Background/Purpose: The age-adjusted prevalence of spondyloarthritis (SpA) has been estimated to be 0.9-1.4% of the US population using the Amor and ESSG criteria. The objective of this study was to determine the prevalence of non-radiographic axial SpA (nr-axSpA) based on the Assessment of Spondyloarthritis International Society (ASAS) axial SpA criteria among undiagnosed patients (pts) with chronic back pain.  

Methods: This is an ongoing, non-drug, multicenter study in the US. Pts previously undiagnosed with SpA, who have had chronic back pain for ≥3 months, with age at onset of <45 years, and at least 1 of the following: 1) positive HLA-B27, 2) current inflammatory back pain, or 3) imaging (MRI or x-ray) evidence of sacroiliitis were included. Pts were either new referrals from other physicians, self-referred, or existing pts in the rheumatologist’s clinic. Medical history, physical exam findings, disease activity measures, and laboratory and imaging test results were collected. The rheumatologist was asked if a diagnosis of axial SpA could be made based on this information and the level of confidence with the diagnosis. Data collected were then analyzed independently to determine which patients are classified as nr-axSpA based on fulfillment of the ASAS criteria for axial SpA, but not modified New York (modNY) criteria for ankylosing spondylitis (AS). 

Results: As of February 2013, 459 pts have been enrolled (31.6% new referrals, 6.5% self-referred, and 61.9% existing pts at the site). 210 (46%) pts fulfilled ASAS criteria, of whom 146 (32%) were classified as nr-axSpA (fulfilled ASAS but not modNY criteria), 59 (13%) classified as AS (fulfilled ASAS and modNY criteria), and 5 (1%) had missing data that did not allow for evaluation of fulfillment of the modNY criteria. Of those who fulfilled ASAS criteria and had available data, 74% were also given a diagnosis of axial SpA by the investigator (95% of those classified as AS and 64% of those classified as nr-axSpA). In addition, 20% of pts who did not fulfill ASAS criteria were given a diagnosis of axial SpA by the investigator, although the overall level of confidence in the diagnosis was lower for these pts. Demographics and disease characteristics for pts classified as nr-axSpA are presented in the Table.

 

Table.  Baseline Demographics and Disease Characteristics of Patients Classified as nr-axSpA*
Characteristic	nr-axSpA N=146
Gender, n (%)
Female	76 (52.1)
Age (years)
Mean (SD)	40 (12.3)
Duration of chronic back pain (months)
Mean (SD)	166 (145.7)a
Age at onset of chronic back pain (years)
Mean (SD)	27 (9.3)a
Human leukocyte antigen-B27 (HLA-B27), n (%)
Positive	110 (75.3)
Bath AS functional index (BASFI)
Mean (SD)	4.4 (2.6)
Bath AS disease activity index (BASDAI)
Mean (SD)	5.9 (2.1)
AS Disease Activity Score (ASDAS)
Mean (SD)	2.3 (0.8)b
hs-CRP or CRP, category
Elevated	48 (34.3)b
Missing	6
*fulfilled ASAS but not modNY criteria
an=144.  bn=140

Conclusion: Almost half of pts enrolled with chronic back pain and at least 1 SpA-related feature were classified as having axial SpA based on ASAS criteria. Two-thirds of these axial SpA pts had nr-axSpA, with an average of 14 years of symptoms, indicating a substantial delay in diagnosis. The majority of pts who fulfilled ASAS criteria were also diagnosed as having axial SpA by US rheumatologists, but a quarter of these patients were not.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/prevalence-of-axial-spondyloarthritis-in-the-united-states-among-patients-with-chronic-back-pain-and-other-spondyloarthritis-related-features/