University of Pennsylvania Medical Center Guidelines for Antibiotic Use

 

Intermediate

 

Pictured above is a graph which shows the relationship between the MIC and the size of the zone of inhibition in the Bauer-Kirby test for the drug-A using the bacteria-Q. "R" represents the cutoff for the interpretation of "resistant", and "S" represents the cutoff for the interpretation of "susceptible"

The portion of the curve which is yellow represents the region where it would be reported that the sensitivity of bacteria-Q to drug-A is intermediate. Refection of the curve onto the X-axis (dotted yellow lines) show that a zone diameter between 16 mm and 23 mm (yellow arrow) would result in "intermediate" being reported out for this bug/drug pair. This correlates approximately with an MIC of between 8 and 64 micrograms.

This interpretation is the most confusing of the three. This is because it may mean several things.

1. It may truly mean intermediate sensitivity in that some isolates may have a "relative resistance" and in fact be susceptible to higher than typical concentrations of antibiotic.

2. It may mean indeterminate in that there may be an overlap in the in vivo response given a particular in vitro testing result.

Examples of scattergram data used in determining the spectrum of sensitivity of isolates are given below to help clarify the intermediate interpretation. These figures are from Fuchs et al., J Clin Micro, 35:125-131 (1997).



Scattergram #1 -- Correlation of TMP/SMX MIC and zone diameters for H. influenzae. Horizontal and vertical lines represent MIC and zone diameter breakpoints. In this case, the isolates fall into two groups. With the exception of 6 isolates, all can be readily interpreted as susceptible or resistant. Six isolates which are resistant by zone diameter but appear intermediate by MIC are likely fall into the resistant group.



Scattergram #2 -- Correlation of TMP/SMX MIC and zone diameters for S. pneumoniae. Horizontal and vertical lines represent MIC and zone diameter breakpoints. In this case, the isolates form a continuum, with the breakpoint for resistant and susceptible not being obvious. In this case one can not predict how the isolates which fall into the intermediate zone will behave in vivo. The breakpoints are set to maximize the predictive value of the test while minimizing errors (ie. it is preferable to call an isolate intermediate, than to incorrectly call it sensitive or resistant).

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