ORs of developing RA overall and ACPA-positive/ACPA-negative RA according to duration of oral contraceptive use. EIRA, Sweden, 1996–2014
| ACPA-status | Duration of OC use* | Ever OC use | Current OC use | Past OC use | |||
| Ca/Co | OR (95% CI)† | Ca/Co | OR (95% CI)† | Ca/Co | OR (95% CI)† | ||
| RA overall | Never | 852/1245 | 1.0 | 852/1245 | 1.0 | 852/1245 | 1.0 |
| ≤7 years | 865/1348 | 0.94 (0.83 to 1.07) | 59/85 | 1.16 (0.77 to 1.76) | 806/1263 | 0.93 (0.82 to 1.06) | |
| >7 years | 835/1481 | 0.81 (0.71 to 0.92) | 134/242 | 0.99 (0.74 to 1.33) | 701/1239 | 0.81 (0.71 to 0.93) | |
| p-trend | 0.0014 | 0.9982 | 0.0021 | ||||
| ACPA-positive | Never | 565/1245 | 1.0 | 565/1245 | 1.0 | 565/1245 | 1.0 |
| ≤7 years | 556/1348 | 0.89 (0.76 to 1.03) | 39/85 | 1.18 (0.73 to 1.90) | 517/1263 | 0.88 (0.75 to 1.02) | |
| >7 years | 570/1481 | 0.80 (0.69 to 0.93) | 95/242 | 0.95 (0.69 to 1.32) | 475/1239 | 0.80 (0.68 to 0.93) | |
| p-trend | 0.0037 | 0.8011 | 0.0039 | ||||
| ACPA-negative | Never | 287/1245 | 1.0 | 287/1245 | 1.0 | 287/1245 | 1.0 |
| ≤7 years | 309/1348 | 1.04 (0.86 to 1.25) | 20/85 | 1.15 (0.61 to 2.18) | 289/1263 | 1.04 (0.86 to 1.25) | |
| >7 years | 265/1481 | 0.82 (0.67 to 0.99) | 39/242 | 1.09 (0.68 to 1.74) | 226/1239 | 0.83 (0.67 to 1.01) | |
| p-trend | 0.0356 | 0.7056 | 0.0636 | ||||
26 cases and 55 controls lacked information on duration of oral contraceptive use.
↵*Duration of OC use categorised according to median value among controls.
†Adjusted for age, residential area, smoking (pack-years) and alcohol consumption (low (including non-drinkers), medium and high).
ACPA, anticitrullinated protein antibodies; Ca/Co, number of cases/controls; EIRA, Epidemiological Investigation of Rheumatoid Arthritis; OC, oral contraceptives; RA, rheumatoid arthritis.