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Linear association between age and serum concentrations of YKL-40. Linear model applied for patients with rheumatoid arthritis (RA) ≤ 65 years of age (n = 238) and controls (n = 605) is sufficient to explain the age dependent variation (P = 0.96). The y-axis represents serum concentrations of YKL-40. Dotted lines represent 95% confidence intervals.
