<h4>Introduction</h4>This study assesses the impact of functional iron deficiency (FID) on outcomes, including all-cause mortality, hospitalizations and non-fatal cardiovascular events in patients with non-dialysis chronic kidney disease (CKD) and hemodialysis (HD).<h4>Methods</h4>In HD, absolute iron deficiency (AID) was defined as ferritin < 200 μg/L and TSAT (transferrin saturation) ≤ 20%, and FID ferritin ≥200 μg/L with TSAT ≤20%. Here, TF is linked to Iron deficiency anemia.