During the physical exam, your doctor might try to pinpoint the cause of your pain by pressing on places on your hips and buttocks. He or she might move your legs into different positions to gently stress your sacroiliac joints.
An X-ray of your pelvis can reveal signs of damage to the sacroiliac joint. If ankylosing spondylitis is suspected, your doctor might recommend an MRI — a test that uses radio waves and a strong magnetic field to produce very detailed cross-sectional images of both bone and soft tissues.
Because low back pain can have many causes, your doctor might suggest using numbing injections (anesthetics) to help with the diagnosis. For example, if such an injection into your sacroiliac joint stops your pain, it's likely that the problem is in your sacroiliac joint. However, the numbing medicine can leak into nearby structures, and that can reduce the reliability of this test.