Table 3.

Threshold effect analysis of systemic immune-inflammation index and serum ferritin stratified by age (years).

Age (years)	<40	≥40
Model 1, β (95% CI)
Linear effect model	−0.01 (−0.02, 0.01) 0.4806	−0.05 (−0.07, −0.03) <0.0001
Model 2, β (95% CI)
Inflection point (K)	881.06	874.59
< K	0.01 (−0.01, 0.03) 0.4956	−0.07 (−0.10, −0.05) <0.0001
> K	−0.04 (−0.09, 0.00) 0.0658	0.04 (−0.02, 0.10) 0.1861
LLR	0.086	0.002

Model 1: Linear effects model; Model 2: Non-linear effects model.

In the subgroup analysis stratified by each covariate, the model is not adjusted for the stratification variable itself.