Rapid differentiation of Staphylococcus aureus in blood cultures using the STAPH score: a prospective observational study

Abstract

ABSTRACT Rapid and accurate identification of Staphylococcus aureus (SA) in blood culture specimens is crucial for timely clinical intervention. Traditional Gram staining methods, although widely accessible and cost-effective, exhibit variable sensitivities and specificities in the BACTEC system. We aimed to develop and validate the “STAPH score,” a novel semi-quantitative scoring system that integrates Gram stain morphology and time to positivity to differentiate SA from coagulase-negative staphylococci (CoNS) in blood cultures. We analyzed 148 blood culture specimens from septic patients. Each specimen was assessed independently by two examiners using Gram staining and the STAPH score, which assigns points based on five parameters: cluster size, time to positivity, aerobic enlargement, pint (three-dimensional appearance), and the presence of hemorrhage. Sensitivity and specificity were calculated for various cutoff points. Cohen’s kappa coefficient was used to assess inter-examiner agreement. Of the 148 specimens, 49 (33.1%) were identified as SA and 99 (66.9%) as CoNS. At a STAPH score cutoff of 3 points, the sensitivity was 93.9% (95% confidence interval [CI], 83.1%–98.7%) and specificity was 91.9% (95% CI, 84.7%–96.4%). The kappa coefficient at this cutoff was 0.67 (95% CI, 0.55–0.79). A STAPH score <3 effectively ruled out SA with 100% sensitivity, whereas a score of 5 confirmed SA with 100% specificity. The STAPH score is a reliable and efficient tool for the rapid identification of SA in blood cultures within the BACTEC system. By combining Gram staining observations with time to positivity, this method enhances diagnostic accuracy, reduces subjectivity, and supports timely clinical decision-making. IMPORTANCE The rapid and accurate identification of Staphylococcus aureus (SA) in blood cultures is vital for timely and appropriate clinical intervention. This study introduces the “STAPH score,” a novel semi-quantitative scoring system that combines Gram stain morphology and time to positivity. By providing a reliable and efficient method to differentiate SA from coagulase-negative staphylococci, the STAPH score enhances diagnostic accuracy and reduces subjectivity in microscopic examinations. This score, applicable within the BACTEC system, addresses the limitations of traditional Gram staining methods and expensive molecular techniques. The implementation of the STAPH score in clinical practice can lead to faster diagnosis, improved patient outcomes, and optimized antimicrobial therapy. This method is particularly valuable in resource-limited settings where advanced diagnostic tools may not be available.