Possible Sacroiliitis in 27 years old female

Dear Ask The Doctor: 27yo female; simmetrical bilateral pain in buttocks and lower back, sometimes involves both great throcanters too. Gets better with movement, worse at night when lie in bed. Think it is sacroiliitis. Would like to know if there are other possibilities, and even how to diagnose sacroiliitis avoiding x-rays exposing techniques: know of MRI, is there any blood test? sorry for my english, thank you very much for your answers.

Dear Bianca: Sacroiliitis is the inflammation of the Sacro-Iliac (SI) bone, located in the lower part of the spine, in the back of the pelvis, just above of the coccyx or tailbone. The inflammation may be caused by diverse factors as: falling, car accidents, blows directly or to the side of the pelvis,  degenerative arthritis. In a young female patient with sacroiliac pain, the differential diagnoses to be considered are: Hip overuse Syndrome, Iliotibial band Syndrome, Lumbosacral facet Syndrome, Piriformis Syndrome, Trochanteric bursitis, Lumbosacral radiculopathy, Sacroiliac joint infection, Seronegative spondyloarthropathy, Reiter Syndrome. Some studies show that sacroiliac pain would appear to be at least 13% and perhaps as high a 30% in patients with low back and buttock pain. I recommend that you get a clinical and lab re-evaluation to rule out some of the conditions mentioned above (seronegative spondyloarthritis, Reiter Syndrome). The diagnosis of Sacroiliitis is more clinic, meaning, by your doctor's examination than by x-rays.