Association of work ability, sick leave, respiratory symptom score and spirometry with asthma and BMI categories and tests of interaction**
| Health-related outcome, effect estimate or units | Coefficients from regression models (95% CI)* | Coefficients for interaction terms in regression models† for all outcomes and Synergy Index for dichotomous outcomes | |||||
| Asthma, yes vs no | Elevated BMI vs normal weight | Interaction terms in regression models (95% CI)† | Synergy Index (95% CI)‡ | ||||
| Overweight | Obesity | Asthma×overweight | Asthma×obesity | Asthma×overweight | Asthma×obesity | ||
| Dichotomous outcomes | |||||||
| Work Ability Score ≤8, OR | 1.9 (1.4 to 2.5) | 1.2 (0.8 to 1.6) | 1.2 (0.8 to 1.7) | 1.76 (0.90 to 3.41) § | 1.08 (0.51 to 2.28) | 3.3 (0.35 to 32) | 1.26 (0.35–4.5) |
| Sick leave in the last 12 months, OR¶ | 1.4 (1.1 to 1.8) | 1.05 (0.8 to 1.4) | 1.3 (0.9 to 1.8) | 1.28 (0.71 to 2.30) | 1.01 (0.51 to 2.00) | 2.75 (0.06 to 112) | 1.17 (0.21 to 6.45) |
| Sick leave >14 days, OR¶ | 1.5 (0.99 to 2.2) | 0.9 (0.6 to 1.5) | 0.9 (0.6 to 1.6) | 1.35 (0.64 to 2.86) | 1.05 (0.45 to 2.47) | 2.50 (0.05 to 129) | 1.22 (0.13 to 11.4) |
| Symptom score ≥3, OR | 7.3 (5.5 to 9.7) | 0.9 (0.7 to 1.3) | 1.7 (1.2 to 2.4) | 1.05 (0.54 to 2.04) | 0.71 (0.35 to 1.44) | 0.94 (0.61 to 1.47) | 1.37 (0.87 to 2.17) |
| Continuous outcomes | |||||||
| Symptom score (0–9), score | 2.4 (2.2 to 2.7) | −0.03 (−0.3 to 0.3) | 0.6 (0.3 to 0.97) | −0.1 (−0.7 to 0.5) | 0.01 (−0.7 to 0.7) | ||
| Pre-FEV1 % of predicted | −6.6 (−8.2 to −5.1) | 1.7 (−0.2 to 3.5) | −2.9 (−5.1 to −0.7) | −1.7 (−5.1 to 1.7) | −2.2 (−6.2 to 1.8) | ||
| Pre-FVC, % of predicted | −2.3 (−3.6 to −0.96) | −0.08 (−1.6 to 1.5) | −5.2 (−7.0 to −3.4) | −3.6 (−6.6 to –0.6) | −2.4 (−6.0 to 1.2) | ||
| Pre-FEV1/FVC-ratio, % | −0.04 (−0.05 to − 0.03) | 0.01 (0.003 to 0.020) | 0.009 (−0.001 to 0.019) | 1.3 (−0.4 to 2.9) | −0.3 (−2.2 to 1.7) | ||
| Post-FEV1, % of predicted | −5.2 (−6.7 to −3.4) | 1.1 (−0.7 to 2.8) | −2.8 (−4.9 to −0.7) | −2.0 (−5.4 to 1.5) | 0.1 (−4.0 to 4.2) | ||
| Post-FVC, % of predicted | −1.0 (−2.5 to 0.4) | −0.05 (−1.7 to 1.6) | −4.2 (−6.1 to −2.3) | −3.1 (−6.3 to 0.05) § | 0.03 (−3.7 to 3.8) | ||
| Post-FEV1/FVC ratio, n % | −0.03 (−0.04 to − 0.03) | 0.008 (−0.001 to 0.02) | 0.009 (−0.001 to 0.02) | 0.9 (−0.8 to 2.5) | −0.1 (−2.0 to 1.9) | ||
Predilator spirometry: 661 acceptable tests among controls, 596 acceptable tests among cases.
Postdilator spirometry: 559 acceptable tests among controls, 532 acceptable tests among cases.
*A separate regression model was fit for each outcome and included covariates for asthma, BMI categories, age, sex, smoking and education.
†A separate regression model was fit for each outcome and included covariates for asthma, BMI categories, asthma×BMI interaction, age, sex, smoking and education. The coefficients for the interaction terms test for interaction on the multiplicative scale for dichotomous outcomes and on the additive scale for continuous outcomes.
‡Synergy Index is an indicator of an additive interaction for dichotomous outcomes and has a null value of 1.0.
§Results are borderline statistically significant, 0.05<p<0.10.
¶Results for sick leave are limited to the 1143 participants employed in the last 12 months.
**Statistically significant findings are given in bold. The operational definitions of asthma and BMI categories are described in the Methods section.
BMI, body mass index; FEV1, forced expiratory volume after 1 s; FEV, forced expiratory volume; FVC, forced vital capacity.