575. Optimization of a Nitrocefin-Based Rapid Test for the Detection of the Cefazolin Inoculum Effect (CzIE) in Methicillin-Susceptible <i>Staphylococcus aureu</i>s
Bibliographic record
Abstract
Abstract Background The cefazolin (Cz) inoculum effect, defined as a Cz MIC ≥16 µg/mL at 107 CFU/mL in MSSA isolates, has been associated with poor outcomes in adult patients with MSSA bacteremia and pediatric patients with osteomyelitis. Broth microdilution at high inoculum is the gold standard for the detection of the CzIE, but this method is time and labor-intensive. We developed a nitrocefin-based rapid test that can identify the CzIE among MSSA(Figure 1). However, the original protocol for this rapid test includes the use of ampicillin in solution, which is difficult to prepare and unstable once reconstituted. In this study, we aimed to show that simple elution from ampicillin disks available in the typical clinical microbiology lab performs as well as an ampicillin powder-based assay.Figure 1.Original Protocol Nitrocefin Rapid Test Original Protocol for the performance of the Nitrocefin-based rapid test as published by Rincon et at (Rincon et al, JCM, 2021) Methods We included 107 unique MSSA bloodstream isolates which had been previously characterized (Dingle et al., JCM, 2022). The CzIE was determined using gold standard broth microdilution at high inoculum (107 CFU/mL). The published protocol of the nitrocefin test (Rincon et al., JCM, 2021) was modified as follows (Figure 2): instead of adding ampicillin from powder to BHI broth (150 ug/mL), ampicillin disks (10 ug) of 2 different brands (Oxoid and BD) were added to 1 mL of BHI broth of 2 different brands (Oxoid, BD) and incubated for 2 hours at room temperature with frequent shaking (Figure 3). The remaining steps of the protocol were not modified. The original protocol was used as control. Performance metrics were calculated for the complete data set.Figure 2.Modification to Nitrocefin-based rapid test protocol Modification of the Step 1 from the original protocol to include ampicillin disks instead of ampicillin from powder. Figure 3. Experiments and Controls. 2 different brands of BHI broth and 2 different brands of ampicillin disks(10 ug) were compared to the use of ampicillin from powder to a final concentration of 150 ug/mL Results 51.4%(55/107) of the isolates were CzIE+ by reference broth microdilution at high inoculum. When using Oxoid BHI broth, the sensitivity and specificity of the test were 96.3% and 90.3% regardless of the ampicillin form (disk vs powder) or disk brand (Table 1). When using BD BHI broth, the sensitivity and specificity of the test were 69.1% and 94.2%, regardless of the ampicillin form or disk brand.Table 1.ResultsPerformance metrics of the Nitrocefin-based rapid test with the original protocol (Control A and Control B) and the modified protocol using ampicillin disks from 2 different brand (BD: Experiments C and D; Oxoid: Experiments E and F) as well as 2 different brands of BHI broth (BD: Control A, experiments C and E; Oxoid: Control B, experiments D and F) Conclusion The modification of the nitrocefin-based rapid to use ampicillin disks instead of ampicillin powder did not impact the accuracy of the tests as it was able to correctly detect the CzIE in isolates, regardless of the brand of ampicillin disk. Implementation of this test may contribute to therapeutic decisions in deep-seated MSSA infections. Disclosures Jennifer Dien Bard, PhD, Abbott Molecular: Grant/Research Support|BioMerieux: Advisor/Consultant|BioMerieux: Grant/Research Support|BioMerieux: Honoraria|Genetic Signature: Advisor/Consultant|Genetic Signature: Grant/Research Support|Luminex: Grant/Research Support|Salve: Stocks/Bonds|Thermo Fisher: Honoraria Lars Westblade, PhD, Accelerate Diagnostics, Inc.: Grant/Research Support|bioMerieux, Inc.: Grant/Research Support|Hardy Diagnostics: Grant/Research Support|Roche Molecular Systems, Inc.: Advisor/Consultant|Roche Molecular Systems, Inc.: Grant/Research Support|Selux Diagnostics, Inc.: Grant/Research Support|Shionogi, Inc.: Advisor/Consultant|Talis Biomedical: Advisor/Consultant William R. Miller, M.D., Merck: Grant/Research Support|UpToDate: Honoraria
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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 | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 0.004 |
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".