Inflammatory back pain in ankylosing spondylitis: a reassessment of the clinical history for application as classification and diagnostic criteria.

OBJECTIVE: Back pain associated with ankylosing spondylitis (AS) is referred to as inflammatory back pain (IBP). The value of the clinical history in differentiating IBP from mechanical low back pain (MLBP) has been investigated in only a few studies. In this exploratory study, we sought to evaluate the individual features of IBP and to compose and compare various combinations of features for use as classification and diagnostic criteria.

METHODS: We assessed the clinical history of 213 patients (101 with AS and 112 with MLBP) younger than 50 years who had chronic back pain. Single clinical parameters and combinations of parameters were compared between the AS and MLBP patient groups.

RESULTS: Morning stiffness of >30 minutes' duration, age at onset of back pain, no improvement in back pain with rest, awakening because of back pain during the second half of the night only, alternating buttock pain, and time period of the onset of back pain were identified as independent contributors to IBP. Importantly, none of the single parameters sufficiently differentiated AS from MLBP. In contrast, several sets of combined parameters proved to be well balanced between sensitivity and specificity. Among these, a new candidate set of criteria for IBP, which consisted of morning stiffness of >30 minutes' duration, improvement in back pain with exercise but not with rest, awakening because of back pain during the second half of the night only, and alternating buttock pain, yielded a sensitivity of 70.3% and a specificity of 81.2% if at least 2 of these 4 parameters were fulfilled (positive likelihood ratio 3.7). If at least 3 of the 4 parameters were fulfilled, the positive likelihood ratio increased to 12.4.

CONCLUSION: A new set of criteria for IBP performed better than previous criteria in AS patients with established disease. A prospective study is needed to validate the diagnostic properties of the new candidate criteria set in patients with early disease.