How is low back pain diagnosed?
Your doctor will ask questions about your past health, symptoms, and activities. He or she will also do a physical exam. Your answers and the exam can help rule out a serious cause for the pain. In most cases, doctors are able to recommend treatment after the first exam.
Tests such as X-rays, CT scans, and MRIs usually don't help.
But if you have a back pain problem that has lasted longer than 6 weeks, or if your doctor thinks you may have more than muscle pain, it might be time for one of these tests.
Source: http://www.webmd.com