Table 3 Threshold effect analysis of serum calcium level and the presence of RI using piece-wise linear regression

 one-line linear regression model

2.80 (1.4, 5.6) 0.004

 turning point

2.30 mmol/L

 Group1 < 2.3 correlation coefficient (β1)

0.10 (0.0, 2.3) 0.108

 Group2 > 2.3, correlation coefficient (β1)

6.20(1.9, 10.2) < 0.001

 predictive value of RI at turning point

−1.40 (−1.7, −1.1)

 a log likelihood ratio test

0.011