Storing Antimicrobial Discs: Temperatures, Desiccant, and Why Potency Matters

Cartridges containing commercially prepared antimicrobial discs for susceptibility testing are generally packaged to ensure appropriate anhydrous conditions.

Temperature and moisture are the most critical parameters affecting antimicrobial discs’ performance. Many antimicrobial agents are temperature sensitive, and moisture is a major cause of antibiotic degradation, so one should ensure careful handling of antimicrobial discs during transportation, storage, and regular usage.

Why this matters: a disc that has lost potency produces a falsely small zone of inhibition, which can make a susceptible organism read as resistant. The clinical cost is real. A patient could be denied an antibiotic that would actually work, on the strength of a result that was wrong before the plate was ever read. Every step below exists to keep the disc delivering its stated potency at the moment it touches the agar.

Transport

1. Transport the discs in a cold chain. Do not accept antimicrobial discs from suppliers/vendors if it is not received in an ice pack.
2. On receipt, immediately store the discs at 2 to 8°C. If they will not be used soon, store them at -20°C or below.

Storage

1. Store antibiotic discs according to the manufacturer's instructions, in a sealed, desiccated container. There are two storage modes, not one continuous range: a working supply held refrigerated at 2 to 8°C, and bulk or long-term stock held frozen at -20°C or below.
2. Sealed packages of disks that contain drugs from the ß-lactam class should be stored frozen at -20°C or below, except for a small working supply, which may be refrigerated at 2 to 8°C for at most one week.
3. Some labile agents (e.g., imipenem, cefaclor, and clavulanic acid combinations) may retain excellent stability if stored frozen until the day of use.

Practice During Regular Usage

1. Use only those discs that have not reached the manufacturer’s expiration date stated on the label. Discard the discs on the expiration date.
2. Once a cartridge of discs has been removed from its sealed package, place it in a tightly sealed, desiccated container.
3. Remove unopened disc containers from the refrigerator or freezer one to two hours before use.
4. Allow unopened or used cartridges to equilibrate to room temperature (1 to 2 hours) before opening. This is the single most important handling step: opening a cold container in warm room air lets moisture condense directly onto the discs, and moisture is a primary cause of antibiotic degradation. Warming the container first prevents this.
5. Once discs are removed from storage, do not leave them out at room temperature longer than necessary; return them to the refrigerator or freezer promptly after use.
6. Keep antimicrobial discs away from the vicinity of a Bunsen burner or any other heat source, since many agents are heat-sensitive.

Disc-dispensing apparatus

1. Disc-dispensing apparatus should be fitted with a tight cover and supplied with an adequate desiccant.
2. Allow the dispenser to warm to room temperature before opening. Excessive moisture should be avoided by replacing the desiccant when the indicator changes color.
3. Always refrigerate dispensing apparatus containing discs when not in use.

References

1. Balouiri, M., Sadiki, M., & Ibnsouda, S. K. (2016). Methods for in vitro evaluating antimicrobial activity: A review. Journal of pharmaceutical analysis, 6(2), 71–79. https://doi.org/10.1016/j.jpha.2015.11.005
2. CLSI. M02, Performance Standards for Antimicrobial Disk Susceptibility Tests. Clinical and Laboratory Standards Institute; current edition.