Fig. 1

Baseline variables, including red flags and descriptive characteristics, and outcome variables of four data sources. The asterisk indicates the red flag with evidence of positive likelihood ratio > 5.0; the dagger represents the red flag described in the American College of Physicians low back pain clinical practice guideline [9] or by the American College of Radiology [8]; LBP, low back pain; CTAS, Canadian Triage and Acuity Score; ICD, International Classification of Disease; XR, plain-film X-ray; CT, computed tomography; MRI, magnetic resonance imaging; ED, emergency department