A new diagnostic test can accurately distinguish osteoarthritis (OA) from inflammatory arthritis using two synovial fluid biomarkers, according to research published in the Journal of Orthopaedic Research on December 18, 2024.
However, experts question whether such a test would be useful.
“The need would seem to be fairly limited, mostly those with single joint involvement and a lack of other systemic features to specify a diagnosis, which is not that common, at least in rheumatology, where there are usually features in the history and physical that can clarify the diagnosis,” said Amanda E. Nelson, MD, MSCR, professor of medicine in the Division of Rheumatology, Allergy, and Immunology at the University of North Carolina at Chapel Hill. She was not involved with the research.
The test uses an algorithm that incorporates concentrations of cartilage oligomeric matrix protein (COMP) and interleukin 8 (IL-8) in synovial fluid. The researchers hypothesized that a ratio of the two biomarkers could distinguish between primary OA and other inflammatory arthritic diagnoses.
“Primary OA is unlikely when either COMP concentration or COMP/IL‐8 ratio in the synovial fluid is low since these conditions indicate either lack of cartilage degradation or presence of high inflammation,” wrote Daniel Keter and coauthors at CD Diagnostics, Claymont, Delaware, and CD Laboratories, Towson, Maryland. “In contrast, a high COMP concentration result in combination with high COMP/IL‐8 ratio would be suggestive of low inflammation in the setting of cartilage deterioration, which is indicative of primary OA.”
In patients with OA, synovial fluid can be difficult to aspirate in sufficient amounts for testing, Nelson said.
“If synovial fluid is present and able to be aspirated, it is unclear if this test has any benefit over a simple, standard cell count and crystal assessment, which can also distinguish between osteoarthritis and more inflammatory arthritides,” she said.
Differentiating OA
To test this potential diagnostic algorithm, researchers obtained 171 knee synovial fluid samples from approved clinical remnant sample sources and a biovendor. All samples were annotated with an existing arthritic diagnosis, including 54 with primary OA, 57 with rheumatoid arthritis (RA), 30 with crystal arthritis (CA), and 30 with native septic arthritis (NSA).
Researchers assigned a CA diagnosis based on the presence of monosodium urate or calcium pyrophosphate dehydrate crystals in the synovial fluid, and NSA was determined via the Synovasure Alpha Defensin test. OA was confirmed via radiograph as Kellgren‐Lawrence grades 2‐4 with no other arthritic diagnoses. RA samples were purchased via a biovendor, and researchers were not provided with diagnosis‐confirming data.
All samples were randomized and blinded before testing, and researchers used enzyme-linked immunosorbent assay tests for both COMP and IL-8 biomarkers.
Of the 54 OA samples, 47 tested positive for OA using the COMP + COMP/IL-8 ratio algorithm. Of the 117 samples with inflammatory arthritis, 13 tested positive for OA. Overall, the diagnostic algorithm demonstrated a clinical sensitivity of 87.0% and specificity of 88.9%. The positive predictive value was 78.3%, while the negative predictive value was 93.7%.
Unclear Clinical Need
Nelson noted that while this test aims to differentiate between arthritic diagnoses, patients can also have multiple conditions.
“Many individuals with rheumatoid arthritis will develop osteoarthritis, but they can have both, so a yes/no test is of unclear utility,” she said. OA and calcium pyrophosphate deposition (CPPD) disease can often occur together, “but the driver is really the OA, and the CPPD is present but not actively inflammatory,” she continued. “Septic arthritis should be readily distinguishable by cell count alone [and again, can coexist with any of the other conditions], and a thorough history and physical should be able to differentiate in most cases.”
While these results from this study are “reasonably impressive,” more clinical information is needed to interpret these results, added C. Kent Kwoh, MD, director of the University of Arizona Arthritis Center and professor of medicine and medical imaging at the University of Arizona College of Medicine, Tucson, Arizona.
Because the study is retrospective in nature and researchers obtained specimens from different sources, it was not clear if these patients were being treated when these samples were taken and if their various conditions were controlled or flaring.
“I would say this is a reasonable first step,” Kwoh said. “We would need prospective studies, more clinical characterization, and potentially longitudinal studies to understand when this test may be useful.”
This research was internally funded by Zimmer Biomet. All authors were employees of CD Diagnostics or CD Laboratories, both of which are subsidiaries of Zimmer Biomet. Kwoh reported receiving grants or contracts with AbbVie, Artiva, Eli Lilly and Company, Bristol Myers Squibb, Cumberland, Pfizer, GSK, and Galapagos, and consulting fees from TrialSpark/Formation Bio, Express Scripts, GSK, TLC BioSciences, and AposHealth. He participates on Data Safety Monitoring or Advisory Boards of Moebius Medical, Sun Pharma, Novartis, Xalud, and Kolon TissueGene. Nelson reported no relevant disclosures.