The aim of this study was to compare the diagnostic performance of serum creatinine, cystatin C and β2M for discriminating CKD stage 3 from stage 4 when classification was defined using the 2021 CKD-EPI creatinine (eGFRcr) and creatinine–cystatin C (eGFRcr–cys) equations in adults with CKD stages 3–4. Here, B2M is linked to chronic kidney disease.