Infectious Diseases II
ii.
In a prospective randomized controlled study in which patients were randomized to early
notification of PNA FISH results for CoNS or S. aureus within 3 hours or usual care,
intervention was associated with decreased mortality, decreased antibiotic use, and decreased
LOS. The most pronounced benefits occurred in critically ill patients.
Mass spectroscopy: Matrix-assisted laser desorption-ionization/time of flight (MALDI-TOF)
Mechanism: Mass spectroscopy is compared with library standards for identifying pathogen
species and/or resistance mechanisms. Mass spectroscopy is an analytic technique whereby
samples are ionized that produces a mass spectrogram for the sample. Each sample has a unique
mass spectrogram, similar to a fingerprint, which can be matched to a library of reference standards
for identification.
Sensitivity and specificity: 98%–100%
Limitations
Similar to PNA FISH, no antimicrobial susceptibility is reported; however, the technology
could detect genes encoding resistance. MALDI-TOF for resistance genes is currently not
commercially available.
ii.
Cannot be used for polymicrobial cultures
iii.
No library is available for unusual organisms, though pathogen libraries are consistently being
updated.
| d. | Application: Potentially wider clinical applicability than PNA FISH (which is limited by the |
|---|
availability of specific tests for certain pathogens) with early identification of many more pathogens.
Additional library standards for different pathogens are continually being added.
Studies: This strategy has been evaluated in several studies. In a before-and-after study of
MALDI-TOF implementation, a shortened time to pathogen identification and a decrease in
LOS, recurrent infections, and mortality were seen. Of note, the implementation of MALDI-TOF
included communicating the results to the treating clinicians by an antibiotic stewardship team
with evidence-based antibiotic recommendations.
Polymerase chain reaction (PCR)-based detection systems
MRSA PCR test
Mechanism: Novel multiplex real-time assay for mecA gene
ii.
Sensitivity and specificity: 85% and 92%, respectively, for MRSA and MSSA identification
iii.
Application: Earlier de-escalation of anti-MRSA antimicrobials and earlier appropriate
treatment with antistaphylococcal penicillin (oxacillin, nafcillin, cefazolin) for MSSA
| (a) | Most experience and evidence with using the MRSA PCR test is for de-escalation of |
|---|
MRSA-active antibiotics in patients with pneumonia. New evidence also shows this
may be used for de-escalation in patients with intra-abdominal infections, bloodstream
infections, wound cultures, and urinary cultures.
| (b) | Providers may use results from MRSA PCR test for up to 14 days between specimen |
|---|
collections (negative predictive value at 14 days in one study was 95.5%). Another study
demonstrated that MRSA nares screening within 60 days of ICU admission has a high
negative predictive value (98%) and specificity (96%).
iv.
Studies: In a before-and-after study, implementation of the MRSA PCR test resulted in reduced
time to appropriate therapy and duration of unnecessary MRSA coverage. In addition, the
mean hospital costs were decreased, and there was a trend toward decreased LOS. In a very
similar study, the combination of the MRSA PCR test with antimicrobial stewardship efforts
resulted in significant decreases in LOS and cost and a trend toward decreased mortality (18%
vs. 26%).
FilmArray System
Mechanism: Uses multiplex PCR technology to identify bacterial species and resistance genes