The SCANNED rule: derivation and validation of a two-step rule for blood culture stewardship in patients presenting with suspected infection

BACKGROUND: Blood culture over-ordering is a recognized diagnostic stewardship challenge, contributing to contamination, inappropriate antimicrobial escalation, and laboratory burden. Validated tools to guide culture ordering at initial patient assessment are lacking. OBJECTIVE: We aim to derive and externally validate a two-step diagnostic stewardship rule for blood culture ordering in the emergency department (ED) patients with suspected infection. METHODS: We derived the Stepwise Culture Assessment with NEWS and NLR in ED (SCANNED) rule using consecutive blood cultures from a Singapore ED (April-July 2025) and validated it in Vancouver, Canada (2016-2019). The rule uses age-stratified National Early Warning Score (NEWS) thresholds (Step 1), followed by neutrophil-lymphocyte ratio (NLR) ≥5 for remaining patients (Step 2). Thresholds were selected via exhaustive grid search, optimizing culture reduction while constraining the false-negative rate to <5%. RESULTS: We included 1,224 derivation and 808 validation encounters. Blood culture positivity was 9.6% (117/1,224) and 13.8% (112/808), respectively. The optimal rule (NEWS ≥5 if age <75 or NEWS ≥3 if age ≥75; NLR ≥5 for remainder) reduced cultures by 18.8% (95% CI 16.6-20.9%) in derivation and 15.1% (95% CI 12.8-17.6%) in validation, missing <5% of bacteremia cases in both cohorts. Performance was consistent across infection sources, including unclear/occult infections (17.5% reduction, 3.7% false-negative rate). CONCLUSION: The SCANNED rule represents a practical diagnostic stewardship intervention that can reduce unnecessary blood cultures by 15-19% while maintaining >95% sensitivity for bacteremia. It requires no electronic health record integration and can be applied at triage. Prospective validation is needed before clinical implementation.