What laboratory tests are recommended to evaluate iron status?

How to Check Iron Levels

Start with serum ferritin and transferrin saturation as your primary tests, but interpret ferritin thresholds based on inflammatory status: use <15 μg/L without inflammation, but raise the threshold to <30-100 μg/L when inflammation is present. 1, 2

Core Laboratory Tests

The essential tests for evaluating iron status are:

• Serum ferritin - reflects total body iron stores (1 μg/L ≈ 10 mg stored iron)
• Transferrin saturation (TSAT) - indicates iron immediately available for erythropoiesis
• Complete blood count (CBC) with hemoglobin/hematocrit - detects anemia but only at late stages

These three tests form the foundation of iron assessment 2. Hemoglobin and hematocrit are inexpensive and widely available but are late indicators - they only become abnormal after iron stores are already depleted 2.

Critical Interpretation Based on Inflammatory Status

Without inflammation present:

• Iron deficiency: ferritin <15 μg/L OR transferrin saturation <16% 1, 2
• Ferritin is highly reliable when inflammation is absent 1

With inflammation present (elevated CRP, ESR, or clinical signs):

• Raise ferritin threshold to <30 μg/L for definite deficiency 1
• Ferritin 30-100 μg/L + TSAT <16% suggests combined iron deficiency and anemia of chronic disease 1
• Ferritin >100 μg/L + TSAT <16% indicates anemia of chronic disease alone 1

This distinction is crucial because ferritin is an acute-phase reactant that rises with inflammation, potentially masking true iron deficiency 1, 2.

Additional Tests for Complex Cases

Soluble transferrin receptor (sTfR):

• Elevated in iron deficiency, normal/low in anemia of chronic disease 1
• Not affected by inflammation, making it valuable when ferritin interpretation is unclear 1
• Major limitation: lacks standardization across laboratories 3, 4

Mean corpuscular volume (MCV):

• Low MCV indicates microcytic anemia, suggesting iron deficiency (after excluding thalassemia, lead poisoning) 2, 5
• Late marker - only abnormal after prolonged deficiency 6

Red blood cell distribution width (RDW):

• RDW >14% with low MCV suggests iron deficiency anemia
• RDW ≤14% with low MCV suggests thalassemia minor 2

Erythrocyte protoporphyrin:

• Elevated (>70 μg/dL RBCs in adults) indicates insufficient iron for hemoglobin production 2
• Limitation: also elevated by infection, inflammation, and lead poisoning 2

Practical Algorithm

1. Order initial tests: CBC, serum ferritin, transferrin saturation
2. Assess inflammatory status: Check CRP or ESR simultaneously 1
3. Interpret ferritin using appropriate threshold based on inflammation status
4. If ferritin is equivocal (30-100 μg/L with inflammation): add sTfR if available 1
5. For microcytic anemia: add MCV and RDW to differentiate iron deficiency from thalassemia 2

Common Pitfalls to Avoid

• Don't use ferritin alone in inflammatory conditions - it will overestimate iron stores 1, 2
• Don't assume anemia equals iron deficiency - only 37% of anemic women and 25% of anemic children aged 1-5 years actually have iron deficiency 2
• Don't rely on hemoglobin/hematocrit alone - these miss early iron deficiency before anemia develops 2
• Don't use a single ferritin cutoff for all patients - adjust thresholds based on clinical context 1, 7, 8

Special Populations

Chronic kidney disease patients:

• Use ferritin <25 ng/mL (males) or <11 ng/mL (females) for non-dialysis patients 6
• Transferrin saturation is more reliable than ferritin in dialysis patients due to high inflammation rates 6

Premenopausal vs postmenopausal women:

• Different baseline ferritin levels (average 43 μg/L vs 135 μg/L in men) 2
• Consider menstrual losses before extensive GI workup in premenopausal women 7, 8

Patients with inflammatory bowel disease:

• Always measure inflammatory markers (CRP, ESR) alongside iron studies 1
• Use higher ferritin thresholds (100 μg/L) when inflammation is present 1

The 2024 AGA guideline recommends a ferritin cutoff of 45 ng/mL for diagnosing iron deficiency in patients with anemia, balancing sensitivity and specificity 7, 8. However, this may need adjustment upward in inflammatory conditions.