An uninterpretable AST result most often indicates a test failure or QC out of range; what should be done next?

• A. It may indicate a clerical error in results entry.
• B. The sample is contaminated and should be discarded.
• C. It indicates test failure or QC out of range; the test should be repeated with proper controls.
• D. The organism is susceptible to all tested antibiotics.

Answer: (C)

When an AST result is uninterpretable, the priority is to treat it as a technical failure rather than a clinical finding. The best next step is to repeat the test with proper controls to verify that the method is performing correctly and within acceptable QC ranges.

This means repeating the assay using fresh reagents and media, confirming the inoculum density, and running it alongside quality-control strains that have known susceptibility profiles. If the QC results fall within the expected ranges, the repeated test can yield a trustworthy interpretation. If the repeat remains unreadable, escalate to supervision and consider alternative testing approaches or reference testing.

Clerical errors could complicate reporting, but they don’t address the underlying issue of an unreadable result. Contamination might cause problems, but the standard response is to re-run with proper controls to determine if the problem was technical. Saying the organism is susceptible to all antibiotics isn’t supported by an uninterpretable result.