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.