Diagnostic Validity of Computed Tomography for HLA-B27 Negative Ankylosing Spondylitis: A Retrospective Study of 209 Patients

Objective: Radiographic sacroiliitis is considered the hallmark of ankylosing spondylitis (AS). However, large interobserver variations make plain radiographic diagnosis of sacroiliitis notoriously difficult. The heterogeneity of HLA-B27 negative AS makes its diagnosis even more challenging. Several reports have shown that sacroiliitis tends to be underestimated using radiography, and that computed tomography (CT) facilitates AS diagnosis in patients with suspected spondyloarthritis. However, no studies have stressed the clinical utility of CT, particularly in the diagnosis of HLA-B27(-) AS. Methods: We retrospectively evaluated 209 HLA-B27(-) patients with chronic lower back pain and suspected AS. Radiography and CT reports were examined and outcomes were compared. Results: Among 408 sacroiliac (SI) joints examined using both radiography and CT, there was agreement between the two methods in the sacroiliitis grading of 82 (20.1%) SI joints. However, sacroiliitis grade using CT was higher in 276 (67.6%) SI joints and lower in 50 (12.3%) SI joints. CT evaluation of SI joints showed that 78.9% of patients met the radiographic sacroiliitis criteria of the modified New York criteria, while only 26% of patients satisfied the criteria for plain radiography. Surprisingly, 117 patients (57.3%), who did not meet the modified New York AS diagnosis criteria for plain radiography met the criteria for CT. Conclusion: CT was sensitive and useful in providing evidence for the diagnosis of AS. We suggest that HLA-B27(-) patients with equivocal plain radiography results and chronic inflammatory lower back pain be examined using CT.