When and how should I perform a synergy test for combined antimicrobial agents in a serious infection?

Synergy Testing: When and How to Perform

Synergy testing should NOT be performed routinely for serious infections, as there is insufficient evidence that it improves clinical outcomes, and the clinical significance of in vitro synergy remains unclear.

Key Recommendation

The available evidence does not support routine synergy testing in clinical practice. While synergy testing methods exist and are well-defined, the critical limitation is that in vitro synergy does not reliably translate to improved patient outcomes 1, 2, 3.

Evidence Against Routine Use

Clinical Outcome Data

• The only randomized controlled trial examining synergy testing in cystic fibrosis patients with multiresistant organisms showed no improvement in clinical outcomes compared to conventional susceptibility testing 2
• A comprehensive 2014 review concluded there is insufficient evidence to endorse synergy testing for routine clinical use 3
• The disconnect between laboratory findings and clinical benefit makes routine testing unjustifiable from a cost-effectiveness standpoint 2

Methodological Problems

The technical challenges with synergy testing are substantial:

• Poor correlation between methods: Checkerboard and time-kill curve techniques frequently disagree on which combinations show synergy 4, 5
• Lack of standardization: Results vary dramatically based on inoculum size, concentration ratios tested, and interpretation criteria used 4, 5
• Arbitrary testing conditions: In vitro antibiotic ratios often bear no resemblance to actual drug levels achieved in patients 6

When Synergy Testing Might Be Considered (Exceptional Circumstances)

If you are considering synergy testing despite the lack of supporting evidence, it should be reserved only for:

• Life-threatening infections with multidrug-resistant organisms where all standard single-agent options have failed
• Research protocols or institutional studies designed to generate clinical outcome data
• Specific scenarios where institutional data has demonstrated correlation between synergy results and outcomes

How to Perform Synergy Testing (If Absolutely Necessary)

According to EUCAST definitions 1:

Methods Available

1. Checkerboard Titration with FIC Index

   • Calculate Fractional Inhibitory Concentration (FIC) for each drug:
     • FIC_A = (MIC_A in presence of B) / (MIC_A alone)
     • FIC_B = (MIC_B in presence of A) / (MIC_B alone)
   • Sum FIC Index (ΣFIC) = FIC_A + FIC_B
   • Interpretation: ΣFIC ≤0.5 suggests synergy, >4.0 suggests antagonism
   • Caveat: This method shows poor correlation with time-kill curves 5

2. Time-Kill Curves

   • More labor-intensive but considered more reliable
   • Synergy defined as ≥2 log₁₀ decrease in CFU/mL with combination vs. most active single agent at 24 hours 1

3. Isobologram

   • Graphical representation of drug interactions 1

Critical Pitfalls to Avoid

• Do not use microdilution checkerboard for enterococci - it suffers from inherent variation and poor agreement with time-kill curves 5
• Recognize that different methods will give different results - there is no gold standard 4
• Understand that in vitro synergy ≠ clinical benefit - the EUCAST guideline explicitly states "the clinical significance is unclear" 1

Practical Alternative Approach

Instead of synergy testing, focus on:

• Conventional MIC testing with appropriate breakpoints
• Pharmacokinetic/pharmacodynamic optimization of dosing regimens
• Clinical response monitoring as the ultimate guide to therapy effectiveness
• Infectious disease consultation for complex multidrug-resistant infections

The evidence strongly suggests that clinical judgment, conventional susceptibility testing, and appropriate dosing strategies are more valuable than in vitro synergy testing for improving patient outcomes.