Direct Disk Diffusion Susceptibility Testing for Staphylococcus aureus from Blood Cultures: Diagnostic Accuracy and Impact on Antimicrobial Stewardship
Bibliographic record
Abstract
In order to detect multidrug resistant strains of bacteria, our laboratory routinely performs direct susceptibility (DS), in addition to standardized susceptibilities (SS), testing from positive blood cultures. We conducted a prospective study to determine the accuracy, reporting time (RT), and antimicrobial stewardship impact of DS testing for Staphylococcus aureus positive blood cultures. From March–December 2016, first time positive blood cultures for S. aureus were included. Broth from positive blood culture bottles was inoculated to standard media, as well as to Mueller–Hinton agar with a cefoxitin disk. CLSI breakpoints for S. aureus were used to guide interpretation. If DS results were to be reported to clinicians, a penicillin-binding protein 2a (PBP2a) Alere™ test was performed. When the PBP2a result was concordant with the CLSI interpretation, the isolate was reported as either Methicillin-susceptible S. aureus (MSSA) or Methicillin-resistant S. aureus (MRSA). Antibiotic therapy changes made based on reporting of DS results were recorded. In order to determine RT, the following time points were recorded: blood culture positivity, reading of DS, and reporting of SS Of the 100 patients with S. aureus bacteremia, 97 showed pure growth of MSSA or MRSA; 3 patients had mixed infections, all of which were only detected using the DS plates. Average RT was 23 hours and 36h for DS and SS, respectively. There were 32 MRSA isolates, with a cefoxitin zone size range of 6–15 mm (median = 8 mm). PBP2a was performed on 11 isolates; all were positive. Of the 69 MSSA isolates, the cefoxitin range was 22 to 32 mm (median = 27 mm). PBP2a was performed on 26 isolates; all were negative. Direct susceptibility results were reported on 31 patients. Of the 21 patients with MSSA bacteremia, 15 changed therapy from vancomycin/daptomycin to cloxacillin/cefazolin. These results were reported an average of 23 hours prior to SS. DS testing is an accurate and rapid method to determine whether isolates are MSSA or MRSA. We had no major or minor errors. PBP2a testing was concordant for all isolates tested. DS also has the added benefit of detecting mixed S. aureus infections. Clinicians acted on the reported results of DS testing, with 15/21 (71%) of our patients narrowed to a cloxacillin/cefazolin 23 hours before the availability of SS. All authors: No reported disclosures.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.014 | 0.069 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.002 | 0.003 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.002 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".