Table 3.

Mediation analysis in 56 patients with rheumatoid arthritis from the Leiden Early Arthritis Clinic, with radiographic severity of joint damage as the outcome

Step 1: effect of age on possible mediators
	Effect (β)	95 % CI	p value
MRI inflammation	1.018	1.002–1.034	0.027
Synovitis	1.011	1.00–1.024	0.092
BME	1.021	1.00–1.043	0.052
Step 2: effect of age on radiographic joint damage
	Effect (β)	95 % CI	p value
Ageing	1.032	1.010–1.055	0.004
Step 3: effect of age and possible mediator on radiographic joint damage
	Effect (β)	95 % CI	p value
MRI inflammation	1.026	1.004–1.047	0.018
Ageing	1.025	1.004–1.047	0.021
Synovitis	1.069	0.97–1.17	0.15
Ageing	1.029	1.007–1.051	0.011
BME	1.039	1.011–1.067	0.007
Ageing	1.026	1.005–1.047	0.014

Step 1, 2 and 3 are explained in Fig. 1. In step 1 a linear regression is used, in step 2 and 3 a multivariate normal regression analysis is used [27]. The effects are per unit increase, for example per point increase in rheumatoid arthritis magnetic resonance imaging score (RAMRIS) and per year increase in age; for further explanation see legend of Table 2. MRI magnetic resonance imaging, BME bone marrow edema