Although >10% bands on a CBC has been a part of the definition for sepsis, scant data exists regarding the clinical significance of lower percentages of bands. Our aim was to determine whether any associations exist between percentage of bands on an initial CBC and likelihood of a bloodstream infection and in-hospital mortality.

We performed a retrospective study of emergency department adults from January 1, 2016 to September 1, 2019 who had a CBC with manual differential and blood cultures obtained during their initial evaluation. Band percentages were grouped into zero (0% bands), minimal (1–2% bands), mild (3–4% bands), moderate (5–10% bands) and high (>10% bands). The primary outcomes were bloodstream infections and in-hospital mortality.

Increasing rates of bloodstream infections were observed as bands went from zero (95% CI: 9.3%–10.5%) to minimal (17.5%–19.1%, p < 0.0001), minimal to mild (19.2%–22.0%, p = 0.0039), mild to moderate (23.5%–26.7%, p < 0.0001), and moderate to high (33.0%–37.4%, p < 0.0001). Similar observations were seen when comparing mortality. The most common bloodstream infections were due to Gram-negative bacilli.

Elevated bands on an initial CBC were correlated with the likelihood of a concurrent bloodstream infection and in-hospital mortality, even at levels below 10%. Our results suggest that clinical suspicion for a bloodstream infection due to Gram-negative bacilli should rise if bands are elevated on an initial CBC. Therefore, clinicians should consider obtaining blood cultures if bands are elevated on an initial CBC.