M-test
for estimation of antimicrobial MIC values of clinical isolates
The SRA equation can be modified in a way which will permit the
calculation of MIC-values for individual isolates, the so called
M-test. This is due to the fact that the inhibition zone size
extrapolated to zero gives a concentration value from the regression
line which corresponds to the 'critical concentration' of the isolate.
The principle has been described also by others, first by
Shannon et al. in 1975, who determined the penicillin susceptibility
of gonococcal isolates using manual, graphic plots.
Drugeon et al. in 1987 determined the 'critical concentration'
of cefotaxime and ceftriaxone for 91 bacterial isolates. In 1994
Delignette-Muller and Flandrois described the ICD, Inhibitory
Concentration in Diffusion, determined for three aminoglycosides in
70 isolates using the same principle.
In all these methods a series of different concentrations of the antibiotic
in diffusion sources, usually paper discs, is applied. The inhibition
measurements are then used to solve the equation and to calculate
the 'critical concentration', or Q-zero, at zone zero. In the M-test
equation the MIC value is obtained by multiplying the Q-zero value
with a conversion factor. This factor is often 2, but can vary with
drug and bug.
The M-test has also been used successfully for fluconazole and voriconazole
susceptibility testing of Candida species.
Let us now return to inhibition zone diameter histograms. The disc
method is the most common test for antimicrobial susceptibility and
such results are available all over the world. How can this untapped
source of susceptibility results be used for surveillance? Well, there
is a method to obtain an internal calibration for comparative purposes,
so called normalized resistance interpretation, NRI (see next page).
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