What tests are used to evaluate for iron deficiency anemia?

Evaluation of Iron Deficiency Anemia

Begin with a complete blood count (CBC) followed by serum ferritin and transferrin saturation—these three tests form the essential diagnostic triad for iron deficiency anemia. 1

Initial Laboratory Tests

Complete Blood Count (CBC)

• Hemoglobin is the preferred measure over hematocrit because it is more reproducible across laboratories and not affected by storage time or patient variables like serum glucose 2, 1
• Diagnostic thresholds: Hemoglobin <13 g/dL in men or <12 g/dL in non-pregnant women indicates anemia 2
• Red Cell Distribution Width (RDW) >14.0% with microcytosis strongly suggests iron deficiency anemia 1
• Mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) help identify microcytic pattern, though these are late findings 2, 1

Critical Pitfall to Avoid

Do not rely solely on hemoglobin and hematocrit for diagnosis—these are late indicators that only become abnormal after iron stores are completely depleted. 1, 3 Many patients with normal CBC parameters already have depleted iron stores and will rapidly progress to anemia if not detected early 3

Iron Status Assessment

Serum Ferritin (Primary Test)

• Most sensitive and specific test for iron stores 1, 4
• Diagnostic cutoffs:
  • <25 ng/mL in males and <11 ng/mL in females indicates insufficient iron stores 2, 1
  • <30 ng/mL is the standard threshold for diagnosing iron deficiency 4
  • <45 ng/mL provides 92% specificity for iron deficiency 2, 1
  • <15 μg/L has 99% specificity in the absence of inflammation 1

Transferrin Saturation (TSAT)

• Represents iron immediately available for erythropoiesis 2, 1
• Less affected by inflammation than ferritin, making it more reliable in inflammatory states 2
• TSAT <20% indicates iron deficiency 2, 1, 4
• Calculated as: (serum iron × 100) / total iron-binding capacity 1

Special Considerations for Inflammatory Conditions

When inflammation, chronic kidney disease, heart failure, or inflammatory bowel disease is present, use ferritin <100 ng/mL as the diagnostic threshold because ferritin acts as an acute-phase reactant and can be falsely elevated 2, 1, 5

In chronic kidney disease patients specifically:

• Ferritin <100 ng/mL suggests iron deficiency 1, 5
• Consider measuring C-reactive protein to assess inflammation's contribution to elevated ferritin 2

Additional Tests When Diagnosis is Unclear

Reticulocyte Count

• Evaluates bone marrow response to anemia 2
• Low reticulocyte count with adequate iron, folate, and B12 suggests insufficient erythropoietin production or inflammation 2

Advanced Tests (Limited Availability)

• Percent hypochromic red blood cells (PHRBC) >10% indicates iron deficiency but requires specialized equipment and is time-sensitive 2, 1
• Reticulocyte hemoglobin content reflects recent iron availability 1
• Serum transferrin receptor is less affected by inflammation but not routinely available 1

Diagnostic Algorithm

1. Measure hemoglobin to confirm anemia (Hb <13 g/dL men, <12 g/dL women) 2
2. Check serum ferritin as primary marker of iron stores 1, 4
3. Add transferrin saturation to improve diagnostic accuracy 2, 1
4. If ferritin is 45-100 ng/mL and diagnosis unclear, measure additional iron parameters (serum iron, TIBC) 1
5. In inflammatory conditions, use higher ferritin threshold (<100 ng/mL) and rely more heavily on TSAT 1, 5

Critical Pitfalls to Avoid

• Never interpret ferritin in isolation when inflammation is present—chronic disease, infection, hepatitis, cirrhosis, malignancy, or arthritis can elevate ferritin independent of iron status 1, 5
• Do not test iron parameters within 4 weeks of IV iron administration—circulating iron interferes with assays and produces falsely elevated results 1
• In men and postmenopausal women with confirmed iron deficiency anemia, always investigate for gastrointestinal bleeding—approximately 62% have GI lesions requiring evaluation 1, 6
• Recognize that serum iron alone is unreliable due to significant diurnal variation and increases after meals 1

Non-Invasive Testing for Underlying Causes

After confirming iron deficiency anemia, perform non-invasive testing for H. pylori and celiac disease before proceeding to endoscopy 2

For asymptomatic patients:

• Men and postmenopausal women: bidirectional endoscopy is strongly recommended 2
• Premenopausal women: consider empiric iron supplementation first, especially in younger women who prioritize avoiding endoscopy risks 2