Infectious Diseases II

reductions in health care costs and improvements in clinical outcomes in relevant studies. However,

many of these were single-center studies, which makes external validity questionable. The overall

impact of the implementation of rapid diagnostic tests in a single institution is determined by several

factors.

Epidemiology of targeted organisms

Presence and actions of existing antibiotic stewardship teams

Current clinician prescribing patterns

The inability to detect polymicrobial infections is a common limitation to most of the techniques

described.

Newer methods using whole blood (vs. blood culture medium or agar plates) have been developed

(LightCycler SeptiFast, SepsiTest).

A benefit to identification from whole blood is the ability to identify bacteria in patients with recent

or current antibiotic exposure.

Some of the technology discussed has been developed for respiratory cultures, but the clinical adaptation

in that arena is considerably less than that for blood cultures.

Application of rapid diagnostic tests that detect genetic encoding of resistance mechanisms requires

further education and guidelines to assist clinicians in choosing the proper therapy because traditional

susceptibility results are not available. See Table 6 for a reasonable approach based on the detection of

resistance genes and species.

Peptide nucleic acid (PNA) fluorescence in situ hybridization (FISH)

Mechanism: Targets species-specific ribosomal RNA from positive blood cultures

Sensitivity and specificity: 96%–100%

Limitations: Does not provide antimicrobial sensitivity data. Currently available FISH products are

only used for species identification; however, the FDA approved a new product that could detect

the mecA gene for detecting the presence of methicillin resistance. This product is not currently

commercially available.

Separates S. aureus from possible skin flora contamination of coagulase-negative staphylococci

(CoNS).

ii.

Differentiates Enterococcus faecium (which is often resistant to ampicillin and vancomycin)

from Enterococcus faecalis.

iii.

Identifies fluconazole-sensitive Candida spp. for patients empirically treated with

echinocandins.

iv.

Detects Pseudomonas versus non-Pseudomonas gram-negative spp. in patients treated with

combination gram-negative therapy.

Studies

A retrospective study evaluating the outcome and economic benefit of PNA FISH methods

for the early differentiation of CoNS and S. aureus bacteremia in clinical practice showed a

significant cost savings and a decrease in median LOS. Of note, the PNA FISH results were

combined with the efforts of an antibiotic therapy team. Similar results have been shown with

PNA FISH implementations for other pathogens, including Enterococcus spp. and Candida

spp. These results are in contrast to those of another study that evaluated the pre- and post-

staphylococci PNA FISH implementation results without the use of an antibiotic stewardship

team. This study found no significant effects on patient LOS or vancomycin use. This suggests

that rapid identification tests are probably beneficial only when combined with educational

efforts and prospective alerts to notify clinicians of the clinical applicability of the test results.