Three autoantibodies to newly discovered axial spondyloarthritis peptides may improve early diagnosis of the disease, according to a cross-sectional cohort study reported in Arthritis & Rheumatology.
Dr. Veerle Somers
The Assessment in SpondyloArthritis International Society (ASAS) classification criteria were not intended for diagnosis and do not differentiate well between patients with early axial spondyloarthritis (axSpA) and patients with nonspecific chronic low back pain, note the investigators, who conducted their research under senior investigator Veerle Somers, PhD, professor of molecular biology at Hasselt (Belgium) University and vice dean of the School of Life Sciences at Transnationale Universiteit Limburg, also in Hasselt.
“Therefore, for many patients, axSpA diagnosis may be challenging and is often delayed by several years after the occurrence of first clinical symptoms, posing a problem for early treatment initiation,” they wrote.
The investigators used plasma samples from patients with early disease and an axSpA complementary DNA phage display library developed with synovial tissue to screen for IgG antibodies that displayed significantly higher reactivity to plasma pools from the early axSpA patients than healthy controls.
They then assessed presence of the antibodies with enzyme-linked immunosorbent assays in a mixed cohort (76 patients with early axSpA having mean disease duration of 2.8 years, 75 control patients with nonspecific chronic low back pain, 60 patients with RA, and 94 healthy controls) and in an axSpA-only cohort (174 patients, 79 of whom had early disease with mean disease duration of 1.4 years).
Screening identified antibodies to nine novel peptides – eight peptides showing partial homology to human proteins and one novel axSpA autoantigen, double homeobox protein 4 (DUX4) – that were more commonly present in patients with early axSpA than in healthy controls, Dr. Somers and coinvestigators reported.
Subsequent analyses focused on the three antibodies having the highest positive likelihood ratios for differentiating axSpA from chronic low back pain.
Some 14.2% of the combined group of all patients with early axSpA had at least one antibody in this panel, compared with just 5.3% of the patients with chronic low back pain (P = .0484), corresponding to 95% specificity.
Prevalence did not differ significantly from that in patients with RA (10.0%; P = .5025) or healthy controls (8.4%; P = .2292).
The positive likelihood ratio for confirming early axSpA using the three antibodies was 2.7, on par with the historical ratio of 2.5 seen for C-reactive protein (CRP), the currently used laboratory marker, the investigators noted.
Among the patients with chronic low back pain, the posttest probability for axSpA increased from 79% with presence of inflammatory back pain and positive test results for HLA-B27 and CRP to 91% with addition of testing for the three antibodies.
The researchers proposed that, “in combination with other laboratory markers such as HLA-B27 and CRP, antibodies against our [three peptides] ... could provide a novel tool for the diagnosis of a subset of axSpA patients,” but the three-peptide panel needs to be studied more in larger cohorts of early axSpA patients and controls with low back pain.