What does 'essential agreement' mean when comparing a new AST method to a reference method?

• A. The MICs (or zone diameters) are exactly identical.
• B. The MICs (or zone diameters) are within one two-fold dilution (for MIC) or within established acceptance ranges of the reference method.
• C. The results are within a 10% variance.
• D. The new method and reference method disagree by more than two dilutions.

Answer: (B)

Essential agreement measures how closely a new AST method matches the reference method within the normal variability of testing. For MIC, results come in two-fold dilution steps, so being within one dilution means the new method’s MIC is the same or one dilution away from the reference value (for example, if the reference MIC is 2 µg/mL, acceptable results are 1, 2, or 4 µg/mL). For disk diffusion, it means the measured zone diameter falls within the established acceptance range relative to the reference method’s zones, as defined by guidelines. This reflects practical differences due to technique and conditions and ensures the new method is reliably comparable.

Thus, the best description is that results are within one two-fold dilution for MIC or within the defined acceptance ranges for zone diameters. It’s not required to be exactly identical, and a variance like 10% isn’t the standard criterion, while disagreement by more than two dilutions signals poor agreement.