodology. This information has been obtained largely from published data. Use of these guidelines should result in uniform methodology for bactericidal testing that is sufficiently practical for use in the clinical microbiology laboratory. The methodology described in this document does not imply per se that bactericidal testing is clinically relevant, but instead allows such testing to be used as a tool to assess clinical relevance. The techniques described in this document are intended primarily for testing aerobic bacteria that grow well after overnight incubation in either Mueller-Hinton broth or in Mueller-Hinton broth supplemented with human serum as described in Section 2.2.1. Modifications for more fastidious microorganisms such as anaerobes will be described in detail in the future.