Rare diseases

Antibodies of little diagnostic value in early axial spondyloarthritis

Although anti-CD74 antibodies are elevated in patients with early axial spondyloarthritis  (axSpA) they appear to be of little diagnostic value in people under the age of 45, new research from the Netherlands suggests.

The team of researchers from the Rheumatology and Immunology Center in Amsterdam noted that results from two recent studies had shown that anti-CD74 antibodies were elevated in SpA, but their role in a diagnostic setting remained unclear.

In an exploratory cohort of patients (n=38 with ankylosing spondyloarthritis, n=57 healthy controls) they discovered  anti-CD74 IgG antibodies were present in 79.7% of patients with ankylosing spondyloarthritis (AS) vs. 43.9% of healthy controls (p < 0.001). Anti-CD74 IgA antibodies were present in 28.5% of patients with AS vs. 5.3% of healthy controls (p < 0.001).

Levels were then measured in patients with early axSpA (n = 274) and with non-SpA chronic back pain (CBP) (n = 319), participating in the spondyloarthritis caught early (SPACE) prospective cohort.

In these patients anti-CD74 IgG antibody levels were present in 46.4% of the patients with axSpA compared to 47.9% of the patients with chronic back pain (p = 0.71).

Anti-CD74 IgA antibodies were present in 54.7% of the patients with axSpA and 37.0% of the patients with chronic back pain (p < 0.001), equating to a positive predictive value of 58.8% and a negative predictive value of 59.1%.

A regression model showed total serum IgA was associated with axSpA (Odds Ratio 1.19, p < 0.001) whereas anti-CD74 IgA was not (OR 1.01, p = 0.33).

Anti-CD74 IgA was also associated with sacroiliitis on magnetic resonance imaging (OR = 2.50, p = 0.005) and heel enthesitis (OR = 2.56, p = 0.002).

Anti-CD74 IgG and IgA antibodies are of limited value in diagnosing axSpA in patients with early, chronic back pain,” the authors concluded in the study published in Arthritis Research and Therapy.

However, they also said that the study had revealed “two novel pieces of information”.

Firstly, the percentage of patients with non-radiographic axSpA who were positive for anti-CD74 IgG antibodies was lower than seen in other studies.

They said this finding suggested that either antibodies develop over time or that they are associated with radiographic sacroiliitis rather than with the diagnosis of axSpA.

Secondly, the percentage of control patients  with elevated anti-CD47 IgG antibodies (47.9%) was much higher than previously reported.

“This may be due either to the fact that the antibodies are higher in patients with chronic back pain than in healthy individuals, or may be explained by the fact that the anti-CD74 IgG assay has been modified overtime,” they wrote.

They suggested that long-term follow up will show whether anti-CD74 IgA antibodies predict axSpA characteristics such as radiographic damage, extra-articular manifestations or peripheral joint disease.

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