Blood levels of matrix metalloproteinase generated C-reactive protein metabolite (CRPM) are a sensitive measure of chronic inflammation in OA

BackgroundThe heterogeneous nature of osteoarthritis (OA) and the need to subtype patients is widely accepted. The biomarker CRPM, a metabolite of C-reactive protein (CRP), is released to the circulation during inflammation. Blood CRPM levels have shown to be associated with disease activity and response to treatment in rheumatoid arthritis (RA). We hypothesized that circulating levels of CRPMs could be used to identify OA patients with an inflammatory phenotype. We investigated the level and the prognostic effect of CRPM using combined data from two phase III OA and two RA studies (N = 1591). The association between serum CRPM levels and radiographic progression was investigated in knees of OA patients without radiographic OA in contra-lateral knee at baseline by dividing the patients into cases (knees with two-year radiographic progression) and controls (knees without radiographic progression). ResultsThe mean CRPM levels were significantly lower in OA (8.5 [95% CI 8.3-8.8] ng/mL) compared to the RA patients (15.6 [9.5-21.6] ng/mL); however, a significant subset of OA patients (31%) had CRPM levels ≥ 9ng/mL, as 75% of patients with early RA. Furthermore, OA patients with CRPM levels ≥ 9ng/mL were more likely to progress on X-ray over a two-year follow-up period; CRPM was prognostic for contralateral incidence knee OA with an odds ratio of 2.2 [1.0 - 4.7]. ConclusionA subset of OA patients, approximately 30%, appear to have tissue inflammation comparable to that of RA patients, reflected by the level of CRPM. Furthermore, high CRPM levels were prognostic of incident knee OA. These data suggest that CRPM is a blood-based biochemical marker for early identification OA patients with an inflammatory phenotype.