To evaluate the usefulness of needle biopsy in the diagnosis of early sacroiliitis to improve the diagnostic level and outcome of ankylosing spondylitis (AS).

One hundred nine patients in whom early AS was highly suspected, but in whom only sacroiliitis of grade I or lower on radiography/computed tomography (CT) was seen, were recruited for study. CT-guided needle biopsy of the sacroiliac joints was performed, and the patients were followed up for 5-10 years.

Of the 109 patients, magnetic resonance imaging (MRI) was used to confirm the presence or absence of sacroiliitis in 77 patients. Of these, 23 patients were determined to have sacroiliitis on MRI, and 54 had no sacroiliitis on MRI. Needle biopsy was performed on all 109 patients. Features of inflammation were found in 85 patients, which included all 23 patients with MRI evidence of sacroiliitis and 38 of the 54 patients without MRI evidence of sacroiliitis. No features of inflammation were found on needle biopsy in 24 of the patients, including the remaining 16 patients who did not have sacroiliitis on MRI. The sensitivity and specificity of MRI for the early diagnosis of sacroiliitis in these patients were 37.7% and 100%, respectively. Thirty-four patients with pathologic evidence of sacroiliitis were followed up for 5-10 years. At the study end point, 16 of these 34 patients continued to show grade I or lower changes on CT, and 18 had changes of grade II or higher. These 18 patients included 7 of the 8 patients with evidence of sacroiliitis on MRI and 6 of the 20 patients confirmed not to have MRI evidence of sacroiliitis at baseline.

MRI, though of low sensitivity, is specific for the diagnosis of early sacroiliitis. Sacroiliitis can be detected earlier by needle biopsy than by MRI.