Characteristics of cases and controls. EIRA, Sweden, 1996–2014
| Cases (n=2641) N (%), mean±SD | Controls (n=4251) N (%), mean±SD | ||
| ACPA-positive RA 1756 (66.5%) | ACPA-negative RA 885 (33.5%) | ||
| Age at inclusion (years) | 50.9±13.0 | 52.0±13.5 | 51.4±13.4 |
| Age at menarche (years) | 13.2±1.4 | 13.2±1.4 | 13.1±1.5 |
| Parous | 1375 (78.3) | 718 (81.1) | 3376 (79.4) |
| Number of children | 2.2±1.2 | 2.2±0.8 | 2.2±0.9 |
| Age at first birth (years) | 24.8±4.9 | 24.5±4.9 | 25.6±5.0* |
| Age at menopause (years) | 49.6±5.6 | 49.8±5.3 | 50.0±5.4 |
| Oral contraceptive use† | |||
| Ever | 1135 (64.7) | 582 (65.8) | 2862 (67.4)‡ |
| Current | 134 (7.6) | 61 (6.9) | 331 (7.8) |
| Past | 1001 (57.1) | 521 (58.9) | 2531 (59.6)‡ |
| Never | 572 (32.6) | 289 (32.7) | 1267 (29.9) |
| Missing | 46 (2.6) | 13 (1.5) | 115 (2.7) |
| Breast feeding (months)§ | |||
| None | 1 (0.1) | 1 (0.3) | 14 (0.7) |
| 1–6 | 193 (28.7) | 80 (27.6) | 519 (24.7) |
| 7–12 | 192 (28.6) | 83 (28.6) | 574 (27.4) |
| ≥13 | 234 (34.8) | 100 (34.5) | 842 (40.1) |
| Missing | 52 (7.7) | 26 (9.0) | 148 (7.1) |
| Total duration of breast feeding (months) according to parity§ | |||
| One child | 6.4±5.6 | 4.8±2.7 | 6.9±5.4 |
| Two children | 11.7±8.2 | 11.7±8.1 | 12.4±8.3 |
| Three children or more | 22.5±18.1 | 19.5±12.9 | 20.7±13.9 |
| Ever use of PMH¶ | 117 (26.3) | 67 (29.7) | 412 (29.5) |
| BMI ≥25 kg/m2 | 749 (42.7) | 409 (46.2) | 1704 (40.1)‡ |
| University degree | 469 (26.7) | 251 (28.4) | 1425 (33.5)* |
| Ever smoker | 1175 (66.9) | 531 (60.0) | 2266 (53.3)* |
| Pack-years | |||
| Never smokers | 571 (32.5) | 348 (39.3) | 1943 (45.7) |
| 0–10 | 367 (20.9) | 185 (21.0) | 963 (22.7)‡ |
| 10–20 | 316 (18.0) | 132 (14.9) | 531 (12.5)* |
| ≥20 | 409 (23.3) | 149 (16.8) | 530 (12.5)* |
| Other | 82 (4.7) | 63 (7.1) | 243 (5.7)‡ |
| Missing | 11 (0.6) | 8 (0.9) | 41 (0.9) |
| Alcohol consumption | |||
| Non-drinkers | 213 (12.1) | 100 (11.3) | 330 (7.8)* |
| Low | 892 (50.9) | 418 (47.3) | 1991 (46.9) |
| Moderate | 408 (23.3) | 228 (25.8) | 1045 (24.6) |
| High | 235 (13.4) | 138 (15.6) | 864 (20.4)* |
| Missing | 5 (0.3) | 0 (0) | 14 (0.3) |
Baseline characteristics among participants who replied to the questionnaire, excluding cases lacking ACPA-status (35 cases).
Information on age at menarche and age at menopause available for 1211 cases/2596 controls and 757 cases/1548 controls, respectively.
A pack-year is defined as 20 cigarettes smoked every day for 1 year. The category ‘Other’ includes those smoking other tobacco than cigarettes (eg, cigarillos, cigars or pipe tobacco).
Alcohol consumption defined as number of drinks per week (one drink=12 g of alcohol) and categorised according to the quartile distribution among the controls. The two lowest categories (non-drinkers and low consumption) were merged for analyses.
*p Value <0.0001 for the difference between cases and controls.
†Oral contraceptive use after exclusion of four cases/seven controls who initiated use during the index-year. Ever oral contraceptive use is the sum of current and past use.
‡p Value <0.05 for the difference between cases and controls.
§Information on breastfeeding available for 884 cases and 1949 controls (all parous women) from 2006. Quartile distribution among controls, with the two highest categories merged into one.
¶Only among postmenopausal women.
ACPA, anticitrullinated protein antibodies; BMI, body mass index; EIRA, Epidemiological Investigation of Rheumatoid Arthritis; PMH, postmenopausal hormone therapy; RA, rheumatoid arthritis.