What are the best laboratory tests for evaluating iron deficiency in a patient?

Best Laboratory Test for Iron Deficiency Evaluation

Serum ferritin is the single best laboratory test for evaluating iron deficiency, with a threshold of <45 ng/mL providing 92% specificity for diagnosis, though transferrin saturation (TSAT) should be added when inflammation is present or when ferritin results are equivocal. 1, 2

Initial Diagnostic Approach

First-Line Testing

• Order serum ferritin as the primary test because it is the most sensitive and specific marker of iron stores in the absence of inflammation 1, 2, 3
• Add a complete blood count (CBC) to assess for anemia (hemoglobin <13 g/dL in men or <12 g/dL in non-pregnant women) 1, 2
• Calculate transferrin saturation (TSAT) by measuring serum iron and total iron binding capacity (TIBC), as TSAT <20% has high sensitivity for diagnosing iron deficiency and is less affected by inflammation than ferritin 1, 2

Diagnostic Thresholds

• Ferritin <15 μg/L has 99% specificity for iron deficiency in the absence of inflammation 2
• Ferritin <45 ng/mL provides 92% specificity and is the recommended cutoff for diagnosing iron deficiency anemia, particularly when considering gastrointestinal evaluation 1, 2
• TSAT <20% confirms iron deficiency when combined with low ferritin, and among nonpregnant women, TSAT <16% has 93% specificity 1, 2

Critical Consideration: The Inflammation Problem

Ferritin is an acute-phase reactant and can be falsely elevated in inflammatory conditions, chronic disease, infection, hepatitis, cirrhosis, neoplasia, or arthritis. 1, 2

Modified Thresholds for Inflammatory States

• Use ferritin <100 ng/mL as the diagnostic threshold when inflammation, chronic kidney disease, heart failure, or inflammatory bowel disease is present 1, 2
• TSAT becomes more reliable than ferritin in inflammatory conditions because it is less affected by acute-phase reactions 1
• Consider soluble transferrin receptor (sTfR) in patients with inflammatory conditions where ferritin is unreliable, as sTfR is elevated in iron deficiency but not affected by inflammation 1, 2

Algorithmic Approach to Laboratory Diagnosis

Step 1: Initial Panel

• Order CBC with ferritin and TSAT (calculated from serum iron and TIBC) 2

Step 2: Interpret Results

• If ferritin <15 μg/L: Iron deficiency is confirmed 2
• If ferritin 15-45 μg/L and TSAT <20%: Iron deficiency is confirmed 2
• If ferritin 45-100 μg/L with TSAT <20%: Consider iron deficiency, especially if inflammation is present 1, 2
• If ferritin >100 μg/L and TSAT >20%: Iron deficiency is unlikely unless severe inflammation is present 1

Step 3: Additional Testing When Diagnosis is Unclear

• Check inflammatory markers (CRP, ESR) to determine if ferritin elevation is due to inflammation 2
• Consider reticulocyte hemoglobin content (CHr or RET-He) for direct assessment of functional iron availability, though this requires specialized equipment 1, 2
• Bone marrow biopsy showing absence of stainable iron remains the gold standard when results are unclear and definitive diagnosis is critical 2

Red Cell Indices: Helpful But Not Sufficient

• Mean corpuscular volume (MCV) may be normal in early iron deficiency before microcytosis develops 2
• Red cell distribution width (RDW) >14.0% with microcytosis strongly suggests iron deficiency anemia 2
• Do not rely on MCV to exclude iron deficiency, as microcytosis is a late finding and normal MCV does not rule out iron depletion 2

Timing of Laboratory Monitoring

• Do not check iron parameters within 4 weeks of receiving IV iron because 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 effective 1, 2

Common Pitfalls to Avoid

Pitfall 1: Relying Solely on Hemoglobin and Hematocrit

• Hemoglobin and hematocrit are late indicators that only become abnormal after iron stores are depleted and erythropoiesis is impaired 2, 4
• Iron deficiency exists before anemia develops and requires treatment even when hemoglobin is normal 2

Pitfall 2: Ignoring Inflammation

• Do not ignore inflammatory markers when interpreting ferritin levels because chronic disease can mask true iron deficiency by elevating ferritin 1, 2
• Apparently normal ferritin levels may still occur with true iron deficiency when inflammation is present 2

Pitfall 3: Testing Too Soon After IV Iron

• Wait at least 4 weeks after IV iron administration to avoid spurious results from circulating iron interfering with assays 1, 2

Pitfall 4: Missing Underlying Causes

• In men and postmenopausal women with iron deficiency anemia, approximately 62% have gastrointestinal bleeding from lesions requiring further investigation 2
• Perform non-invasive testing for H. pylori and celiac disease after confirming iron deficiency anemia before proceeding to endoscopy 1, 2

Advanced Testing for Special Situations

When Standard Tests Are Inadequate

• 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 and is immediately available on certain auto analyzers (Siemens and Sysmex) 1, 2
• Percent hypochromic red blood cells (>10%) is compatible with iron deficiency in erythropoietin-treated patients but requires specialized equipment 1

Limitations of Advanced Tests

• sTfR is elevated with increased erythropoietic activity (response to IV iron, hemolytic anemia) and ineffective erythropoiesis, limiting its specificity 1
• Reticulocyte hemoglobin content can be abnormal in inflammation and thalassemia 1