What labs are used to diagnose iron deficiency?

Laboratory Tests for Diagnosing Iron Deficiency

The essential laboratory tests for diagnosing iron deficiency are serum ferritin as the primary test, combined with transferrin saturation (TSAT) and a complete blood count (CBC) to assess for anemia. 1, 2

Initial Screening Tests

• Complete Blood Count (CBC) should be the first step, measuring hemoglobin, hematocrit, mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH). 1, 2
• Hemoglobin is preferred over hematocrit because it is more reproducible across laboratories and not affected by storage time or patient variables like serum glucose. 2
• However, hemoglobin and hematocrit are late indicators of iron deficiency—they only become abnormal after iron stores are depleted and erythropoiesis is impaired, so normal values do not rule out iron deficiency. 2, 3
• Red cell distribution width (RDW) >14.0% with microcytosis strongly suggests iron deficiency anemia. 2

Primary Diagnostic Tests for Iron Status

Serum Ferritin (Most Important Test)

• Serum ferritin is the most sensitive and specific test for iron stores in the absence of inflammation. 1, 2
• Ferritin <15 μg/L has 99% specificity for iron deficiency, while ferritin <30 μg/L generally indicates low body iron stores. 1, 2
• A cutoff of 45 μg/L provides 92% specificity and may warrant gastrointestinal investigation, especially in chronic inflammatory conditions. 1
• In the absence of inflammation, ferritin <50 ng/mL indicates iron deficiency regardless of sex. 2

Transferrin Saturation (TSAT)

• TSAT <20% has high sensitivity for diagnosing absolute or functional iron deficiency and is less affected by inflammation than ferritin. 1, 2
• TSAT is calculated as (serum iron × 100)/total iron binding capacity (TIBC). 2, 4
• TSAT reflects iron immediately available for hemoglobin synthesis. 1, 2

Supporting Tests

• Serum iron measures circulating iron bound to transferrin but has significant diurnal variation and increases after meals, making it less reliable as a standalone test. 2
• Total iron-binding capacity (TIBC) measures transferrin's capacity to bind iron and is elevated in iron deficiency. 2, 4

Special Considerations for Inflammatory Conditions

• Ferritin is an acute-phase protein and may be falsely elevated in inflammation, chronic disease, infection, hepatitis, cirrhosis, neoplasia, or arthritis. 1, 2
• When inflammation is present, use ferritin <100 μg/L as the diagnostic threshold because apparently normal ferritin levels may still occur with true iron deficiency. 1, 2
• Ferritin >150 μg/L is unlikely to occur with absolute iron deficiency, even in the presence of inflammation. 1

Advanced or Specialized Tests (When Standard Tests Are Unclear)

• Soluble transferrin receptor (sTfR) reflects cellular iron deficits and is less affected by inflammation, though most hospitals do not routinely offer this test. 1, 2
• Reticulocyte hemoglobin content (CHr or RET-He) provides direct assessment of functional iron availability to erythropoietic tissue and is immediately available on certain auto-analyzers (Siemens and Sysmex). 1, 5
• Reticulocyte hemoglobin content is an early marker of iron-deficient erythropoiesis and increases within days of initiating iron therapy. 5
• Bone marrow biopsy showing absence of stainable iron remains the gold standard when results are unclear and definitive diagnosis is important. 1, 5

Diagnostic Algorithm

1. Order CBC with ferritin and TSAT as the initial panel. 1, 2
2. If ferritin <15 μg/L (or <30 μg/L), iron deficiency is confirmed. 1, 2
3. If ferritin is 15-45 μg/L, add serum iron and TIBC to calculate TSAT; TSAT <20% confirms iron deficiency. 1
4. In patients with chronic inflammatory conditions (CKD, heart failure, IBD), use ferritin <100 μg/L as the threshold. 1, 2, 6
5. If ferritin is apparently normal but clinical suspicion remains high, check inflammatory markers (CRP, ESR) and consider sTfR or reticulocyte hemoglobin content. 1

Timing of Laboratory Monitoring

• Do not check iron parameters within 4 weeks of receiving IV iron, as circulating iron interferes with assays and gives falsely elevated results. 1, 2
• Recheck iron parameters 4-8 weeks after starting supplementation to assess response. 1, 2
• Hemoglobin should increase by 1-2 g/dL within 4-8 weeks if treatment is working. 1, 2

Common Pitfalls to Avoid

• Do not rely solely on hemoglobin and hematocrit—many iron-deficient individuals have normal CBC parameters until severe depletion occurs. 2, 3
• Do not ignore inflammatory markers when interpreting ferritin levels—chronic disease can mask true iron deficiency by elevating ferritin. 1, 2
• Do not assume dietary insufficiency in men and postmenopausal women with iron deficiency—approximately 62% have gastrointestinal bleeding requiring further investigation. 2
• Do not test iron parameters too soon after IV iron administration—wait at least 4 weeks to avoid spurious results. 1, 2
• In patients with unexplained iron deficiency, test for tissue transglutaminase (TTG) antibodies to screen for celiac disease (assuming they have not been excluding gluten for at least 6 weeks). 1
• Check for IgA deficiency if TTG is negative but clinical suspicion for celiac disease remains, as IgA deficiency can cause falsely negative TTG readings. 1