A more efficient strategy would be to use likelihood ratios to inform imaging decisions. For instance, the prevalence, or pretest probability, of cancer in a primary care population is about 0.7% (39). A history of cancer is the strongest risk factor for a spinal tumor (positive likelihood ratio, 15) (39). Unexplained weight loss, lack of improvement after 1 month, and age older than 50 years are weaker risk factors (positive likelihood ratio, 2.7 to 3.0). On the basis of these likelihood ratios, the probability of cancer in a patient with a history of cancer would increase to approximately 9%, or high enough to warrant immediate imaging (a strong clinical suspicion for cancer would give a similar result ). In patients with any of the other 3 risk factors, the posttest probability increases only marginally, to 1.2%. Imaging could be reasonably deferred in most cases unless symptoms did not improve after several weeks (38, 74). For patients with no signs of neurologic compromise who have risk factors for vertebral compression fracture, ankylosing spondylitis, herniated disc, or spinal stenosis, a trial of therapy before imaging would also be warranted. Diagnostic rules based on the evaluation of multiple risk factors could help better inform imaging decisions, but they are in the early stages of development (72).