Low back pain guidelines recommend classifying patients into three categories: nonspecific low back pain, radicular syndromes, or severe pathology. Some guidelines use a simpler classification into two categories. This approach has been in standard practice for 30 years. Most guidelines do not recognize a category for non-spinal causes of low back pain. Such causes include kidney and gastrointestinal disorders, other musculoskeletal conditions such as coxarthrosis, and fractures behind the lumbar spine. This diagnostic system has limitations and may be too simplistic to guide the diagnostic process.