Antimicrobial susceptibility testing using disc diffusion methods, B.

 

 


 

 

 

 

Single strain regression analysis, SRA

One reason for using species-related interpretive breakpoints was the fact that different species often gave different regression lines, i.e. correlations between MIC values of isolates and their inhibition zone diameter values in disc diffusion tests.

Calculating species-specific regression lines is possible using Single-strain Regression Analysis, SRA.

SRA was originally developed in order to provide a tool for defining interpretive zone diameter values corresponding to the MIC-limits for susceptibility issued by reference authorities.

SRA is a formula describing the relation between disc content, MIC value and inhibition zone diameter and is based on the original equations developed during the 1950, as exemplified by the work of Cooper. The difference between the SRA equation and the regular regression line equation in comparison with the original ones is that the SRA equation retains the amount of antibiotic in the diffusion source as a variable. This adds an interesting feature. When the MIC value of an isolate is known (for instance an official reference or control strain) then a series of disc contents producing different inhibition zone diameters will make it possible to calculate the constants of the equation. Thereby, the regression line is also defined. Moreover, this regression line was determined using only one single strain and if this isolate behaves like most isolates of that species it will also represent the regression line for that particular bacterial species.

SRA therefore makes it possible to determine the regression lines for different species and thereby also to set correct zone breakpoints for different species corresponding to the MIC limits for susceptibility.

For an explanation of SRA, see Kronvall & Ringertz, 1991.

Examples of breakpoint calculations using SRA:
Cephalothin, 1984
Doxycycline, 1985
Ceftazidime, 1985
Chloramphenicol, 1988
Norfloxacin , 1989, and Ciprofloxacin, 1989
Gentamicin, 1981, and for enterococci, 1991
Trovafloxacin, 1999, and for anaerobes, 2000
Fusidic acid and C.difficile, 2000,
and for Staphylococcus aureus, 2002

 

SRA was also applied to cefoperazone results from other authors to calculate interpretive breakpoints corresponding to MIC limits for different species, not possible using regular regression analysis.

The examples given above seem to indicate that calibration of the disc diffusion test might be possible in the individual laboratory. Such procedures are everyday tasks for the clinical chemist, but the word calibration is not yet in the vocabulary of the clinical microbiologist,
(see next page).

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