Table 4.

Threshold effect analysis of serum cotinine levels on LSM using two-piecewise linear regression model.

Log2-transformed cotinine	LSM adjusted β (95% CI) p value
Fitting by the standard linear model	0.05 (0.03, 0.08) < 0.001
Fitting by the two-piecewise linear model
Inflection point	4.53
< K segment effect	0.02 (− 0.02, 0.06) 0.268
> K segment effect	0.29 (0.10, 0.47) 0.002
Log likelihood ratio	0.012