Validation of blood culture gram staining for the detection of Staphylococcus aureus by the ‘oozing sign’ surrounding clustered gram-positive cocci: a prospective observational study

BACKGROUND: Staphylococcus aureus bacteraemia is a common and significant infection, associated with high rates of mortality. Therefore, early identification is important for the initiation of appropriate treatment. The objective of this study was to evaluate the accuracy of blood culture Gram staining along with the finding of an ‘oozing sign’ to diagnose either Staphylococcus aureus or coagulase-negative staphylococci. METHODS: This single-centre, prospective observational study was performed from May 2017 to November 2017. We used routine blood culture bottles (BacT/ALERT FA and BacT/ALERT SN; bioMérieux, Inc., Durham, NC). Bacterial species were identified and the minimum inhibitory concentration was determined by using the MicroScan WalkAway 96 SI system (Beckman Coulter, Tokyo, Japan). Bottles showing growth were removed, and Gram staining was performed. RESULTS: A total of 118 samples, including 55 aerobic and 63 anaerobic bottle samples, were analysed. The overall sensitivity of Gram staining was 78.7% (95% CI: 65.8–94.3%), and the specificity was 95.0% (95% CI: 84.7–98.4%). CONCLUSION: The ‘oozing sign’ observed in Gram staining may be useful for the rapid prediction of S. aureus in BacT/ALERT blood culture bottles. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1186/s12879-018-3412-2) contains supplementary material, which is available to authorized users.