Antimicrobial susceptibility testing using disc diffusion methods, E.

 


 

 

 

 

 

 

NRI
Normalized Resistance Interpretation

For comparative investigations of antimicrobial susceptibility

 


Let us ask ourselves: Whick part of an inhibition zone diameter or MIC distribution (species-wise) for a given antimicrobial is unaffected by the development of resistance?

Answer: The high-zone (or the low MIC) side of the most susceptible population of strains, representing the wild-type population. When resistance occurs in an isolate of that species, the position of that isolate in the distribution changes to lower zone sizes or higher MIC values.

So, if we can use the upper zone size slope (or low MIC side) of the distribution for a reconstruction of the whole wild-type distribution, then we have obtained an internal calibrator which will enable us to compare results from anywhere, from any laboratory in the world.

This can be done using the Normalized Resistance Interpretation method, NRI.

A summary of the procedure is presented in IJAA.
A detailed analysis of parameter setting for the NRI calculations was performed by Joneberg et al.

MIC distributions with regular double dilution steps provided too few points for solving the regression, but Etest results with intermediate values included, were precise enough for NRI calculations to work. This was shown in studies of tigecycline susceptibility.

US Patent No. 7,465,559, European patent No 1383913.

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Page updated by G. Kronvall, 2015-10-09