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) with red cell indices. 1

Initial Screening Tests

Start with a CBC to assess for anemia and evaluate red cell indices. 1

• Hemoglobin is the preferred measure over hematocrit because it is more reproducible across laboratories and not affected by storage time or patient-specific variables like serum glucose 1
• Diagnostic thresholds: Hemoglobin <13 g/dL in men or <12 g/dL in non-pregnant women indicates anemia 1
• Red Cell Distribution Width (RDW) >14.0% with microcytosis strongly suggests iron deficiency anemia 1
• Mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) provide sensitive indication of iron deficiency in the absence of chronic disease or hemoglobinopathy 2

Important caveat: Hemoglobin and hematocrit are late indicators of iron deficiency—they only become abnormal after iron stores are depleted and erythropoiesis is impaired. 1

Primary Diagnostic Tests for Iron Status

Serum Ferritin (Most Important Test)

Serum ferritin is the most sensitive and specific test for assessing iron stores. 1

• Ferritin <15 μg/L has 99% specificity for iron deficiency in the absence of inflammation 1
• Ferritin <30 μg/L generally indicates low body iron stores 1
• The AGA recommends using a cutoff of 45 ng/mL (rather than 15 ng/mL) when diagnosing iron deficiency in patients with anemia, providing 92% specificity 3
• For basic screening: Ferritin <25 ng/mL in males and <11 ng/mL in females indicates insufficient iron stores 1

Critical pitfall: Ferritin is an acute-phase reactant and may be falsely elevated in inflammation, chronic disease, infection, hepatitis, cirrhosis, neoplasia, or arthritis. 1 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. 3, 1

Transferrin Saturation (TSAT)

TSAT is calculated as (serum iron × 100)/TIBC and reflects iron immediately available for hemoglobin synthesis. 1

• TSAT <20% has high sensitivity for diagnosing iron deficiency and is less affected by inflammation than ferritin 1
• Among nonpregnant women, TSAT <16% has 93% specificity for iron deficiency 1
• TSAT is particularly useful when ferritin results are equivocal or inflammation is present 1

Supporting Tests

Total iron-binding capacity (TIBC) measures the blood's capacity to bind iron with transferrin; this value increases when iron stores are depleted. 1

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. 1

Diagnostic Algorithm

Order CBC with ferritin and TSAT as the initial panel. 1

1. If ferritin <15 μg/L (or <30 μg/L), iron deficiency is confirmed 1
2. If ferritin is 15-45 μg/L, add serum iron and TIBC to calculate TSAT; TSAT <20% confirms iron deficiency 1
3. In patients with chronic inflammatory conditions (CKD, heart failure, IBD, cancer), use ferritin <100 μg/L as the threshold 1

Advanced or Specialized Tests

Reticulocyte hemoglobin content (CHr or RET-He) provides direct assessment of functional iron availability to erythropoietic tissue and is less affected by inflammation than ferritin. 1

Soluble transferrin receptor (sTfR) reflects cellular iron deficits and is less affected by inflammation, though most hospitals do not routinely offer this test. 1

Bone marrow biopsy showing absence of stainable iron remains the gold standard when results are unclear and definitive diagnosis is important. 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

Recheck iron parameters 4-8 weeks after starting supplementation to assess response. 1

Hemoglobin should increase by 1-2 g/dL within 4-8 weeks if treatment is working. 1

Common Pitfalls to Avoid

• Do not rely solely on hemoglobin and hematocrit for diagnosing iron deficiency, as these are late indicators 1
• Do not ignore inflammatory markers when interpreting ferritin levels—chronic disease can mask true iron deficiency by elevating ferritin 1
• Do not test iron parameters too soon after IV iron administration—wait at least 4 weeks to avoid spurious results 1
• Recognize that serum iron has substantial day-to-day variation, greater than for hemoglobin or hematocrit, which limits its diagnostic utility when measured in isolation 1
• In men and postmenopausal women with iron deficiency anemia, do not assume dietary insufficiency; approximately 62% have gastrointestinal bleeding from lesions requiring further investigation 1

Additional Testing for Underlying Causes

After confirming iron deficiency anemia, perform non-invasive testing for H. pylori and celiac disease (with tissue transglutaminase antibodies) before proceeding to endoscopy. 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