Iron Studies

Iron studies include four main components, each measuring a different aspect of iron metabolism. Serum iron measures the amount of iron bound to transferrin in the blood — it is highly variable, affected by recent meals, diurnal variation (higher in the morning), and acute illness. It should never be interpreted alone.

TIBC (total iron-binding capacity) measures the total capacity of transferrin to bind iron — it reflects the amount of transferrin available. In iron deficiency, the liver upregulates transferrin production, raising TIBC. In anemia of chronic disease, inflammatory cytokines suppress transferrin production, lowering TIBC. This directional difference is one of the key distinguishing features between the two conditions.

Transferrin saturation is calculated as (serum iron / TIBC) × 100. It represents the percentage of iron-binding capacity that is currently occupied. Normal is approximately 20–50%. Below 16–20% suggests inadequate iron delivery to the bone marrow — either from true iron deficiency or from functional iron deficiency in anemia of chronic disease.

Ferritin is a storage protein that reflects total body iron stores. It is the most clinically useful single iron study — but it is also an acute-phase reactant. Ferritin rises with inflammation, infection, liver disease, and malignancy, independent of iron stores. This means a normal or elevated ferritin does not exclude iron deficiency in a patient with concurrent inflammation.

The classic iron deficiency pattern is: low serum iron, high TIBC, low transferrin saturation (below 16–20%), and low ferritin (below 30 ng/mL). The high TIBC reflects the liver's upregulation of transferrin in response to iron deficiency — the body is trying to capture more iron from the circulation. The low ferritin is the most specific finding for iron deficiency when it is unambiguously low.

Ferritin below 30 ng/mL is highly specific for iron deficiency — the body does not deplete storage iron until stores are genuinely low. However, sensitivity is limited because ferritin rises with inflammation. A ferritin of 30–100 ng/mL is indeterminate in the setting of chronic disease — it may represent iron deficiency masked by the acute-phase response.

Iron deficiency progresses through three stages before anemia develops: iron depletion (low ferritin, normal hemoglobin and MCV), iron-deficient erythropoiesis (low ferritin, low transferrin saturation, normal hemoglobin, possibly elevated RDW), and iron deficiency anemia (low ferritin, low transferrin saturation, low hemoglobin, microcytosis). Symptoms — fatigue, reduced exercise tolerance, restless legs, pica — can occur in the pre-anemia stages.

Once iron deficiency is confirmed, the cause must be identified. In premenopausal women, menstrual losses are the most common cause. In men and postmenopausal women, GI blood loss must be excluded — iron deficiency in these populations should prompt GI evaluation unless a clear alternative cause is identified.

Anemia of chronic disease (ACD, also called anemia of inflammation) is caused by inflammatory cytokines — particularly IL-6, IL-1, and TNF-alpha — that suppress erythropoiesis, reduce iron availability through upregulation of hepcidin (which blocks intestinal iron absorption and iron release from macrophages), and blunt the EPO response.

The ACD pattern is: low serum iron, low or normal TIBC, low transferrin saturation, and normal or elevated ferritin. The low TIBC distinguishes ACD from iron deficiency (where TIBC is high). The normal or elevated ferritin reflects the acute-phase response — ferritin is elevated by the same inflammatory cytokines that cause ACD.

The challenge arises when both conditions coexist — iron deficiency superimposed on ACD. In this setting, ferritin may be normal or only mildly elevated despite true iron deficiency, and TIBC may be normal rather than elevated. Transferrin saturation below 20% and a low serum iron are the most reliable indicators of functional iron deficiency in this setting.

Soluble transferrin receptor (sTfR) is elevated in iron deficiency (reflecting increased transferrin receptor expression on iron-starved erythroid precursors) but is not elevated in ACD. The sTfR/log ferritin ratio can help distinguish the two conditions in ambiguous cases, though it is not universally available.

Elevated ferritin has a broad differential that extends well beyond iron overload. The most common causes of elevated ferritin are inflammation, infection, liver disease (ferritin is synthesized in the liver and released with hepatocellular injury), malignancy, and metabolic syndrome. True iron overload — hereditary hemochromatosis, transfusional iron overload — is a less common cause of elevated ferritin.

Hereditary hemochromatosis (most commonly HFE gene mutations, particularly C282Y homozygosity) causes progressive iron accumulation in the liver, heart, pancreas, joints, and skin. The classic presentation is liver disease, diabetes, cardiomyopathy, arthropathy, and skin bronzing — but most patients are now identified at an earlier, asymptomatic stage through elevated ferritin and transferrin saturation on routine labs.

The key distinguishing feature of iron overload is an elevated transferrin saturation — typically above 45–50% in hereditary hemochromatosis. In inflammatory states, ferritin is elevated but transferrin saturation is normal or low (because hepcidin blocks iron release from macrophages). An elevated ferritin with an elevated transferrin saturation should prompt evaluation for hemochromatosis with HFE genotyping.

Oral iron is the first-line treatment for iron deficiency in most patients. Ferrous sulfate, ferrous gluconate, and ferrous fumarate are the most commonly used forms. The recommended dose is 150–200 mg of elemental iron per day in divided doses. Absorption is enhanced by vitamin C and impaired by food, calcium, antacids, and PPIs.

GI side effects — nausea, constipation, abdominal cramping — are common with oral iron and are the most frequent reason for non-adherence. Alternate-day dosing (every other day rather than daily) has been shown in some studies to improve absorption and reduce side effects by allowing hepcidin levels to fall between doses.

IV iron is indicated when oral iron is not tolerated, not absorbed (malabsorption syndromes, gastric bypass), or insufficient to meet the demand (active bleeding, pre-operative optimization, CKD on erythropoiesis-stimulating agents). Multiple IV iron formulations are available — ferric carboxymaltose, ferumoxytol, iron sucrose, and low-molecular-weight iron dextran — with different dosing schedules and infusion times. Anaphylaxis risk is low with modern formulations but premedication and monitoring protocols vary by institution.

Response to iron replacement should be assessed at 4–6 weeks: reticulocytosis typically occurs within 1–2 weeks, hemoglobin begins to rise within 2–4 weeks, and full correction of anemia takes 2–3 months. Iron stores (ferritin) take longer to replenish — continue iron supplementation for 3–6 months after hemoglobin normalizes to replenish stores.