Table 1.

Characteristics of study sample

Variable	RA cases (n = 520)	RA-free OA controls (n = 303)	P
Gender, % female (n)	83.3 (433)	80.2 (243)	ns
Age at inclusion, years (range)	51.6 ± 11.2 (19 to 80)	57.9 ± 13.5 (21 to 83)	< 0.0001
Age of onset, years	40.8 ± 12.7	50.7 ± 12.8	< 0.0001
Duration of disease, years	10.8 ± 8.3	7.2 ± 6.8	< 0.0001
RF, IU/ml	149.8 ± 67.2	-	-
RF-positive, % (n)	53.8 (280)	-	-
anti-CCP antibody, RU/mla	67.5 ± 53.7	1.5 ± 6.4	< 0.0001
anti-CCP positive, % (n)b	78.6 (239)	3.2 (4)	< 0.0001
CRP, μg/ml	19.6 ± 23.7	5.1 ± 9.2	< 0.0001

Values denote means ± standard deviations unless indicated otherwise.

CCP = cyclic citrullinated peptide; CRP = C-reactive protein; ns = not significant; RF = rheumatoid factor.

^a anti-CCP antibody serum level was determined in 428 individuals.

^b Values below 4.2 RU/ml were considered as anti-CCP negative.