377. An antibiotic algorithm in patients with positive blood cultures for gram-positive cocci in clusters based on S. aureus, S. epidermidis, and mecA rapid detection
Session: Poster Abstract Session: MRSA, MSSA, Enterococci
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C

Background:  Rapid detection of S. aureus and its susceptibility to methicillin from positive blood culture bottles can be used as a tool for timely initiation of anti-staphylococcal antibiotics. Our aims were to 1) evaluate the performance of rapid differentiation of S. aureus and S. epidermidis by nucleic acid detection, and 2) develop an algorithm to guide antibiotic prescribing decision in patients with positive blood cultures for gram-positive cocci (GPC) in clusters.

Methods: A rapid identification of S. aureus, S. epidermidis, and methicillin susceptibility by Verigene Gram Positive Blood Culture Test was retrospectively reviewed over an 11-month period from January to November 2012.

Results:  A total of 395 positive blood culture samples consisted of 419 isolates including 79 S. aureus (40 MRSA), 170 S. epidermidis (130 MRSE), 133 other coagulase-negative staphylococci (CoNS), 33 other GPC organisms, and 4 non-GPC bacteria were reviewed. Of 395 samples, 29 contained mixed organisms, and 5 had no growth. The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and diagnostic accuracy of detection of S. aureus, S. epidermidis and their mecA are shown in the Table 1. Among the 23 mixed cultures containing S. epidermidis, only 17 had a positive S. epidermidis amplification result, and 18 correctly identified methicillin susceptibility. To guide empirical antibiotic therapy, a proposed algorithm is shown in the Figure 1.

Test

Sensitivity (%)

Specificity (%)

PPV

(%)

NPV

(%)

Diagnostic accuracy (%)

S. aureus

98.7

100

100

99.7

99.7 

mecA of S. aureus

100

97.4

97.6

100

98.7 

S. epidermidis

93.5

96.9

95.8

95.2

95.4 

mecA of S.epidermidis

91.5

100

100

77.1

93.4

Conclusion:  Microarray-based detection of S. aureus and its mecA was highly sensitive and specific. The results should lead to a prompt initiation of optimal anti-staph therapy before susceptibility data are available. While detection of S. epidermidis and its mecA is less sensitive. However, detection of S. epidermidis in only 1 bottle of a blood culture set could help avoid the unnecessary use of vancomycin as empiric therapy in patients with low risk of S. epidermidis infections. Mixed cultures of S. epidermidis decreased the reliability of S. epidermidis detection.

Kanokporn Mongkolrattanothai, MD, Pediatrics, Texas Tech University Health Science Center School of Medicine, LUBBOCK, TX, Janie Robles, PharmD, Texas Tech University Health Science Center School of Pharmacy, Lubbock, TX and Phillip Watkins, MS, Clinical Research Institute, Texas Tech University Health Science Center, LUBBOCK, TX

Disclosures:

K. Mongkolrattanothai, None

J. Robles, None

P. Watkins, None

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