, according to a comparative study first presented at the 2019 European League Against Rheumatism and now published.
“The major finding is that patients classified with one set of the criteria are essentially the same as those classified with the other,” according to Anne Boel, a researcher in the department of rheumatology at Leiden (the Netherlands) University Medical Center, and first author of the study.
The study addresses a controversy that has persisted since the introduction of ASAS criteria for defining axial spondyloarthritis (axSpA) with definite structural changes on conventional radiographs. It was unclear whether this ASAS diagnosis, called radiographic axSpA (r-axSpA), was the same as ankylosing spondylitis (AS) as defined by the older modified New York (mNY) criteria.
In this study, patients from eight cohorts were evaluated with the two classification sets. In addition to having radiographic sacroiliitis, all patients had to have back pain for at least 3 months, which is also mandatory for both classification sets.
Of the 3,434 fulfilling the ASAS criteria for r-axSpA, 96% fulfilled the mNY criteria for AS. Of the 3,882 meeting the mNY criteria for AS, 93% fulfilled the ASAS criteria for r-axSpA.
On the basis of this level of agreement, the authors called the terms r-axSpA and AS “interchangeable.” In the small proportion of cases when there was disagreement, the reason was considered to be minor and not to alter the conclusion that the disease entities are the same.
“Patients cannot be classified according to the ASAS criteria if they first develop back pain at age 45 years or older, so this is one difference between the two criteria sets that would affect classification,” Ms. Boel explained in an interview.
When tallied, 7% of the 4,041 patients with axSpA with radiographic sacroiliitis evaluated met only the mNY criteria, 3% met only the ASAS criteria, 89% met both sets of criteria, and 1% met neither, according to the published data.
Of those who met the mNY criteria but not the ASAS criteria, 99.7% would have potentially fulfilled the ASAS criteria for r-axSpA except for older age at onset. The remainder was attributed to an absence of inflammatory back pain or another spondyloarthritis feature.
Of the 3,434 patients fulfilling the ASAS criteria, 90% fulfilled the mNY criteria because of the presence of inflammatory back pain. Most of those without inflammatory back pain had a mobility restriction and so still met the mNY criteria. A small proportion without inflammatory back pain or mobility restriction fulfilled the ASAS criteria because of other SpA features.
The study resolves a persistent debate over whether AS patients identified by mNY criteria are the same as r-axSpA identified by ASAS criteria, according to the authors, reiterating that these data show that they can be considered the same disease.
This finding is particularly relevant when evaluating studies that have classified patients by either the mNY or the ASAS criteria.
This finding “has important implications for the axSpA research field,” the authors concluded. “Acknowledging that both criteria sets identify the same patients implies that older literature on AS and newer literature on r-axSpA can be directly compared.”
The study had no specific funding source. Ms. Boel reported having no potential conflicts of interest. Coauthors reported ties with pharmaceutical companies outside of this study.
SOURCE: Boel A et al. Ann Rheum Dis. 2019 Jul 30.