Normal C-reactive protein in active psoriatic arthritis: results from real-world clinical practice.

Background: The value of normal C-reactive protein (CRP) in psoriatic arthritis (PsA) is debatable.

Objectives: To test the hypothesis that CRP is frequently normal in contemporary real-world PsA patients, despite active disease.

Design: In this cross-sectional study, patients were divided into two groups: CRP ⩽ 0.5 mg/dl (normal) and CRP > 0.5 mg/dl (increased). Having as dependent variable the CRP status, these groups were compared for disease-related features, including composite disease activity indices [clinical Disease Activity in PSoriatic Arthritis (cDAPSA) and minimal disease activity (MDA)] and patient-reported outcomes (PROs). Agreement between CRP status and cDAPSA/MDA scores was calculated (Cohen's kappa).

Methods: Data from consecutive PsA patients attending two outpatient rheumatology clinics (January 2019-June 2021) were analysed.

Results: From 128 patients enrolled (51.6% females; mean ± standard deviation age: 53.4 ± 11.7 years; 23.4%, 48.4% and 64.1% treated with glucocorticoids, conventional synthetic disease-modifying anti-rheumatic drugs (csDMARDs) and biologic DMARDs, respectively), two-thirds (66.4%,  = 85) had normal CRP values. CRP status was not associated with any of the disease-related parameters and PROs, but only with ESR [odds ratio: 1.04 (95% confidence interval: 1.01-1.06),  = 0.005]. Among patients with normal CRP, 45.9% (39/85) were on non-MDA state, while 21.2% (18/85) had cDAPSA-moderate and 5.9% (5/85) had cDAPSA-high disease activities. Conversely, 54.2% (39/72) of patients on non-MDA state and 52.3% (23/44) of those with cDAPSA-moderate or cDAPSA-high disease activity had normal CRP values. Cohen's kappa between normal CRP and MDA, cDAPSA-remission, and cDAPSA-remission/low disease activity was -0.26, -0.21 and -0.22, respectively, displaying total disagreement.

Conclusion: Normal CRP in PsA should not be used as surrogate marker of remission or low/MDA, therefore needs to be interpreted with caution in clinical decision-making.