National population estimates of patient visits for low back pain in the United States, 2003–2004. LBP indicates low back pain; NPE, national population estimate; SE, standard error. National population estimates are for both years combined and were computed by applying patient visits weights to the numbers reported in the National Ambulatory Medical Care Survey. Patient visits involving back symptoms attributed to a lump, mass, or tumor are not included as LBP visits. Patient visits other than initial visits for acute problems or follow-up visits for chronic problems are not represented under the "all LBP visits" and "primary LBP visits" headings.