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Evaluation of MicroPhage KeyPath MRSA/MSSA Blood Culture Test

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Title: Controlled Multicenter Evaluation of a Bacteriophage Based Method for the Rapid Detection 10

of Staphylococcus aureus in Positive Blood Cultures 11

Running Title: Evaluation of MicroPhage KeyPath Blood Culture Test 12 13

T. Bhowmick1, S. Mirrett2, L.B. Reller2, C. Price3, C. Qi4, M.P. Weinstein1 and T.J. Kirn1# 14

1UMDNJ-Robert Wood Johnson Medical School, 1 Robert Wood Johnson Place, New Brunswick, NJ 15

08901; 2Duke University School of Medicine and Duke University Health System, 130 Research 16

Drive, Durham, NC 27710; 3 Denver Health and Hospital, Denver CO and University of Colorado 17

School of Medicine, 13001 E 17th Place, Aurora, CO 80045; 4Northwestern University, Feinberg 18

School of Medicine, 420 East Superior Street, Chicago, IL 60611. 19

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#Corresponding author: T.J. Kirn 21

Email: [email protected] 22

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Copyright © 2013, American Society for Microbiology. All Rights Reserved.J. Clin. Microbiol. doi:10.1128/JCM.02967-12 JCM Accepts, published online ahead of print on 6 February 2013

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Abstract 24

Staphylococci are a frequent cause of bloodstream infections (BSIs). Appropriate 25

antibiotic treatment for BSIs may be delayed because conventional laboratory testing 26

methods take 48-72 hours to identify and characterize isolates from positive blood 27

cultures. We evaluated a novel assay based on bacteriophage amplification that identifies 28

S. aureus and differentiates between methicillin-susceptible and methicillin-resistant S. 29

aureus (MSSA and MRSA, respectively) in samples taken directly from signal positive 30

BACTECTM blood culture bottles within 24 hours of positive signal, with results 31

available within 5 hours. The performance of the MicroPhage KeyPathTM MRSA/MSSA 32

Blood Culture Test was compared to conventional identification and susceptibility testing 33

methods. At four sites, we collectively tested a total of 1165 specimens of which 1116 34

were included in our analysis. Compared to standard methods, the 35

KeyPathTM MRSA/MSSA Blood Culture Test demonstrated a sensitivity, specificity, 36

positive predictive value and negative predictive value of 91.8%, 98.3%, 96.3% and 37

96.1%, respectively for correctly identifying S. aureus. Of those correctly identified as S. 38

aureus (n=334), 99.1% were correctly categorized as either MSSA or MRSA. Analysis 39

of a subset of the data revealed that the KeyPathTM MRSA/MSSA Blood Culture Test 40

delivered results a median of 30 hours sooner than conventional methods (a median of 41

46.9 hours vs a median of 16.9 hours). Although the sensitivity of the test in detecting S. 42

aureus-positive samples is not high, its accuracy in determining methicillin resistance and 43

susceptibility among positives is very high. These characteristics may enable earlier 44

implementation of appropriate antibiotic treatment for many S. aureus BSI patients. 45


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(Subsets of these results were presented at the 2010 ICAAC meeting in Boston, MA and 46

the 2011 ASM meeting in New Orleans). 47

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Introduction 49

Staphylococci are a frequent cause of bloodstream infections (BSIs). The incidence of S. 50

aureus bacteremia (SAB) has increased over the past decade, largely due to the 51

increasing use of intravascular catheters and invasive devices (29). In a recent United 52

States retrospective multicenter study, 31% of bloodstream isolates representing true 53

infection were Staphylococcus, of which 74% were S. aureus and 26% coagulase 54

negative staphylococci (CoNS) (29). In a prospective cohort of patients with nosocomial 55

BSIs in the United States, S. aureus was the most commonly isolated organism 56

representing true infection, accounting for 19% of cases (36). 57

Complications of SAB are common and their incidence has also become more frequent, 58

ranging from 10-50% (30). The mortality rate of SAB ranges from 11- 20% and relapse 59

rates are high, especially if appropriate therapy is not administered or is delayed (14, 29, 60

35). It has been estimated that 33-60% of SAB bloodstream isolates in the United States 61

are methicillin-resistant (MRSA) (21, 29). 62

Patients with suspected BSI are often started empirically on broad-spectrum antimicrobial 63

agents, which may entail added healthcare costs and adverse events. The most common 64

broad-spectrum antibiotic prescribed for presumptive MRSA BSIs in the United States is 65

vancomycin. Several studies have reported increased rates of treatment failure for 66

patients with MSSA BSI treated with vancomycin (6, 17, 19, 32, 34). The selection of 67

the most effective antibiotic treatment for SAB is often delayed because conventional 68

microbiologic testing methods take 48 to 72 hrs to identify S. aureus and provide 69

definitive susceptibility results. In this study, we evaluated a novel, phenotypic phage-70

based assay that requires no instrumentation to identify S. aureus and differentiates 71


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between MSSA and MRSA isolates directly from positive blood cultures within 5 hours. 72

The KeyPathTM MSSA/MRSA Blood Culture Test (Microphage, Longmont, CO) uses 73

mixed lytic bacteriophage to differentiate S. aureus from other bacteria and predicts 74

susceptibility to penicillinase resistant β-lactams. The test consists of two simultaneous 75

reactions: one (the ID reaction) contains bacteriophage specific for S. aureus, the other 76

(the RS reaction) contains the bacteriophage plus cefoxitin. The bacteriophage amplify 77

in the ID reaction if S. aureus is present in the sample. The bacteriophage amplify in the 78

RS reaction if the S. aureus present are able to grow in the presence of cefoxitin. 79

Because bacteriophage growth is dependent on host growth, resistant strains (MRSA) are 80

positive and susceptible strains (MSSA) are negative for phage amplification in the RS 81

reaction. Bacteriophage amplification is detected by phage-specific monoclonal 82

antibodies in a dual-strip self-contained immunoassay cartridge. This assay represents 83

one possible approach to reducing the analytical time required to identify MSSA and 84

MRSA from positive blood culture bottles. Rapid, reliable identification and 85

susceptibility can enable clinicians to initiate optimal antimicrobial therapy sooner and 86

thereby may lead to reductions in healthcare costs and adverse events and better patient 87

outcomes. 88

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Materials and Methods 91

Patient Samples. This clinical trial was performed at the following four sites: University 92

of Medicine and Dentistry of New Jersey - Robert Wood Johnson Medical School, 93

Northwestern Memorial Hospital, Duke University Health System, and Denver Health & 94

Hospital. All sites received approval to participate from their respective Institutional 95

Review Boards. Each site was instructed to identify positive BACTECTM Culture System 96

(BD Diagnostics, Sparks, MD) blood culture bottles, 0 – 24 hours after detection (i.e. 97

signal-positive). There were no limits placed on the number of samples tested from each 98

patient enrolled in the study. Gram-stains were performed on an aliquot of culture broth 99

from each bottle. Subsequently, samples underwent both conventional testing (see 100

below) and the KeyPathTM MRSA/MSSA Blood Culture Test. Clinical information about 101

each patient, such as recent administration of antibiotics and antiviral medications, was 102

collected by physician chart review. 103

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Inclusion and Exclusion criteria. Enrollment in the trial was limited to signal-positive 105

BACTECTM blood cultures drawn from adult patients (at least 18 years of age). Samples 106

had to be enrolled in the trial within 24 hours of the BACTECTM alarm signaling a 107

positive blood culture. Samples were excluded if they were mislabeled or misidentified. 108

109

Standard Method Tests. Conventional methods for the identification of organisms in 110

positive blood cultures, including tube coagulase, catalase, and Staphaurex® (Remel, 111

Lenexa, KS) tests, were performed on all specimens. The comparative standard for S. 112

aureus identification was defined as concordant results between the catalase, coagulase, 113


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and Staphaurex® tests (all positive). For blood cultures that were judged to be positive 114

for S. aureus, additional testing was performed to distinguish between MRSA and MSSA 115

using both Cefoxitin and Oxacillin Sensi-discs™ (BD, Franklin Lakes, NJ). These 116

assays were interpreted using the following criteria: Cefoxitin Sensi-disc ™ Test with a 117

zone of inhibition (ZOI) ≥ 21mm was considered susceptible (MSSA) and a ZOI <21 was 118

considered resistant (MRSA)(7). Oxacillin Sensi-disc™ with a ZOI ≥ 13mm was 119

considered susceptible (MSSA), a ZOI of 11-12 of intermediate susceptibility, and an 120

ZOI < 10 resistant (MRSA). The Oxoid PBP2a Latex Agglutination Test (Thermo 121

Scientific, Basingstoke, UK) was also performed. The gold standard for distinguishing 122

MRSA from MSSA was based on the results of the Cefoxitin Sensi-disc™ test. 123

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MicroPhage MRSA/MSSA Blood Culture Test. An aliquot of broth from the positive 125

blood culture bottle was transferred to a tube. KeyPathTM reaction media (ID and RS 126

media) containing phage cocktail and broth were added to respective reaction tubes (ID 127

and RS) containing dry reagents. The device contains both an identification and 128

susceptibility component which are read in tandem. 10 μL of the blood culture sample 129

was then added to each of the two separate reaction tubes. The tubes were incubated for 130

5 hours ± 20 minutes at 35±2°C to allow phage amplification from phage susceptible 131

bacteria (S. aureus), if present. One of the two tubes (RS) contained cefoxitin (a 132

surrogate for methicillin). At the end of the incubation period, aliquots were taken from 133

the reaction tubes and applied to a lateral flow device that detects phage antigens as a 134

surrogate for the presence of bacteria. Cultures that yielded positive results (i.e. phage 135

amplification) in both tubes were interpreted as positive for MRSA, those in which only 136


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the tube which did not contain cefoxitin gave a positive result were interpreted as positive 137

for MSSA and those that gave negative results for the ID reaction were interpreted as 138

negative for S. aureus. When performed in batches of 5-10, the total time from specimen 139

acquisition to result was approximately 7 hours with a hands on time for the laboratory 140

technician of approximately 60-90 minutes. 141

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Time to result. Three of the study sites (Duke, RWJUH and Denver Health) recorded 143

the date and time of reporting the initial Gram stain, routine bacterial identification and 144

final susceptibility results in the hospital laboratory information system. In addition, 145

these 3 sites recorded the date and time of completing the KeyPathTM MRSA/MSSA 146

Blood Culture Test for each positive blood culture specimen included in the study 147

(n=708 for conventional methods, 706 for the KeyPathTM MRSA/MSSA Blood Culture 148

Test). Time data was not recorded for 2 of the KeyPathTM assays. 149

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Data Analysis. We compared the performance of the KeyPathTM MRSA/MSSA Blood 151

Culture Test to the conventional methods as a consensus standard. Analysis was also 152

stratified based on exposure of patients to antimicrobials and antiviral medications. For 153

analysis of turn-around times, start time was defined as the point at which the blood 154

culture bottle signaled positive. The gold standard methods were conducted per standard 155

protocol in each laboratory while the KeyPathTM MRSA/MSSA Blood Culture Test was 156

routinely batched for clinical trial purposes. Finish time for the KeyPathTM 157

MRSA/MSSA Blood Culture Test assay was defined as the time when the research 158

technologist recorded the results. Standard method finish time was defined as the time 159


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when the results were entered into the computer system by clinical laboratory staff. The 160

median of the time to result for each method was compared. 161

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Discrepant Analysis. Sample repeat testing was not performed on any discrepant 163

samples at the sites. Three samples were initially invalid on the MRSA/MSSA Blood 164

Culture Test Detector and were re-run immediately with the remaining volumes from 165

Reaction Tubes. However, these data were excluded from analysis. 166

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This study was registered with ClinicalTrials.gov, Identifier number NCT01184339. 168

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Results 170

A total of 1165 samples were collected at four sites for KeyPathTM MRSA/MSSA Blood 171

Culture Test and predicate testing. The overall prevalence of S. aureus was 32%, and 172

ranged from 21% to 43% between sites. Among S. aureus, 53% of samples were MRSA, 173

with a range of 39% to 69% between sites. 174

Forty-nine positive blood culture bottles were excluded for the following reasons: 175

5 missing catalase, coagulase or Staphaurex® results; 8 disagreements between catalase, 176

coagulase or Staphaurex® results; 3 Invalid KeyPathTM MRSA/MSSA Blood Culture Test 177

detector results and 33 invalid KeyPathTM MRSA/MSSA Blood Culture Test results 178

(performed>24hrs after signal-positive). Thus, 1116 samples were included in the data 179

set used to characterize the ability of the KeyPathTM MRSA/MSSA Blood Culture Test to 180

accurately identify S. aureus. All samples included in the analyzed data set had valid 181

coagulase (4 or 24 hour), catalase, Staphaurex® and KeyPathTM MRSA/MSSA Blood 182

Culture Tests performed. 183

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Samples that had Gram stain results other than gram-positive cocci were considered to be 185

negative for S. aureus even if no further testing was performed. At the start of the study, 186

all specimens (regardless of Gram stain result) underwent subsequent testing; however, as 187

the trial progressed, to enrich the population for S. aureus positive samples, only those 188

specimens identified as Gram-positive cocci underwent subsequent testing as described 189

above. All of the non-Gram-positive cocci that underwent KeyPathTM MRSA/MSSA 190

Blood Culture Test testing were correctly categorized as not S. aureus. 191

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193

S. aureus Detection 194

A total of 366 samples were identified as S. aureus by the standard methods. Thirty S. 195

aureus (SA) isolates were incorrectly characterized as “not S. aureus” (NSA) by the 196

KeyPathTM MRSA/MSSA Blood Culture Test (false negatives) and 13 NSA isolates were 197

mischaracterized as SA (false positive) by the KeyPathTM MRSA/MSSA Blood Culture 198

Test (Figure 1). The overall accuracy (concordance) of standard (predicate) methods and 199

Microphage KeyPathTM MRSA/MSSA Blood Culture Test for detection of S. aureus was 200

96.2%. Sensitivity and specificity of the MRSA/MSSA Blood Culture Test vs. standard 201

(predicate) methods was 91.8% and 98.3%, respectively. For the MRSA/MSSA Blood 202

Culture Test, the PPV was 96.3% and NPV was 96.1%. Further evaluation of discrepant 203

results revealed that half the false negatives when isolated as colonies and retested, gave 204

results that matched those obtained by the standard method. The false negatives were not 205

concentrated in any one PFGE type. All false positive results were CoNS except for one 206

enterococcus. No discrepant results were observed from blood cultures which grew 207

multiple organisms. There was no difference of test performance characteristics among 208

the different sites in the study (data not shown). 209

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Differentiating MRSA from MSSA 211

Two of the 366 S. aureus isolates were not evaluated by disk diffusion and were excluded 212

from resistance and susceptibility analyses. Of the remaining 364 isolates, 334 were 213

correctly identified as S aureus by the KeyPathTM MRSA/MSSA Blood Culture Test; 214

these samples were further analyzed for correct categorization as MRSA or MSSA. Of 215


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these 334 S. aureus isolates, 180 were categorized as MRSA and 154 as MSSA based on 216

the standard method results. 99.1% of these were correctly categorized as either MRSA 217

or MSSA using the Microphage KeyPathTM MRSA/MSSA Blood Culture Test (Figure 2). 218

Two S. aureus isolates judged to be cefoxitin-resistant with the disk test (cefoxitin disk 219

test zone diameters 9 mm and 10mm) and PBP2a positive were misclassified as 220

susceptible by the KeyPathTM MRSA/MSSA Blood Culture Test (1.3% very major error 221

rate). One cefoxitin susceptible isolate (cefoxitin disk test zone diameter >30) which was 222

also PBP2a negative was misclassified as resistant by the KeyPathTM MRSA/MSSA 223

Blood Culture Test (0.8% major error rate). Repeat KeyPathTM MRSA/MSSA testing on 224

two of the isolates (one incorrectly classified as S and one incorrectly classified as R) 225

yielded results concordant with the standard method. Sensitivity and specificity of the 226

KeyPathTM MRSA/MSSA Blood Culture Test vs. cefoxitin disk testing for determination 227

of MSSA was 88.4% and 98.7%, respectively with a PPV of 92.7% and a NPV of 97.9%. 228

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For S. aureus, MRSA, and MSSA detection, there were no significant differences for 230

accuracy, sensitivity, specificity, PPV, or NPV between antimicrobial or antiviral-treated 231

patient populations when comparing MRSA/MSSA Blood Culture Test to standard 232

methods (data not shown). 233

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Time to results 235

Time to result (TTR) data was recorded for a subset of isolates from three sites as 236

specified in the Materials and Methods section. For 706 isolates, for which TTR data 237

was available for conventional testing, the median TTR was 46.9 h (range 21.8h to 238


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76.3h). For 708 isolates, for which KeyPathTM MRSA/MSSA Blood Culture Test TTR 239

data was available, the median TTR was 16.9h (range 13.4h to 20.9h). 240

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Bottle type 242

The KeyPathTM test was performed on samples from BactecTM Plus Aerobic/F (n = 812), 243

Plus Anaerobic/F (n = 296) and Standard 10 Aerobic/F (n = 8) bottle types. No 244

significant differences in performance between bottle types were observed.245


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Discussion 246

S. aureus is a virulent organism which causes significant morbidity and mortality (8, 12, 247

23). With the increasing incidence of S. aureus bacteremia (SAB) and the rising 248

prevalence of MRSA, it is imperative to consider approaches that may improve the 249

timeliness of appropriate therapeutic interventions. Using standard microbiologic 250

methods, initiation of directed therapy requires definitive identification and susceptibility 251

testing which can take up to 72 hrs. Although Gram stain is used for presumptive 252

diagnosis based on bacterial morphology, it is unable to discriminate between similar 253

appearing organisms and does not provide susceptibility results. Because approximately 254

30% of S. aureus isolates in US hospitals are methicillin resistant, the most common 255

antimicrobial used for presumptive S. aureus infection is vancomycin. Multiple studies 256

have suggested that patients who have MRSA bacteremia compared to MSSA bacteremia 257

have a longer length of stay in the hospital resulting in higher hospital costs (1, 9, 30, 31, 258

33) and higher mortality as a result of inappropriate initial antibiotics (30). 259

Clearly, nafcillin is superior to vancomycin for treating MSSA infection (6, 17, 34, 35) 260

but is ineffective for treating MRSA infection (19). In vitro studies suggest β -lactam 261

antibiotics achieve bactericidal activity significantly faster than glycopeptides (10, 20, 35, 262

37). Receipt of nafcillin or cefazolin is independently associated with decreased 263

treatment failure (35) and mortality (32) compared to vancomycin in the treatment of 264

MSSA bacteremia, a benefit that persists even if receipt is delayed until definitive culture 265

results are available. 266

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These facts, coupled with the potential to increase the emergence of Staphylococci and 268

Enterococci with increased resistance to vancomycin, make initial therapy with 269

vancomycin for patients with presumptive S. aureus infection suboptimal. More rapid 270

definitive identification of S. aureus, along with ß-lactam susceptibility testing should 271

allow earlier initiation of ß-lactams if appropriate, thereby reducing the overall usage of 272

vancomycin in these patients. While it is unclear whether appropriate initial antibiotic 273

therapy for the treatment of SAB affects mortality (10) earlier therapy appears to 274

decrease the length of bacteremia and fever in the patient (16, 18) and delayed treatment 275

is an independent predictor of infection-related mortality and associated with increased 276

length of hospital stay (22). 277

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There are several accelerated SAB identification assays that are commercially available. 279

These methods employ a variety of technologies including molecular amplification 280

and/or detection, antigen detection and culture-based methods (3, 26, 27, 28). None of 281

them are performed directly on blood specimens but rather require a signal positive blood 282

culture or the growth of a colony on media. The MicroPhage KeyPath™ assay is the first 283

to use a bacteriophage as the surrogate marker for the presence of bacteria in a positive 284

blood culture, providing a phenotypic result in as little as 5.5 hours. In our study, it 285

performed comparably to the aforementioned assays and does not require additional 286

instrumentation. Moreover, it provides a phenotypic assessment of susceptibility to 287

methicillin which may potentially avoid some of the limitations of molecular testing for 288

the mecA gene. Such limitations have recently been illustrated in reports that describe the 289

inability of such assays to accurately categorize some S. aureus isolates. A recent 290


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analysis revealed that of 217 patients tested positive by rapid PCR for MRSA carriage, 291

12.7% had S. aureus isolates with a staphylococcal cassette chromosome (SCC) element 292

that lacked the mecA gene; these patients were mislabeled as being MRSA carriers 293

because the rapid test was targeted to detect the SCC rather than mecA gene (3). The 294

KeyPathTM assay determines susceptibility through phenotypic testing using cefoxitin 295

rather than genotypic assessment. Although the PCR test evaluated in the above 296

referenced study is not designed to test directly from signal positive blood cultures, it 297

serves to illustrate the potential benefit of phenotypic susceptibility analysis compared to 298

molecular approaches. 299

300

Earlier identification of organisms should allow institution of appropriate antimicrobial 301

therapies. A pharmacoeconomic analysis of several publications determined the impact 302

of PCR to identify MRSA and concluded that mortality rate when appropriate empiric 303

antimicrobials were administered was significantly lower than the mortality in patients 304

who required a change in therapy. Cost analysis indicated PCR testing for MRSA would 305

be less costly than empiric therapy as long as the cost of the test was less than $2400 (4). 306

However, given that this was a composite of multiple other studies and a theoretical 307

model which proposes a 60-min turn-around time from positive blood culture result to 308

administration of optimal antibiotic therapy, it is difficult to apply the results in an actual 309

laboratory/hospital setting. 310

311

Furthermore, studies that analyzed rapid diagnostics used in combination with direct 312

communication of results to the prescribing physician by the laboratory or an 313


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antimicrobial stewardship team have shown that communication is essential for affecting 314

outcomes (2, 5, 11, 13, 15, 24). The results of this combination were shorter length of 315

stay (2, 11, 24, 25), shorter time to switch to a β -lactam antibiotic (2, 5, 11, 24, 25), and 316

decreased mortality (24). All of these studies were carried out in large academic centers 317

and it is unclear if similar benefits would be realized in a mixed practice or community 318

practice based center. Even with the faster turn-around time compared to conventional 319

methods of the assay we evaluated, timely communication of the results so that clinicians 320

can implement changes if needed, is essential to potentially improving patient care. 321

322

Since the KeyPathTM MRSA/MSSA Blood Culture Test has a sensitivity of 92%, labs 323

may wish to delay reporting of negative results until they are confirmed by standard 324

methods, during which patients would continue on empiric therapy. Labs could continue 325

to use standard of care methods with the KeyPathTM as a supplementary laboratory test. 326

Accuracy of susceptibility determination is high once an organism is identified as S. 327

aureus. On repeat testing of isolates that were not correctly identified initially as SA or 328

NSA, 50% of isolates produced results concordant with the gold standard result. This 329

was likely a result of the fact that the repeat tests were performed on bacterial isolates as 330

opposed to the initial method of testing directly from signal positive blood culture bottles 331

which may contain lower concentrations of organisms or interfering substances. Other 332

possibilities for incorrect identification include technical errors related to reading strips, 333

transfer of reagents, and inadequate mixing of reagents with the blood culture specimen. 334

An isolate identified as S. aureus by both the KeyPathTM MRSA/MSSA Blood Culture 335

Test in conjunction with a positive catalase and tube coagulase or Staphaurex test (to 336


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confirm S. aureus), would provide reliable and actionable data. In this study, all testing 337

with the KeyPathTM assay was performed in batch mode. In practice, it would be 338

practical to do the same to streamline the work flow of the laboratory technician. The 339

test can be easily performed without any specific technical training and expertise, unlike 340

fluorescence in situ hybridization testing. Another option would be to perform the test on 341

demand when a blood culture bottle signaled positive and Gram stain (routinely 342

performed) revealed gram positive cocci. While not impossible, this may be more 343

difficult to integrate into laboratory work flow and lead to errors if a laboratory 344

technician needs to track multiple incubation periods for the assay in addition to their 345

routine assignments. 346

347

Study Limitations 348

Only one type of blood culture system BACTECTM was evaluated. In addition, this rapid 349

test was not compared directly to other rapid methods. 350

351

In summary, the KeyPathTM MRSA/MSSA Blood Culture Test provides moderate 352

sensitivity (91.8%) and excellent specificity (98.3%) for the identification of MRSA and 353

MSSA directly from positive blood cultures with a significantly shorter turn-around time 354

compared to conventional testing methods. 355

356

Acknowledgements 357

This work was supported by Microphage Inc. (Longmont, CO).358


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499

500


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Figure 1. Performance characteristics of the KeyPathTM MRSA/MSSA Blood Culture 501

Test for identification of S. aureus. Of 366 isolates identified as S. aureus by the 502

standard methods (catalase, tube coagulase and Staphaurex®), the 503

KeyPathTM MRSA/MSSA Blood Culture Test correctly identified 336 directly from the 504

positive BACTECTM blood culture bottle. Thirteen non S. aureus blood culture isolates 505

were misidentified by the KeyPathTM MRSA/MSSA Blood Culture Test as S. aureus and 506

30 S. aureus isolates were misidentified as non S. aureus. SA=S. aureus, NSA=Not S. 507

aureus, SN=sensitivity, SP=specificity, PPV=positive predictive value, NPV=negative 508

predictive value. 509

510

Figure 2. Categorical agreement between the KeyPathTM MRSA/MSSA Blood Culture 511

Test and cefoxitin disk test for prediction of oxacillin susceptibility. Two S. aureus 512

isolates judged to be cefoxitin resistant using the disk test were misclassified as 513

susceptible by the KeyPathTM MRSA/MSSA Blood Culture Test (very major error). One 514

cefoxitin susceptible isolate was misclassified as susceptible by KeyPathTM 515

MRSA/MSSA Blood Culture Test (major error). Repeat testing on two of the isolates 516

(one incorrectly classified as S and one incorrectly classified as R) yielded results 517

concordant with cefoxitin disk testing. 518

519


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Figure 1

SA NSA

Gold Standard

SA 336 13 PPV = 96%

NSA 30 737 NPV = 96%

SN = 92% SP = 98%

KeyPathTM


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Figure 2

R SR 178 1

Cefoxitin Disk

R 178 1

S 2 153

Category Agreement 99.4% 98.7%

KeyPathTM


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Downloaded from //jcm.asm.org/content/jcm/early/2013/01/31/JCM... · 2013. 1. 31. · 97 (BD Diagnostics, Sparks, MD) blood culture bottles, 0 24 hours after detection (i.e. 98 signal-positive)