Which laboratory test is most appropriate for diagnosing iron deficiency?

Diagnostic Testing for Iron Deficiency

Serum ferritin is the single most powerful laboratory test for diagnosing iron deficiency, with a level <30 µg/L confirming depleted iron stores in patients without inflammation. 1

Primary Diagnostic Approach

Order serum ferritin as your first-line test. The interpretation depends on the ferritin threshold and presence of inflammation 1:

• Ferritin <15 µg/L provides 99% specificity for absolute iron deficiency—diagnosis is confirmed without additional testing 1, 2
• Ferritin <30 µg/L indicates depleted iron stores and confirms iron deficiency in patients without active inflammation 1, 2
• Ferritin <45 µg/L offers optimal sensitivity and specificity for routine clinical practice and should prompt investigation for iron deficiency 1, 2

When Ferritin Is Equivocal (30–100 µg/L)

Add transferrin saturation (TSAT) and C-reactive protein (CRP) to your initial panel when ferritin falls in the borderline range. 1, 2

• TSAT <20% confirms iron deficiency even when ferritin appears normal due to inflammation 1, 2
• TSAT <16% provides even stronger confirmation of iron-deficient erythropoiesis 1, 2
• Measure CRP concurrently because ferritin is an acute-phase reactant that rises with inflammation, infection, malignancy, or liver disease 1, 2

Interpretation Algorithm for Borderline Ferritin

When ferritin is 30–100 µg/L 1, 2:

• Elevated CRP + TSAT <20% → True iron deficiency coexisting with anemia of chronic disease; treat both conditions
• Normal CRP + TSAT <20% → Pure iron deficiency; initiate iron therapy
• Elevated CRP + TSAT ≥20% + ferritin >100 µg/L → Anemia of chronic disease with functional iron deficiency; prioritize treating the underlying inflammatory condition

Complete Initial Laboratory Panel

Order these tests together as your minimum diagnostic workup 1, 2:

• Complete blood count with red-cell indices (MCV, MCH, RDW)
• Reticulocyte count
• Serum ferritin
• Transferrin saturation (calculated as serum iron ÷ TIBC × 100)
• C-reactive protein

Red Blood Cell Indices as Supporting Evidence

Use CBC parameters to support but not replace ferritin testing 1, 3:

• MCV <80 fL with RDW >14% strongly suggests iron deficiency rather than thalassemia trait (which typically shows RDW ≤14%) 1
• MCH <27 pg is more reliable than MCV because it's less affected by specimen storage 1
• Elevated RDW (>14%) is an early and sensitive marker that can reveal iron deficiency even when MCV remains normal 1, 2

When to Add Soluble Transferrin Receptor (sTfR)

Measure sTfR when ferritin and TSAT provide conflicting information. 1, 2

• An elevated sTfR confirms true iron deficiency because it is not influenced by inflammation 1, 2
• This test is particularly useful in patients with chronic inflammatory conditions where ferritin may be falsely elevated 2

Critical Pitfalls to Avoid

Do not rely on ferritin alone when inflammation is present—always calculate TSAT because functional iron deficiency can exist with high ferritin but low TSAT. 1, 2

Do not use hemoglobin or MCV as screening tests for iron deficiency—they have low sensitivity and specificity; ferritin is far superior. 1, 4

Do not order hemoglobin electrophoresis as a first-line test—reserve it for cases where microcytosis persists despite normal iron studies or when the patient belongs to a high-risk ethnic group for thalassemia. 1

Do not assume ferritin >100 µg/L excludes iron deficiency in inflammatory states—in conditions like inflammatory bowel disease, chronic kidney disease, or chronic heart failure, ferritin up to 100 µg/L may still represent true iron deficiency. 1, 2

Do not overlook combined deficiencies—iron deficiency can coexist with vitamin B12 or folate deficiency, recognizable by an elevated RDW with mixed micro- and macrocytic cells. 1, 2

Special Population Thresholds

Adjust your ferritin threshold based on clinical context 1, 2:

• Inflammatory bowel disease in remission: ferritin <30 µg/L confirms iron deficiency
• Active IBD inflammation: use ferritin <100 µg/L as screening threshold, confirm with TSAT <20%
• Chronic kidney disease or chronic heart failure: ferritin <100 µg/L (rather than <30 µg/L) is the recommended screening threshold