What is the diagnostic process for sacro-iliac joint pain?

The diagnostic process for sacroiliac joint pain is built on a weak foundation. No single test is reliable enough to stand alone.

The evidence shows that the physical exam tests doctors use to diagnose SIJ pain have poor agreement between examiners. A 2017 study found kappa values near zero for common tests like the drop test and straight leg raise, meaning two clinicians examining the same patient often disagree on whether the test is positive [1]. This is not a minor flaw. It means the first step in diagnosis is essentially subjective.

The best available reference standard is a diagnostic injection. When a doctor injects anesthetic into the SI joint under imaging guidance and the patient gets more than 75% pain relief, that is considered confirmation the joint is the source [11]. Fluoroscopy-guided injections hit the joint 97-98% of the time, CT guidance 100% [11]. But even this has limits: the injection itself can miss, and the placebo response to any injection is substantial.

Imaging is rarely helpful for mechanical SIJ dysfunction. MRI is sensitive for inflammatory sacroiliitis (the kind seen in autoimmune disease) but not specific: one study of 691 patients referred for suspected sacroiliitis found that 44% had non-inflammatory findings like disc degeneration or hip disease instead [29]. Another study showed that even among asymptomatic people with uveitis, 37.5% had MRI findings that looked like sacroiliitis [6]. So an abnormal MRI does not prove the SIJ is causing your pain.

My call: the diagnostic process for SIJ pain is a probabilistic exercise, not a definitive one. A cluster of positive exam tests plus a positive diagnostic injection is the best you can do, but the evidence base is thin and the tests themselves are poorly validated. Confidence: moderate that the injection reference standard works, low that any physical exam test alone is trustworthy.