ATLANTA – Adenosine deaminase 2 above the upper limit of normal is 86% sensitive and 94% specific for distinguishing macrophage activation syndrome from active systemic juvenile idiopathic arthritis, making it perhaps the most potent blood marker yet identified to differentiate the two, according to a at the annual meeting of the American College of Rheumatology.
The upper limit of normal was 27.8 U/L, two standard deviations above the median of 13 U/L (interquartile range, 10.6-16.1) in 174 healthy children. The work was published simultaneously in.
In children with active systemic juvenile idiopathic arthritis (JIA), adenosine deaminase 2 (ADA2) “beyond the upper limit of normal is strong evidence for concomitant” macrophage activation syndrome (MAS). “Our work represents a new method to diagnose this condition,” said lead investigator, MD, PhD, a pediatric rheumatologist at Boston Children’s Hospital.
The hope, he said, is that the finding will lead to quicker recognition and treatment of MAS, a devastating complication of systemic JIA in which rampant inflammation begets further inflammation in a downward spiral that ultimately proves fatal in about 20% of cases. The problem is that the clinical features of MAS overlap with those of active systemic JIA, which makes early diagnosis difficult.
Ferritin and other common markers are not very specific unless “the cutoff is raised significantly to distinguish MAS from general inflammation. Most labs will not tell you ‘this is an active systemic JIA range; this is an MAS-like range.’ It’s hard for them to define that for you. ADA2 is more black and white; if you go above the upper limit, you most likely have MAS,” Dr. Lee explained at the meeting.
Potentially, “we can combine this test with other tests to define a single MAS panel,” he said.
ADA2 is measured by a simple, inexpensive enzyme assay that’s been around for 20 years, but it hasn’t caught on because the protein’s function is unknown and the clinical relevance of ADA2 levels has been uncertain. With the new findings, “it is our hope that ADA2 testing will become more available,” Dr. Lee said.
The protein appears to be a product of monocytes and macrophages, and a genetic deficiency has recently been linked to congenital vasculitis, which made Dr. Lee and colleagues curious about ADA2 in other rheumatic diseases. The first step was to define normal limits in healthy controls; the 13 U/L median in children proved to be a bit higher than in 150 healthy adults.
The team then found that levels were completely normal in 25 children with active Kawasaki disease, and only mildly elevated in 13 children with systemic lupus and 13 with juvenile dermatomyositis. The Kawasaki children, in particular “were highly inflamed, so this protein is not just simply a marker of inflammation,” Dr. Lee said.
They next turned to 120 children with JIA, with a mix of systemic and nonsystemic cases. “The ones with very high levels, far beyond the upper limit of normal, were” almost exclusively the 23 children with systemic JIA and clinically diagnosed MAS. “As long as [JIA children] didn’t have MAS, their levels were pretty much close to normal,” he said.
In eight MAS children with repeat testing, levels fell below the upper limit of normal with treatment and remission, but children prone to repeat MAS seemed to hover closer to the limit even when they were well.
Blood sample testing showed that interleukin-18 and interferon-gamma were the main drivers of ADA2 expression in the periphery, “which makes sense because these two cytokines are very involved in the process of MAS,” Dr. Lee said.
The work was funded by the National Institutes of Health, among others. Dr. Lee didn’t have any disclosures.
SOURCE: Lee PY et al. Arthritis Rheumatol. 2019;71(suppl 10), Abstract