What iron studies (serum ferritin, serum iron, total iron‑binding capacity, transferrin saturation) and management steps are recommended for a patient with suspected iron‑deficiency anemia?

Iron Deficiency Anemia: Iron Profile and Management

Serum ferritin is the single most powerful test for diagnosing iron deficiency, with a level <12 μg/dL (or <30 ng/mL in modern units) confirming absolute iron deficiency in patients without inflammation. 1

Essential Iron Studies to Order

When evaluating suspected iron deficiency anemia, obtain the following panel:

• Serum ferritin – the most powerful diagnostic test for iron deficiency 1
• Transferrin saturation (TSAT) – calculated as (serum iron ÷ TIBC) × 100, with <16% confirming iron deficiency in adults without inflammation 2, 3
• Total iron-binding capacity (TIBC) – elevated when iron stores are depleted, reflecting increased transferrin production 2
• Serum iron – though less reliable due to high day-to-day variability and diurnal fluctuations 2
• Complete blood count – to assess hemoglobin, mean corpuscular volume (MCV), and red cell distribution width (RDW) 1
• C-reactive protein (CRP) – essential to identify inflammation that can falsely elevate ferritin 2

Diagnostic Thresholds

Without Inflammation (Normal CRP)

• Ferritin <30 ng/mL confirms absolute iron deficiency 2, 4
• TSAT <16% confirms iron deficiency 2, 3
• The combination of low ferritin and low TSAT unequivocally establishes iron deficiency with depleted stores 2

With Inflammation (Elevated CRP)

• Ferritin <100 ng/mL indicates iron deficiency, even though this appears "normal" 2, 4
• TSAT <20% is the diagnostic threshold in inflammatory states 2, 3
• Ferritin 100–300 ng/mL with TSAT <20% defines functional iron deficiency, where iron is sequestered and unavailable for red cell production 2, 3

Critical Diagnostic Pitfalls to Avoid

• Never rely on serum iron alone – it has poor diagnostic accuracy due to diurnal variation, post-prandial changes, and day-to-day fluctuations 2
• Normal serum iron does NOT exclude iron deficiency – ferritin and TSAT are far more reliable 2
• Ferritin can be falsely elevated by inflammation, malignancy, or hepatic disease; if ferritin is 12–100 μg/dL with concurrent inflammation, iron deficiency may still be present 1
• Microcytosis may be absent in combined deficiency (e.g., with folate deficiency), recognized by elevated RDW 1
• More than 50% of individuals with confirmed iron deficiency have a normal MCV 2

Mandatory Investigation of Underlying Causes

In All Patients

• Dietary history – identify iron-deficient diets, though borderline deficiency is common and should not preclude full GI investigation 1
• Medication review – document aspirin, NSAIDs, and anticoagulants; stop NSAIDs whenever possible 1
• Family history – screen for thalassemia, sideroblastic anemia, telangiectasia, and bleeding disorders 1

In Men and Postmenopausal Women

GI investigation is mandatory to exclude malignancy as a source of occult blood loss 1, 2:

• Upper GI endoscopy – reveals a cause in 30–50% of patients 1
• Small bowel biopsies during endoscopy – 2–3% of patients with iron deficiency anemia have celiac disease 1
• Colonoscopy – should be performed unless upper endoscopy reveals carcinoma or celiac disease, as dual pathology occurs in ~10% of cases 1
• Noninvasive testing for Helicobacter pylori and celiac disease (antiendomysial antibody if endoscopy cannot be performed) 1, 4

In Premenopausal Women

• Assess menstrual blood loss – heavy menstrual bleeding (soaking through a pad/tampon every 1–2 hours or periods >7 days) is a common cause 2, 5
• In younger patients with plausible heavy menstrual bleeding, a reasonable approach is to treat the bleeding and provide iron supplementation 4
• However, do not presume dietary or menstrual causes without full evaluation if anemia is severe or refractory 1

Additional Considerations

• Renal function – calculate eGFR to assess for chronic kidney disease, which affects iron metabolism 2
• Malabsorption disorders – celiac disease, inflammatory bowel disease, post-bariatric surgery 2, 5
• Ongoing blood loss – GI bleeding, frequent blood donation, high-impact athletic activity causing hemolysis 2

Treatment Algorithm

Step 1: Determine Iron Deficiency Type

CRP Status	Ferritin	TSAT	Diagnosis
Normal	<30 ng/mL	<16%	Absolute iron deficiency
Elevated	<100 ng/mL	<20%	Absolute iron deficiency
Elevated	100–300 ng/mL	<20%	Functional iron deficiency

2, 4

Step 2: Choose Iron Supplementation Route

Oral Iron (First-Line for Absolute Iron Deficiency Without Inflammation)

• Ferrous sulfate 325 mg daily (65 mg elemental iron) or on alternate days 4, 5
• Alternate-day dosing markedly improves fractional absorption by avoiding hepcidin-mediated blockade 2, 4
• Administer on an empty stomach (≥1 hour before or ≥2 hours after meals) to maximize absorption 2
• Reduce dose to 50–100 mg elemental iron per administration if GI side effects occur; higher doses do not increase absorption 2
• Expected response: hemoglobin increase of 1–2 g/dL within 4–8 weeks 2, 4
• Approximately 50% of patients have decreased adherence due to adverse effects (nausea, constipation, diarrhea) 4

Intravenous Iron (Preferred or Required in Specific Situations)

Indications for IV iron 2, 4, 5:

• Gastrointestinal intolerance to oral iron
• Lack of hematologic response after 4–8 weeks of adequate oral therapy
• Chronic kidney disease with eGFR <30 mL/min/1.73 m²
• Heart failure (NYHA class II–III) with ferritin <100 ng/mL or ferritin 100–300 ng/mL plus TSAT <20%
• Active inflammatory bowel disease
• Documented malabsorption (celiac disease, post-bariatric surgery)
• Ongoing blood loss exceeding oral replacement capacity
• Second and third trimesters of pregnancy
• Functional iron deficiency (TSAT <20% with ferritin 100–300 ng/mL in inflammatory states)

IV iron formulations 2:

Formulation	Maximum Single Dose	Special Considerations
Ferric carboxymaltose	1,000 mg	Rapid administration; low hypersensitivity risk
Ferric derisomaltose	1,000 mg or 20 mg/kg (max 1,500 mg)	FDA-approved for total-dose infusion
Iron sucrose	200 mg	No test dose required
Low-molecular-weight iron dextran	High-dose infusion	Requires test dose due to anaphylaxis risk

• Hypersensitivity to newer IV iron formulations is rare (<1%) 4
• IV iron bypasses hepcidin-mediated blockade of intestinal absorption in inflammatory states 2

Step 3: Monitor Response

• Oral iron: Evaluate response in 2–4 weeks; expect hemoglobin increase of 1–2 g/dL within 4–8 weeks 2, 4
• IV iron: Repeat iron studies 4–8 weeks after the last infusion – do NOT measure within 4 weeks, as circulating iron falsely elevates results 2, 3
• If no response to oral iron after 4–8 weeks, consider malabsorption, ongoing loss, or switch to IV iron 2

Step 4: Treatment Targets After Repletion

• Ferritin ≥50 ng/mL (without inflammation) or ≥100 ng/mL (with inflammation) 2
• TSAT ≥20% to ensure adequate iron availability for erythropoiesis 2, 3
• Hemoglobin ≥12 g/dL in adult females and ≥13 g/dL in adult males 2

Special Considerations

Functional Iron Deficiency in Chronic Disease

• In chronic inflammatory conditions (CKD, heart failure, IBD, cancer), hepcidin traps iron in storage sites, making it unavailable despite elevated ferritin 2, 3
• Oral iron is ineffective in functional iron deficiency because hepcidin blocks intestinal absorption 2
• IV iron is required to bypass this blockade and directly deliver iron to bone marrow 2
• If IV iron fails, consider erythropoiesis-stimulating agents (ESAs) with continued iron supplementation, maintaining TSAT >20% 2

Why Oral Iron May Fail

• Only a small proportion of ingested iron is absorbed; 200 mg daily often fails to meet needs in established deficiency 2
• Iron absorption declines sharply as ferritin exceeds ~200 ng/mL or TSAT >20% 2
• Daily dosing sustains elevated hepcidin for up to 24 hours, markedly reducing subsequent uptake 2
• Proton-pump inhibitors and H₂-blockers significantly impair iron absorption 2
• Among early non-responders to oral iron, only ~21% improve with an additional 4 weeks of oral therapy, whereas ~65% respond when switched to IV iron 2