Pain-inflammation discordance in rheumatoid arthritis: fibromyalgia, psychological, and functional correlates of a continuous metric.

BACKGROUND AND OBJECTIVE: Pain-inflammation discordance (PID) - a state in which perceived pain substantially exceeds the level expected from objective inflammatory markers - is clinically important in rheumatoid arthritis (RA) but has been studied predominantly using arbitrary categorical thresholds. We developed a continuous, sample-relative z-score-based PID metric, evaluated preliminary construct validity evidence, and identified associated psychological, functional, and disease-related correlates in a cohort of 200 Egyptian RA patients (mean age 38.5 ± 9.2 years; 91% female; 40% with comorbid fibromyalgia; 88% with moderate-to-high disease activity). The metric is derived from within-sample z-standardization and should be regarded as a research tool rather than a directly portable clinical instrument. METHODS: Two hundred consecutive RA patients (2010 ACR/EULAR criteria) were cross-sectionally assessed. The PID score was computed as the difference between z-standardized pain VAS and z-standardized ESR, both standardized within this sample (VAS pain: mean 50.3 ± 26.2 mm; ESR: mean 46.0 ± 24.4 mm/h). Preliminary construct validity was evaluated through known-groups comparisons (fibromyalgia status), discriminant validity testing (tender vs. swollen joint counts), and distributional normality. Fibromyalgia was assessed using the 1990 ACR criteria in a single-examiner setting, with acknowledged limitations of tender-point overlap with active synovitis. Psychological instruments included BDI-II, HAM-A, PSQI, and MAF (Cronbach's α 0.79-0.91) in validated Arabic translations. Benjamini-Hochberg FDR correction was applied across 12 empirical correlational tests; the mathematically predetermined ESR-PID correlation was excluded from the FDR pool. RESULTS: The PID score was normally distributed (SD = 1.29; range - 4.20 to + 3.08; Shapiro-Wilk p = 0.41). Known-groups comparison showed higher PID in fibromyalgia-positive versus fibromyalgia-negative patients (0.65 ± 1.15 vs. - 0.44 ± 1.19; Cohen's d = 0.93), though this comparison carries inherent circularity. The strongest validity evidence was the discriminant TJC/SJC dissociation: TJC correlated with PID (r = 0.233, p_FDR = 0.004) while SJC did not survive FDR correction (r = 0.149, p_FDR = 0.059). After FDR correction, significant correlates included fibromyalgia (r = 0.415), TJC (r = 0.233), sleep quality (r = 0.215), morning stiffness (r = 0.189), depression (r = 0.174), and anxiety (r = 0.174). Fibromyalgia explained 17.3% of PID variance; 82.7% remained unexplained. DAS28 showed no association with PID (r =  - 0.005, p = 0.949), a finding that is partly explained by the opposing mathematical relationships of DAS28 components with PID. Fibromyalgia's association with PID was consistent across moderate and high disease activity strata; evidence within the remission (n = 7) and low-activity (n = 17) strata is underpowered and should not be used to draw inferential conclusions. CONCLUSIONS: This cross-sectional study provides preliminary construct validity evidence for a continuous PID metric, most compellingly through the discriminant TJC/SJC dissociation. Fibromyalgia was the strongest correlate, explaining 17.3% of PID variance, with the majority of variance remaining unexplained. These associative findings support screening for fibromyalgia and psychological comorbidities in RA patients with pain disproportionate to inflammatory markers, though the cross-sectional design precludes causal inference or direct therapeutic recommendations. External validation, test-retest reliability studies, and longitudinal designs are required before clinical application. Key Points • Continuous PID score quantifies pain-inflammation discordance in RA. • Higher PID is significantly associated with depressive symptoms. • Discordant pain is primarily associated with tender rather than swollen joint counts and poor sleep quality. • Psych assessment is vital when pain exceeds objective inflammation.