Subacute presentation may be with or without fever. There is often malaise, pain, limp and some decreased range of motion, but in general symptoms are vague. All patients have SI joint pain, but this may be overlooked because of their reluctance to move. SI joint pain may not be elicited during exam or may be attributed to another source such as septic hip, psoas abscess, malignancy, sciatica, herniated disc, pyelonephritis, ankylosing spondylitis, or appendicitis. Subacute disease is difficult to diagnose and the delay in proper diagnosis may lead to increased morbidity, increased joint destruction, potential rupture and abscess formation.
Physical exam is of the most importance to localize pain and decreased motion.
WBC and differential may not be useful. Synovial fluid cultures are technically difficult to obtain because of the joint anatomy. Open biopsy and culture may be required to establish diagnosis and identify the infectious agent. There have been several studies on the utility of bone and gallium scans in the diagnosis of osteomyelitis and sacroilitis. This evaluation of the three-phase bone scan commented on the benefit obtained from the flow and blood pool images.