Antifungal Susceptibility Test Interpretive Criteria

Why It Matters

Recognized STICs guide clinical laboratories in reporting reliable susceptibility results, influencing treatment decisions and drug development strategies. Absence of FDA breakpoints for many agents creates uncertainty for clinicians interpreting antifungal susceptibility data.

Key Takeaways

• FDA recognizes CLSI M27M44S and M38M51S standards
• Only specific antifungals have FDA‑approved susceptibility breakpoints
• Amphotericin B and several oral agents lack recognized STIC
• Itraconazole and rezafungin include CLSI exceptions
• FDA does not endorse epidemiological cutoff values

Pulse Analysis

Regulatory endorsement of antifungal susceptibility testing standards is a cornerstone for consistent laboratory practice. By aligning with the latest CLSI guidelines, the FDA provides a clear framework for which drugs can be reported with validated breakpoints, ensuring that clinicians receive actionable data for agents such as anidulafungin, caspofungin, and voriconazole. This alignment also streamlines the validation process for diagnostic manufacturers, reducing redundancy and accelerating the adoption of standardized methods across U.S. clinical labs.

The table of recognized drugs highlights a fragmented landscape: while injectable echinocandins and fluconazole have established interpretive criteria, many oral agents—including amphotericin B, griseofulvin, and terbinafine—remain without FDA‑approved breakpoints. For these drugs, laboratories must rely on alternative metrics or extrapolate from non‑FDA sources, which can lead to variability in reporting and potential misinterpretation of resistance patterns. The noted exceptions for itraconazole and rezafungin illustrate that even when a drug is covered by CLSI standards, specific adjustments may be required to reflect FDA expectations.

Understanding that the FDA does not recognize epidemiological cutoff values (ECVs) is critical for both clinicians and researchers. ECVs serve as epidemiological markers rather than clinical decision points, and their exclusion from FDA‑recognized criteria underscores the agency’s focus on clinically validated breakpoints. As resistance trends evolve, stakeholders will likely advocate for expanded STIC coverage, prompting future revisions of CLSI documents and FDA guidance. In the interim, laboratories must navigate the current framework carefully, ensuring that susceptibility reports are both compliant and clinically meaningful.