Soluble transferrin receptor (sTfR) level is a marker of iron deficiency and reflects degree of availability of iron for cells. Evaluation of suspected iron deficiency in patients who may have inflammation, infection, or chronic disease and other conditions in which ferritin concentration does not correlate with iron status, including: Cystic fibrosis patients who frequently have inflammation or infections. Evaluating insulin-dependent diabetics who may have iron-deficiency resulting from gastric autoimmunity and atrophic gastritis.