For a patient with ambiguous iron study values, which test can best determine the presence of iron deficiency?

The soluble transferrin receptor (sTfR) is a valuable test for identifying iron deficiency, particularly in cases where iron study values yield ambiguous results. This protein is a marker for tissue iron deficiency, as it reflects the demand for iron in erythropoiesis. When iron stores are low, more transferrin receptors are produced to capture available iron, resulting in increased levels of sTfR in the circulation.

This test is particularly useful because it is not influenced by the acute phase response, making it reliable even when inflammatory conditions may complicate iron studies. It provides insight into the actual functional iron status of an individual.

In contrast, zinc protoporphyrin levels may rise in conditions of iron deficiency but can also be elevated in other disorders such as lead poisoning, thus lacking specificity. The assessment of peripheral blood sideroblasts is not typically employed to diagnose iron deficiency; rather, it is used in the context of conditions like sideroblastic anemia. Mean cell hemoglobin, while providing some information related to the adequacy of hemoglobin production, does not directly assess iron stores or iron availability in the same informative way as soluble transferrin receptor levels do. Therefore, the measurement of sTfR is optimal for pinpointing iron