Utility of Repeated C-Reactive Protein Testing Nonradiographic Axial Spondyloarthritis
Patients with nonradiographic spondyloarthritis with normal CRP levels have a significant chance of demonstrating elevated CRP levels on subsequent tests.
In patients with nonradiographic axial spondyloarthritis (axSpA) with no signs of inflammation on MRI and normal C-reactive protein (CRP) levels, as well as high disease activity indicative of potential benefits from tumor necrosis factor inhibitor (TNFi) therapy, CRP testing should be repeated after ≥4 weeks, according to data published in Arthritis Research & Therapy.
The investigators sought to examine the likelihood of a CRP test indicating elevated CRP levels in patients with nonradiographic axSpA who have had a prior normal CRP test. They conducted a post-hoc analysis of data from the phase 3, double-blind, randomized, placebo-controlled RAPID-axSpA study (ClinicalTrials.gov identifier: NCT01087762). The study enrolled patients who were MRI-positive for axSpA or had elevated CRP levels (more than the upper limit of normal, 7.9 mg/L). At 1 week, patients were randomly assigned in a 1:1:1 ratio to placebo, certolizumab pegol (CZP) 400 mg every 4 weeks, or CZP 200 mg every other week. Patients randomly assigned to placebo who did not meet Assessment of Spondyloarthritis International Society 20% response criteria (ASAS20) at either 14 or 16 weeks received either CZP 200 mg every other week or CZP 400 mg every 4 weeks. Moreover, participants who received placebo who did not attain an ASAS20 response were randomly assigned to CZP at 24 weeks.
Participant CRP levels were assessed at baseline and at 9 time points until 24 weeks. Among 106 patients who received placebo with baseline CRP measurements, 25% had normal CRP levels at baseline, of whom 50% had ≥1 test revealing an elevated CRP level to 16 weeks. Among 75% of patients with elevated baseline CRP levels, 31% had ≥1 normal CRP test result to 16 weeks. Based on linear mixed models, no changes in mean CRP levels were revealed in those receiving placebo from baseline to 24 weeks.
The investigators concluded that in patients with nonradiographic axSpA who have CRP levels less than upper limit of normal, the CRP test should be repeated after ≥4 weeks, as there is a considerable possibility that subsequent testing will reveal elevated CRP levels, which would allow a patient to access treatment options, including TNFi therapies.
Landewé R, Nurminen T, Davies O, Baeten D. A single determination of C-reactive protein does not suffice to declare a patient with a diagnosis of axial spondyloarthritis ‘CRP-negative.' Arthritis Res Ther. 2018;20(1):209.