LETTER
Staphylococcus aureus is the most common pathogen involved in skin and soft tissue
infections, and it is the principal cause of surgical site infections (1
–
3). Most of the laboratory methods for detecting S. aureus and methicillin-resistant
S. aureus (MRSA) from wounds require incubation time and do not support rapid decisions
for selection of the most appropriate procedural or therapeutic interventions (4
–
6). Therefore, wound infections often have negative impacts on patient outcomes— most
commonly a delay or deterioration of wound healing potentially leading to sepsis (4,
7).
The BD Max StaphSR assay (BD Diagnostic Systems, Québec, Canada) performed on the
BD Max system (BD Diagnostic Systems, Sparks, MD) is an FDA-cleared molecular test
for detection of S. aureus DNA and MRSA DNA from nasal swab specimens collected from
patients at risk of infection due to nasal colonization (8, 9). The BD Max is an automated
sample-in and answer-out instrument that combines sample extraction, PCR setup, and
real-time PCR on a walkaway platform. This PCR-based test can provide results in approximately
2.5 h. The objective of this study was to evaluate the BD Max StaphSR assay for the
detection of S. aureus and MRSA from ESwab (Copan Diagnostics, Murrieta, CA)-collected
wound samples and to compare the results to culture, our standard of care procedure.
ESwab-collected wound samples are not FDA cleared for use with the BD Max StaphSR
assay.
A total of 250 ESwab-collected wound samples were included in this study. All samples
were tested by two different protocols: the standard of care traditional culture and
the BD Max StaphSR assay on the BD Max system. For the standard of care culture, all
ESwab-collected wound samples were inoculated onto BBL Trypticase soy agar with 5%
sheep blood (blood agar), MacConkey II agar, chocolate II agar, Columbia CNA (colistin-nalidixic
acid) agar with 5% sheep blood, and thioglycolate (THIO) broth (BD Diagnostic Systems,
Sparks, MD). Swabs were rolled on the first quadrant of each medium, and plates were
streaked for isolation using the 4-quadrant technique. Swabs were then wrung out in
THIO. Culture plates and THIO were incubated at 35°C and observed for growth at 24
and 48 h. Bacterial colonies were identified by matrix-assisted laser desorption ionization–time
of flight (MALDI-TOF) mass spectrometry, the Vitek2 GP identification card, and/or
the Pastorex Staph-Plus latex agglutination test (Bio-Rad, Hercules, CA).
The BD Max StaphSR assay protocol entailed transferring an aliquot of 200 μl from
the transport medium of the residual ESwab-collected sample into a BD Max sample buffer
tube (SBT). SBTs were vortexed on a multiposition vortex mixer for 60 s and placed
in the BD Max instrument rack along with the BD Max StaphSR assay reagent strip and
reagent tubes. The entire assembly was placed in the BD Max instrument with the BD
Max PCR cartridges, and the runs were initiated. Additionally, 100 μl of each ESwab
sample was transferred into BBL Trypticase soy broth (TSB) with 6.5% NaCl (BD Diagnostic
Systems) and incubated at 35°C for 24 h. TSB samples were saved at refrigerated temperature
until culture, and PCR results were analyzed and compared. TSB samples from samples
with discrepant results were plated onto both BBL CHROMagar S. aureus and BBL CHROMagar
MRSA II (BD Diagnostic Systems, Sparks, MD). Colony growth was observed after 24 h
of incubation at 35°C (10). Discrepant samples were also tested by an in-house-validated
mecA and femA PCR, using previously published primers or probes (11) and by the Xpert
MRSA/S. aureus skin and soft tissue infection (SSTI) assay on the GeneXpert system
(Cepheid, Sunnyvale, CA), following the manufacturer's recommendations (1, 12, 13).
Of 250 ESwab-collected wound samples tested, 159 were negative and 83 were positive
for S. aureus (Table 1) and 194 were negative and 54 were positive for MRSA (Table
2) by both tests. All 54 MRSA-positive samples were also positive for S. aureus. A
total of 9 samples with discrepant results between standard of care culture and the
BD Max StaphSR assay were identified. Among them, eight were negative by culture and
positive by the BD Max StaphSR assay for S. aureus (Table 1). Of these culture-negative
samples, one was also positive by the BD Max StaphSR assay for MRSA. Another discordant
sample was positive for MRSA by culture and negative by the BD MAX StaphSR assay (Table
2). Discrepant results were resolved with additional tests as described in Table 3.
TABLE 1
Comparison of S. aureus results from culture and the BD Max StaphSR assay
Culture result
No. of results from BD Max StaphSR
Positive
Negative
Positive
83
0
Negative
8
159
TABLE 2
Comparison of MRSA results from culture and the BD Max StaphSR assay
Culture result
No. of results from BD Max StaphSR
Positive
Negative
Positive
54
1
Negative
1
194
TABLE 3
Discrepant result analysis
Sample
Result(s) from
a
:
Standard culture
BD Max StaphSR
CHROMagar S. aureus/MRSA
mecA/femA PCR
Xpert SSR S. aureus/MRSA
S. aureus
MRSA
1
−
+/FP
−/TN
−/−
−/−
−/−
2
−
+/FP
−/TN
−/−
−/−
−/−
3
−
+/FP
−/TN
−/−
−/−
−/−
4
MRSA (+)
+/TP
−/FN
−/−
+/+
+/+
5
−
+/TP
−/TN
−/−
+/−
+/−
6
−
+/TP
+/TP
−/−
+/+
+/+
7
−
+/FP
−/TN
−/−
−/−
−/−
8
−
+/TP
−/TN
−/−
−/+
+/−
9
−
+/FP
−/TN
−/−
−/−
−/−
a
FP, false positive; FN, false negative; TP, true positive; TN, true negative.
Skin and soft tissue infections (SSTIs) are among the most common bacterial infections
managed by clinicians, S. aureus being the most common pathogen isolated from wound
infection (1, 2, 5, 7). Uncomplicated SSTIs were largely managed in outpatient settings
by simple incision and drainage procedures for treatment. Traditional wound management
has shown to be an imprecise form of therapy for community-associated methicillin-resistant
S. aureus infections. This results in increased treatment failure, recurrent infections,
local or generalized spread, and other complications (3, 4, 7). Our study evaluated
the BD Max StaphSR assay to detect S. aureus and MRSA in wound samples. The BD Max
StaphSR assay and culture displayed an excellent overall agreement for the detection
of MRSA (99.2%) and S. aureus (96.8%) from ESwab-collected wound samples. All five
S. aureus false-positive samples detected by the BD Max StaphSR assay presented threshold
cycle (CT
) values of ≥38, which could indicate very low bacterial load being detected. Moreover,
the BD Max StaphSR assay has the ability to yield faster results than culture and
can potentially facilitate earlier treatment for wound infections.