context: For 15% to 30% of people with long-term lower back pain, one of the sacroiliac ( SI) joints, which connect the pelvis with the spine, is the source. But it's not always easy for people or their doctors to identify the SI joint as the problem. Although the sacroiliac is one of the largest joints in your body, it's buried deep inside your pelvis. Its location makes it hard for doctors to spot any issues during a physical exam. It can also be tough for imaging tests to capture the high-quality pictures that can show it. SI joint problems can be confused with other painful conditions, like sciatica and hip arthritis, and routine exams can't always pinpoint what's really going on. Doctors usually rely on the results of three or more tests to diagnose SI joint dysfunction. You can have most of them in your doctor's office. Tests in Your Doctor's Office Your doctor will start by asking you about your medical history and doing a physical exam, including checking your posture and how well you walk. Then he'll do some tests to see where the pain may be coming from and how well your SI joint moves. Some of the more common tests include: Cranial shear test: While you lie on your stomach on an exam table, the doctor puts pressure on your tailbone to check for pain. Flamingo test: The doctor asks you to stand on the leg where you have pain and hop. Gaenslen's test: While you lie face up, your doctor bends and presses one knee toward your chest while pressing the other leg, outstretched, off the side of the table. Gillet test (one-leg stork test): You stand with your feet about 12 inches apart, one leg raised and bent. The doctor stands behind you and pushes on each side of your tailbone with his thumbs, checking for correct movement. Pelvic compression test: While you lie on your side, the doctor presses down on your upper hip. Pelvic distraction test: While you lie face up on the table, the doctor presses down on both sides of your pelvis at the same time. Patrick's test ( FABER test): You lie face up and put the foot of the side where you hurt against the opposite knee. The doctor presses against the knee and the opposite pelvis at the same time. Sacroiliac shear test: While you lie face down, the doctor puts one hand on top of the other over your tailbone and pushes down, looking for motion in your joint. Thigh thrust test: While you lie face up on the table, the doctor puts one hand under your buttock while bending your hip and knee at a 90-degree angle. Imaging Tests Your doctor may also want you to have imaging tests to get pictures of the inside of your low back and pelvis. X-rays can help him look for changes in the SI joint. Computed tomography ( CT), a powerful X-ray scan, can give him even more details. Sometimes doctors use magnetic resonance imaging ( MRI), which uses powerful magnets and radio waves to make pictures of organs and structures inside your body. It can help to rule out other problems that can cause similar symptoms. Injection Test The surest way for a doctor to know if you have SI joint dysfunction is through an injection of numbing medicine into your joint. An X-ray or ultrasound guides the doctor to where to put the needle in. If the pain goes away after the shot, you know the joint is the problem. It's slightly riskier than other tests, so doctors usually do it only if the cause still isn't clear after other tests.