It is estimated anemia affects over 30% of the world's population, with iron deficiency (ID) the overwhelmingly most common cause. Whether absolute due to blood loss and/or iron sequestration to underlying morbidity, the need for repletion especially in females, is a formidable medical issue. The diagnosis of iron deficient erythropoiesis has been traditionally based on the biochemical parameters ferritin and percent transferrin saturation (TSAT), mean cell volume and hemoglobin (Hb) concentration. In recent years, reticulocyte Hb content has emerged as a parameter helpful in identifying iron deficient erythropoiesis and informing a need, or lack thereof, for replacement. 556 consecutive, non-selected patients referred for diagnosis and/or treatment of anemia were included in this diagnostic study to compare the performance of reticulocyte hemoglobin equivalent (RET-He) versus traditional biochemical markers for diagnosis and treatment of IDA. CBC, serum ferritin, iron and TSAT were performed as clinically indicated. RET-He was measured with a Sysmex XN-450 analyzer on the residual CBC sample. 556 patients were studied at baseline and 150 were subsequently treated with intravenous (IV) iron. 240/556 were seen at follow-up, with 57 treated and 183 not treated with IV iron. At baseline, ret-He, positively correlated with Hb (Spearman correlation (rho)=0.365, P < 0.001), MCV (rho=0.576, P < 0.001), MCH (rho=0.777, P < 0.001), serum iron (rho=0.526, P < 0.001) and TSAT (rho=0.492, P < 0.001). Serum iron, and TSAT (but not serum ferritin or MCV, or absolute reticulocyte count) positively correlated with Hb concentrations. Based on either a serum ferritin 1.0 g/dl, with sensitivity of 84% and specificity of 78%. The Figure presents data for the 21/57 patients who had RET-He < 28.5 pg and Hb < 10.3 g/dL vs the 36/57 who did not. The present data show that an abnormally low ret-He value (< 28.5 pg) identifies patients who will respond to iron replacement, obviating delays to obtain standard iron parameters. Baseline and changes in ret-He also associate with Hb response. Given the enormous prevalence of ID in the general population the use of the ret-He, which is available with the CBC on the autoanalyzer, informs need for iron replacement, or lack thereof, represents an increase in convenience for patient and physician, decreases costs, streamlines care and represents an improvement in the treatment paradigm of one of the commonest maladies on the planet.