KL and chronic kidney disease: Based on the fact that Klotho was negatively associated with WBC, Neu, NLR, PLR, PIV, SII, SIRI, TG, serum creatinine, serum urea nitrogen and uric acid, and positively associated with eGFR in the general and CKD populations (Additional file 1: Tables S2, S3), RCS was applied to examine the nonlinear relationship and threshold effect of Klotho with them.