The Hidden Variable in AST Reliability

Antimicrobial susceptibility testing (AST) depends on many controlled variables: inoculum density, incubation temperature, media composition, and pH. One variable that receives comparatively little attention — despite being equally important — is antibiotic potency at the point of use.

An antibiotic that has degraded, even partially, before inoculation will produce falsely elevated MIC values, incorrect zone diameters, and QC failures that are easily misattributed to the organism, the media, or the method.

The core problem is that powder-form stability data — the data printed on the label — does not predict solution-form stability. These are chemically and kinetically different situations, and treating them as equivalent is one of the most common sources of preventable AST error.

Why Dissolution Changes Stability

In the dry state, most antibiotics are protected from the primary degradation mechanisms. The crystalline matrix limits molecular mobility, water activity is near zero, and reactive species have limited access to labile chemical bonds.

Once dissolved, all of this changes. The antibiotic molecule is now freely mobile, surrounded by water, and exposed to the full range of solution-phase degradation pathways:

Temperature strongly governs the rate of all these reactions. As described by the Arrhenius equation, even a modest increase in storage temperature — say from −20°C to 4°C — can dramatically accelerate degradation for susceptible compounds.

Stability Data by Antibiotic Class

The following data are drawn from peer-reviewed stability studies. Values are approximate and depend on solvent, concentration, pH, and container type. They are provided to illustrate the order of magnitude differences between powder and solution stability, not as absolute storage guidelines for any specific product.

β-Lactams — Imipenem

Imipenem is one of the most extensively studied antibiotics with respect to solution instability, and one of the least stable of the clinically relevant β-lactams. Smith et al. (1990) characterised the degradation kinetics in aqueous solution using HPLC and mathematical modelling, identifying pH-dependent oligomerisation and β-lactam hydrolysis as the primary mechanisms.

β-Lactams — Ampicillin

Ampicillin is another β-lactam with well-documented solution instability. The primary degradation mechanism involves hydrolytic opening of the β-lactam ring, with the rate highly sensitive to temperature and solvent composition.

An important — and counterintuitive — finding from the literature is that ampicillin in dextrose solutions degrades faster at −20°C than at 5°C, due to eutectic concentration effects that increase reactant concentration in the unfrozen fraction during freezing. Ampicillin in 5% dextrose showed 46% degradation after 24 hours at both −20°C and 27°C, compared to only 29% at 5°C in the same timeframe.

β-Lactamase Inhibitors — Clavulanic Acid

Clavulanic acid is among the least stable antibacterial compounds in solution. Unlike the β-lactam antibiotics, clavulanic acid lacks a traditional side chain and relies on an oxazolidine ring structure that is highly susceptible to hydrolytic attack. Published stability studies consistently show rapid potency loss under most storage conditions once dissolved.

In the context of AST, clavulanic acid is primarily encountered in combination disks or as part of combination MIC testing (amoxicillin-clavulanate, ticarcillin-clavulanate). Any potency loss in the clavulanic acid component can shift the apparent MIC of the combination upward, potentially producing false resistance classifications for ESBL-producing organisms.

Overview: Comparative Stability Across Classes

The Temperature Effect in Context

The relationship between storage temperature and degradation rate is not linear — it follows Arrhenius kinetics, meaning that a reduction in temperature produces a disproportionately large reduction in reaction rate. This is why −80°C offers substantially better protection than −20°C, even though the absolute temperature difference is only 60°C.

For imipenem in particular, the degradation half-life at 25°C is approximately 14.7 hours. The absence of meaningful stabilisation at −20°C (compared to 4°C) for this compound suggests that the reactive mechanisms involved may not be fully quenched at this temperature — a finding consistent with other highly reactive β-lactam systems.

Freeze-Thaw Cycles: An Underestimated Risk

Repeated freeze-thaw cycles represent a compounding source of degradation that is often overlooked. Each cycle contributes additional degradation through several mechanisms:

• Eutectic concentration effects: as a solution freezes, solutes become concentrated in the residual unfrozen fraction, increasing local reactant concentrations and accelerating degradation
• Ice crystal formation: can damage the structural integrity of some macromolecular antibiotics and cause mechanical shear on peptide bonds
• pH shifts during freezing: the freezing process can transiently alter the local pH, especially in poorly buffered solutions, triggering additional pH-sensitive degradation
• Thawing at elevated temperature: if tubes are thawed in a warm water bath or at room temperature rather than on ice, an additional period of elevated-temperature degradation occurs before use

Impact on AST Results

Even modest antibiotic degradation — below the threshold detectable by most routine QC checks — can affect AST results in clinically meaningful ways.

For broth microdilution, a 25% reduction in antibiotic potency is equivalent to diluting down by approximately one two-fold concentration step. This can shift a result from susceptible to intermediate, or from intermediate to resistant, for organisms near the breakpoint. For disk diffusion, reduced antibiotic potency produces smaller inhibition zones — again pushing borderline results toward false resistance.

The CLSI M07 guideline acknowledges antibiotic quality as a critical pre-analytical variable and specifies requirements for antibiotic preparation and storage. EUCAST similarly emphasises the importance of antibiotic integrity as a prerequisite for method reliability. Neither standard can compensate for antibiotic degradation that occurs before inoculation.

Best Practice Recommendations

• 1
  Store powders correctly. Keep dehydrated antibiotic powders at the manufacturer-specified temperature (often −20°C or below for highly labile compounds), in a desiccated, light-protected, airtight container. Powder stability data applies only to the dry powder — not to dissolved forms.
• 2
  Prepare fresh working solutions where possible. For highly labile antibiotics (imipenem, clavulanic acid, ampicillin in dextrose), prepare working solutions from powder on the day of use. Avoid preparing large volumes in advance.
• 3
  Aliquot stock solutions into single-use volumes. Prepare concentrated stock solutions in small single-use aliquots. This eliminates the need to refreeze and rethaw the same material, preventing cumulative degradation across multiple cycles.
• 4
  Use −70°C to −80°C for unstable antibiotic stocks. For imipenem, clavulanic acid, and other highly labile compounds, −80°C provides substantially better long-term protection than −20°C. Published data confirm this difference is clinically meaningful.
• 5
  Monitor QC organisms rigorously. Trending QC data over time is the most practical in-laboratory tool for detecting antibiotic potency drift. A gradual upward shift in QC MIC values or a progressive reduction in zone diameters should trigger investigation of antibiotic storage conditions before attributing the change to other variables.
• 6
  Account for solvent effects. For ampicillin specifically, avoid dissolving in dextrose-containing solutions — published data show faster degradation in dextrose than in saline, even at −20°C. Use water or 0.9% NaCl as the solvent where possible.
• 7
  Thaw on ice, not at room temperature. When thawing frozen aliquots, do so on ice or at 2–8°C rather than at ambient temperature. Even a short period of room-temperature exposure during thawing contributes measurable degradation for the most labile compounds.

Final Thoughts

Antibiotic stability in solution is not a pharmacist's problem or a clinical problem — it is a laboratory science problem that directly affects the reliability of every AST result your laboratory produces.

The data reviewed here highlight a consistent message: freezing is not always stabilising, −20°C is not always sufficient, and the same antibiotic can behave very differently depending on its solvent, concentration, pH, and the number of freeze-thaw cycles it has experienced.

For routine AST workflows, the practical implications are straightforward: prepare fresh, aliquot small, store cold, and use QC trending as your early warning system. For the most labile compounds — imipenem chief among them — these practices are not optional. They are the difference between reliable susceptibility data and results that reflect the storage conditions more than the organism.

References

1. Lautenbach E et al. Comprehensive stability analysis of 13 β-lactams and β-lactamase inhibitors in in vitro media. Antimicrob Agents Chemother. 2022. doi:10.1128/aac — PMC
2. Sornsuvit C et al. Influence of Concentration and Temperature on Stability of Imipenem Focused on Solutions for Extended Infusion. J Infect Dis Pharmacother. 2021. DOI
3. Chin A et al. Degradation kinetics of imipenem in normal saline and human serum. Antimicrob Agents Chemother. 1986;29(5):936–941. AAC
4. Prat-Vallribera C et al. Short-term stability studies of ampicillin and cephalexin in aqueous solution and human plasma: Application of least squares method in Arrhenius equation. J Chromatogr B. 2012;897:36–44. PubMed
5. Savello DR, Shangraw RF. Stability of sodium ampicillin solutions in the frozen and liquid states. Am J Hosp Pharm. 1971;28(10):754–759. AJHP
6. Holmes CJ et al. Effect of freezing and microwave thawing on the stability of six antibiotic admixtures in plastic bags. Am J Hosp Pharm. 1982;39(1):104–8. PubMed
7. Smith GB, Dezeny GC, Douglas AW. Stability and kinetics of degradation of imipenem in aqueous solution. J Pharm Sci. 1990;79(8):732–740. DOI