Infectious Diseases II

Patient Case

A 64-year-old woman is admitted to the medical ICU with possible community-acquired pneumonia. The

patient is initiated on ceftriaxone and azithromycin. Chest radiography reveals focal infiltrate. On admission,

she is dyspneic with a respiratory rate of 33 breaths/minute. Her vital signs and laboratory values are as

follows: blood pressure 90/50 mm Hg, heart rate 101 beats/minute, WBC 18 x 103 cells/mm3, and lactate 4.2

mmol/L. A PCT is obtained on admission. The results are available 12 hours after antibiotics are initiated.

The PCT result is 0.1 mcg/L. Which is the most appropriate option, given the PCT result?

ill patients with infections. An understanding of the microbiology laboratory methods for pathogen

identification and susceptibility testing will further equip critical care pharmacists with the ability to use the

most appropriate antimicrobial regimens for the treatment of critical care–associated infections. Standards

for antimicrobial identification, sensitivity testing, and determination of MIC breakpoints are determined

by the Clinical & Laboratory Standards Institute (CLSI). CLSI is a volunteer-driven standards development

organization that promotes the development and use of laboratory consensus standards and guidelines

within the healthcare community.

Disk diffusion (Kirby-Bauer)

Test agar plates are swabbed with a standardized concentration of the test organisms; paper disks

containing a defined antibiotic concentration are then placed on the plates. The diameter of the

zone of inhibited growth is inversely proportional to the MIC value.

Usually reported qualitatively as sensitive, intermediate, or resistant

Two-fold serial dilutions of antibiotics are prepared (in either broth or agar), which are then

inoculated with a standardized concentration of test organisms.

The MIC is determined.

Precision of this method is within one 2-fold concentration in either direction.

E-test

A calibrated plastic strip with a range of continuous MIC values (with fifteen 2-fold dilutions) is

placed on an inoculated agar plate. The MIC can be determined by the intersection between the

zone of inhibition and the edge of the calibrated strip.

Complement other systems to determine the MIC for specific antibiotics and test for resistance and

for confirmatory testing

Studies have shown that the reading of the exact MIC on an E-test strip is subject to the user’s

interpretation and could lead to a one- or two-dilution error in either direction compared with

standardized methods.