Evaluation of direct Etest for antimicrobial susceptibility testing of bacteria isolated from synovial fluid of horses using enrichment bottles.
Authors: Dumoulin M, Martens A, Van den Abeele A-M, Boyen F, Oosterlinck M, Wilderjans H, Gasthuys F, Haesebrouck F, Pille F
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary When horses present with synovial fluid infections, rapid and accurate identification of bacterial susceptibility to antimicrobials is crucial for clinical decision-making, yet traditional culture and susceptibility testing methods introduce significant diagnostic delays. Dumoulin and colleagues evaluated whether the Etest could be applied directly to bacteria grown in BACTEC enrichment bottles without subculture or inoculum standardisation, comparing results from 94 positive broths against reference agar dilution methods, standard Etest, and disc diffusion testing using 13 antimicrobial agents. The direct Etest achieved 91% categorical agreement with reference methods for determining susceptibility or resistance, substantially outperforming disc diffusion and delivering results at least 24 hours earlier than conventional approaches, though it proved less precise for exact minimum inhibitory concentration (MIC) values (69% essential agreement versus 89% for standard Etest); notably, trimethoprim and sulfadiazine showed unacceptable inaccuracy with the direct method and should be excluded from testing panels. For practitioners managing equine joint infections, the direct Etest offers a pragmatic balance between speed and clinical reliability for most antimicrobials—enabling earlier therapeutic refinement without waiting for subculture—although cases requiring precise MIC determination should still utilise the standard Etest despite the additional 24-hour delay.
Read the full abstract on PubMed
Practical Takeaways
- •Direct Etest from enrichment broths can provide reliable susceptibility results for synovial fluid cultures in 24 hours faster than standard methods, supporting quicker therapeutic decisions for joint infections
- •This method works well for most antimicrobials except trimethoprim and sulfadiazine, so avoid reporting results for these agents when using direct Etest
- •For cases where precise MIC values matter more than speed, request standard Etest instead, as it provides better quantitative accuracy (89% vs 69% essential agreement)
Key Findings
- •Direct Etest achieved 91% overall categorical agreement with reference agar dilution method for predicting susceptibility/resistance of bacteria from equine synovial fluid
- •Direct Etest showed 69% essential agreement (MICs within ±1 log dilution) compared to 89% for standard Etest
- •Direct Etest results were available at least 24 hours earlier than standard methods because it eliminated need for subculture and inoculum standardization
- •Trimethoprim and sulfadiazine showed high inaccuracy rates with Etest and are unsuitable for this testing approach