University of Pennsylvania Medical Center Guidelines
for Antibiotic Use
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
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