Which iron studies (CBC with red‑cell indices, serum ferritin, serum iron, total iron‑binding capacity, transferrin saturation, and soluble transferrin receptor) should be ordered when iron‑deficiency anemia is suspected, and what is the recommended treatment regimen?

Iron Profile in Iron Deficiency Anemia

Essential Laboratory Panel

Order a complete blood count with red‑cell indices (MCV, MCH, RDW), serum ferritin, transferrin saturation (TSAT), and C‑reactive protein (CRP) as the initial diagnostic panel for suspected iron‑deficiency anemia. 1

• The CBC with indices provides hemoglobin, MCV, MCH, and RDW—all critical for confirming anemia severity and distinguishing iron deficiency from other microcytic conditions 1
• Serum ferritin is the single most powerful test for diagnosing iron deficiency, with a threshold < 30 µg/L confirming depleted iron stores in patients without inflammation 1, 2
• A ferritin cut‑off of 45 µg/L provides optimal sensitivity and specificity for routine clinical practice 1, 2
• Transferrin saturation < 20 % is the primary confirmatory marker for iron deficiency, especially when ferritin lies between 30–100 µg/L or may be falsely elevated by inflammation 1
• CRP (or ESR) must be measured concurrently because ferritin is an acute‑phase reactant that can be falsely elevated by inflammation, infection, malignancy, or liver disease 1
• A reticulocyte count distinguishes inadequate marrow response (low/normal in iron deficiency) from hemolysis (elevated) 1

Interpretation of Ferritin Thresholds

Without Inflammation (Normal CRP)

• Ferritin < 15 µg/L provides 99 % specificity for absolute iron deficiency, confirming the diagnosis without further testing 1
• Ferritin < 30 µg/L confirms depleted iron stores consistent with iron‑deficiency anemia 1
• Ferritin > 100 µg/L essentially rules out iron deficiency when inflammation is absent 1

With Inflammation (Elevated CRP/ESR)

• Ferritin 30–100 µg/L together with elevated CRP/ESR suggests a mixed picture of true iron deficiency plus anemia of chronic disease; confirmation requires TSAT < 20 % 1
• Ferritin > 100 µg/L combined with TSAT < 20 % and elevated CRP defines anemia of chronic disease with functional iron deficiency, not true iron deficiency 1
• In inflammatory states, ferritin values up to 100 µg/L may still represent iron deficiency 1
• Ferritin > 150 µg/L makes absolute iron deficiency unlikely even in the presence of inflammation 1

Red Blood Cell Indices

• Microcytosis (MCV below the reference range) is typical of iron deficiency anemia but may be absent when multiple deficiencies coexist 1
• MCH is more reliable than MCV as a marker of iron deficiency because it is less dependent on storage conditions and is reduced in both absolute and functional iron deficiency 2
• An elevated RDW (> 14 %) signals iron deficiency and can uncover concurrent micro‑ and macro‑cytosis that mask MCV changes 1
• A low MCV combined with RDW > 14 % strongly suggests iron deficiency anemia, while RDW ≤ 14 % suggests thalassemia minor 2

Extended Testing When Initial Results Are Inconclusive

• Soluble transferrin receptor (sTfR) measurement is recommended when ferritin and TSAT give conflicting information; an elevated sTfR confirms true iron deficiency because it is not affected by inflammation 1, 3
• Percentage of hypochromic red cells and reticulocyte hemoglobin content provide more precise iron‑status evaluation when standard tests are borderline 1, 4
• Vitamin B12 and folate levels should be obtained to exclude megaloblastic anemia, which can coexist with iron deficiency and produce a mixed micro‑ and macrocytic picture with elevated RDW 1
• When reticulocytes are elevated, order haptoglobin, lactate dehydrogenase, and bilirubin to evaluate for hemolysis, which excludes pure iron‑deficiency states 1

Treatment Regimen

Oral Iron Therapy

First‑line treatment is oral iron supplementation with ferrous sulfate 200 mg three times daily (providing 100–200 mg elemental iron daily) for at least three months after correction of anemia to replenish iron stores. 2

• Alternative formulations include ferrous gluconate and ferrous fumarate if ferrous sulfate is not tolerated 2
• Ascorbic acid can be added to enhance iron absorption 2
• A hemoglobin rise of ≥ 10 g/L within 2 weeks confirms iron deficiency even when initial iron studies are equivocal 1, 2
• Therapy should continue until ferritin exceeds 50 µg/L to fully replenish stores and prevent recurrence 1

Intravenous Iron Therapy

• Switch to intravenous iron (iron sucrose, ferric carboxymaltose, or iron gluconate) if oral iron does not raise hemoglobin by ≥ 2 g/dL within four weeks, if malabsorption is documented, or if gastrointestinal side effects prevent adherence 1, 2
• Serum ferritin should be monitored during IV therapy and kept below 500 µg/L to avoid iron overload, especially in children and adolescents 1, 2
• In iron‑refractory iron‑deficiency anemia (IRIDA) caused by TMPRSS6 mutations, oral iron is ineffective and intravenous iron is required 1, 2

Monitoring and Follow‑up

• Monitor hemoglobin concentration and red cell indices at three‑monthly intervals for one year and then after a further year 2
• Provide additional oral iron if hemoglobin or MCV falls below normal 2
• Check serum ferritin and transferrin saturation to assess iron stores after treatment 2

Critical Pitfalls to Avoid

• Do not rely on ferritin alone when inflammation is present; always calculate TSAT because functional iron deficiency can coexist with high ferritin but low TSAT 1
• Do not overlook combined deficiencies; iron deficiency can coexist with vitamin B12 or folate deficiency, recognizable by an elevated RDW 1
• Do not discontinue iron supplementation once hemoglobin normalizes; iron stores must be restored (target ferritin > 50 µg/L) to prevent rapid recurrence 1
• Occult gastrointestinal blood loss must be excluded before attributing iron deficiency to dietary insufficiency in adults 1
• In adults (men with hemoglobin < 110 g/L; non‑menstruating women with hemoglobin < 100 g/L), expedited bidirectional endoscopy is required to evaluate for gastrointestinal malignancy 1

Special Population Considerations

• In patients with inflammatory bowel disease (IBD) in remission, ferritin < 30 µg/L reliably indicates iron deficiency 1
• During active IBD inflammation, ferritin < 100 µg/L is used as a screening threshold; iron deficiency is confirmed with TSAT < 20 % 1
• In chronic kidney disease, chronic heart failure, and inflammatory bowel disease, a ferritin threshold < 100 µg/L (instead of < 30 µg/L) is recommended for screening iron deficiency 1