Table 3

Multiple logistic regression of LBP taking into account LDD status effect

CoefficientSignificanceOR (95% CI)
Risk factor*EstimateStandard errorWald's χ2p ValueFor unitFor 90% vs 10%
Constant−4.1640.58450.903<0.0001
Regression coefficients
LSUM0.0610.01321.801<0.00011.063 (1.036 to 1.091)3.193 (1.958 to 5.232)
Weight0.0210.0086.7730.00911.021 (1.005 to 1. 037)1.869 (1.162 to 2.985)
Exercise0.4810.2085.3190.02101.617 (1.074 to 2.435)1.617 (1.074 to 2.435)
LBP(DZ_sib)0.7820.2539.5290.00202.186 (1.329 to 3.595)2.186 (1.329 to 3.595)
LBP(MZ_sib)1.7570.31730.777<0.00015.794 (3.112 to 10.789)5.794 (3.112 to 10.789)
• * All variables showing significant association with LBP_1 in the preliminary analysis (table 1) were included into a multiple logistic regression analysis. However, only variables making independent significant contributions are presented in the table. To account for contribution of the affection status of siblings in the logistic regression model, including twins of different zygosity, the following definitions were introduced: LBP(DZ_sib)=1 if an individual has affected dizygotic sibling, otherwise 0; LBP(MZ_sib)=1 if an individual has affected monozygotic sibling, otherwise 0.

• ‘90% vs 10%’ means that OR was estimated contrasting the variable upper vs lower 10 percentiles of the distribution. The corresponding results of univariate analysis of the LSUM, weight and affected twin zygosity effect on individual probability to have LBP1 are shown in the supplementary table. There were 49% LBP1 individuals in the fourth quartile of LSUM distribution, compared with only 6.9% in the first one. Comparison of the first affected monozygotic vs dizygotic twin effect on the probability that the second twin will have LBP1, were statistically reliably significant (χ2=10.99, df=1, p=0.0009).

• DZ, dizygotic; LBP, low back pain; LDD, lumbar disc degeneration; LSUM, summary rank for four LDD traits; MZ, monozygotic.