Can normal serum iron coexist with depleted iron stores?

Ferritin and serum iron answer different clinical questions: ferritin primarily reflects iron reserves, whereas serum iron reflects iron currently circulating and available for transport. For a 42-year-old woman, a reassuring single serum iron result therefore does not by itself establish adequate iron stores.

Clinical evidence

• Ferritin is the preferred initial marker of storage iron. Against absent or reduced bone-marrow iron in adults receiving medical care, ferritin <30 μg/L had pooled sensitivity of 79% and specificity of 98%.
• A low ferritin strongly supports iron deficiency, while a single serum iron result is not considered adequate to diagnose or exclude depleted stores.
• Serum iron has substantial within-person biological variation (about 30%), compared with approximately 10–25% for ferritin. Recent food intake, collection timing, and other pre-analytic factors can materially affect an isolated result.
• Serum iron is most useful as part of transferrin saturation (TSAT), rather than as a stand-alone test.

Mechanistic and interpretive considerations

• Serum iron represents circulating iron availability, not stored iron. Thus, normal circulating iron at one time point can coexist with reduced iron reserves.
• Inflammation reduces serum iron, producing hypoferremia even when depleted stores are not the cause.
• Conversely, ferritin is an acute-phase protein: inflammation, infection, liver disease, and related states can elevate ferritin and obscure iron deficiency. In anemia of inflammation, ferritin <100 μg/L or TSAT <20% can support deficiency when interpreted with the overall clinical picture.

Clinical implications

• Ferritin should be interpreted alongside symptoms, blood count indices, inflammatory context, and, when indicated, TSAT and inflammatory markers rather than in isolation.

Bottom line

• The claim is supported: ferritin is substantially more reliable than a single serum iron measurement for assessing iron stores, and an “optimal” serum iron value does not rule out depleted iron stores.