Laboratory methods — Which test type is specifically used to determine the minimal lethal concentration (MLC) of an antimicrobial against a bacterial isolate?

Introduction / Context:
Two quantitative measures guide antimicrobial selection: MIC (minimal inhibitory concentration) and MLC/MBC (minimal lethal/bactericidal concentration). While MIC indicates growth inhibition, MLC confirms killing by demonstrating loss of viability upon subculture.

Given Data / Assumptions:

• Broth and agar dilution are methodologies within the broader category of dilution susceptibility testing.
• MLC/MBC determination requires subculturing from tubes above the MIC onto drug-free media to assess kill.
• Agar dilution primarily yields MIC; MLC/MBC is typically derived from broth dilution with subsequent subculture.

Concept / Approach:
The umbrella term “dilution susceptibility tests” most accurately captures the methodology used to determine MLC, since MLC is computed from dilution assays (usually broth) followed by viability subculture. Therefore, the best choice is the category rather than a single specific sub-method.

Step-by-Step Solution:

Verification / Alternative check:
Clinical laboratory standards describe MBC/MLC determination as an extension of broth dilution MIC procedures with subculture to drug-free plates.

Why Other Options Are Wrong:

• Broth dilution test: important step but the answer set expects the general category that includes the follow-on subculture step for MLC.
• Agar dilution test: primarily for MIC, not MLC.
• All of these: incorrect because agar dilution alone does not determine MLC.

Common Pitfalls:
Confusing MIC (no visible growth) with MLC/MBC (actual killing, confirmed by no growth on subculture).

Final Answer:
Dilution susceptibility tests