How to Determine Colistin MIC Accurately: EUCAST–CLSI Joint Recommendations Explained

📅 Published: Updated from EUCAST guidelines, March 22, 2016

Colistin (also known as polymyxin E) remains a last-resort antibiotic against multidrug-resistant Gram-negative bacteria. However, testing its minimum inhibitory concentration (MIC) is technically challenging. Due to colistin's unique physicochemical properties, standard antimicrobial susceptibility testing (AST) methods can yield inaccurate or misleading results unless carefully modified.

To address this, the CLSI–EUCAST Polymyxin Breakpoints Working Group published detailed recommendations for reliable MIC determination.

🚨 Why Is Colistin MIC Testing So Tricky?

Colistin binds readily to plastic surfaces, especially polystyrene, which can artificially lower its concentration in test wells, leading to falsely elevated MICs or false resistance. Moreover, some commercial formulations use inactive prodrugs or additives like surfactants, further compromising accuracy.

✅ CLSI–EUCAST Recommended Method for MIC Testing of Colistin

The only validated reference method is ISO-standard broth microdilution (BMD) as defined by ISO 20776-1.

🔬 Key Methodological Points:

1. Target Organisms

   • Enterobacteriaceae

   • Pseudomonas aeruginosa

   • Acinetobacter spp.

2. Broth Media

   • Use cation-adjusted Mueller-Hinton broth (CA-MHB) only

   • No additives, surfactants, or stabilizers (especially NO polysorbate-80)

3. Microdilution Trays

   • Must be made from untreated, plain polystyrene

   • Do NOT use treated or coated plates, as they may alter drug binding

4. Polymyxin Salts

   • Use colistin sulfate or polymyxin B sulfate

   • Do NOT use colistin methanesulfonate (CMS) – it's an inactive prodrug

🚫 Not Recommended (Yet): Other Methods

The following methods are not currently endorsed by EUCAST or CLSI for colistin MIC testing:

• ❌ Agar dilution

• ❌ Disk diffusion

• ❌ Gradient diffusion (e.g., E-test)

These approaches are still under review or require further validation due to poor reproducibility with colistin.

🧠 Summary Table: Colistin MIC Testing at a Glance

Feature	Recommendation
Reference method	Broth Microdilution (ISO 20776-1)
Media	Cation-adjusted Mueller-Hinton broth
Additives	None (no surfactants like P-80)
Plate material	Untreated polystyrene
Drug form	Colistin sulfate only (NOT CMS)
Acceptable for	Enterobacteriaceae, P. aeruginosa, Acinetobacter
Not validated methods	Disk diffusion, gradient diffusion, agar dilution

🧬 Why This Matters in Clinical Practice

Accurate colistin MICs are critical for patient safety, especially in critically ill patients with MDR infections. Inaccurate testing can lead to therapeutic failures, unnecessary toxicity, or inappropriate exclusion of colistin from treatment regimens.

📌 Final Takeaway

Until further validation, only ISO broth microdilution (under strict conditions) is considered the gold standard for colistin MIC determination. All other methods should be used with extreme caution — or not at all.