This study was undertaken to evaluate the role of serum transferrin receptor measurements in the assessment of iron status. Repeated phlebotomies were performed in 14 normal volunteer subjects to obtain varying degrees of iron deficiency. Serial measurements of serum iron, total iron-binding capacity, mean cell volume (MCV), free erythrocyte protoporphyrin (FEP), red cell mean index, serum ferritin, and serum transferrin receptor were performed throughout the phlebotomy program. There was no change in receptor levels during the phase of storage iron depletion. When the serum ferritin level reached subnormal values there was an increase in serum receptor levels, which continued throughout the phlebotomy program. Functional iron deficiency was defined as a reduction in body iron beyond the point of depleted iron stores. The serum receptor level was a more sensitive and reliable guide to the degree of functional iron deficiency than either the FEP or MCV. Our studies indicate that the serum receptor measurement is of particular value in identifying mild iron deficiency of recent onset. The iron status of a population can be fully assessed by using serum ferritin as a measure of iron stores, serum receptor as a measure of mild tissue iron deficiency, and hemoglobin concentration as a measure of advanced iron deficiency.

A specialized serum ferritin assay has been developed for the detection of iron deficiency in epidemiologic studies. An enzyme immunoassay (EIA) was employed to eliminate the need for radioisotopes. The problem of low sensitivity inherent with the EIA for serum ferritin was eliminated by the use of monoclonal immunologic reagents. The working range of the assay is 1-100 micrograms/L with a sensitivity of 0.5 micrograms/L. Excellent agreement in serum ferritin levels was observed between the present method and the two-site immunoradiometric assay (IRMA), while the variability at low ferritin concentrations was significantly less with the EIA. Because only 10 microliter of serum is required for each assay, duplicate measurements can be performed on a single capillary tube of blood. When an automatic microtiter plate reader for optical density measurements is used, 80-100 duplicate determinations can be completed by one technologist in a single working day.