What is the appropriate workup for microcytic anemia?

Workup for Microcytic Anemia

Begin with serum ferritin as the first-line laboratory test, followed by complete iron studies (transferrin saturation, total iron-binding capacity, serum iron) if ferritin is inconclusive, and consider hemoglobin electrophoresis when thalassemia is suspected based on clinical context. 1, 2, 3

Initial Laboratory Assessment

Step 1: Serum Ferritin (First-Line Test)

• Ferritin <45 μg/dL confirms iron deficiency in patients with anemia 2
• If ferritin is low, proceed directly to identify the source of iron deficiency (see below)
• Critical caveat: Ferritin is an acute phase reactant and may be falsely elevated in inflammatory conditions 2, 3

Step 2: When Ferritin is Inconclusive (20-100 μg/dL or elevated CRP)

In patients with inflammation or ferritin 20-100 μg/dL, order confirmatory tests:

• Transferrin saturation (TSAT) - low in iron deficiency
• Total iron-binding capacity (TIBC) - elevated in iron deficiency, low in anemia of chronic disease
• Serum iron - low in both iron deficiency and chronic disease
• Reticulocyte hemoglobin content OR soluble transferrin receptor - particularly useful when inflammation is present 1, 2

Key distinction: Iron deficiency shows low iron + high TIBC, while anemia of chronic disease shows low iron + low TIBC 3

Severity-Based Approach by MCV

The degree of microcytosis guides urgency and differential:

• MCV 75-79 fL: Iron deficiency most likely, but workup completion rate is lowest in this group (higher risk of underdiagnosis) 4
• MCV 65-74 fL: Strongly consider both iron deficiency and thalassemia trait
• MCV <65 fL: Thalassemia becomes more likely; hemoglobin electrophoresis is essential 4, 3

When to Suspect Thalassemia

Order hemoglobin electrophoresis when:

• MCV disproportionately low relative to degree of anemia
• Normal or elevated ferritin with microcytosis
• Family history or ethnicity suggesting hemoglobinopathy risk
• Hemoglobin A2 >3.5% confirms beta-thalassemia trait 3

Common pitfall: In one study, 55% of patients with suspected thalassemia never received appropriate follow-up testing 4. Don't stop at iron studies alone when MCV is severely depressed.

Identifying the Source of Iron Deficiency

Once iron deficiency is confirmed, systematically evaluate:

History-Specific Elements

• Dietary iron intake assessment - vegetarian/vegan diet, pica 2
• Menstrual blood loss in premenopausal women - quantify duration and heaviness 2
• Medication review - NSAIDs, anticoagulants, aspirin causing GI blood loss 2
• GI symptoms - dyspepsia, dysphagia, change in bowel habits, melena 2

Gastrointestinal Evaluation

• Test for Helicobacter pylori - can cause iron malabsorption 2
• Celiac disease screening - tissue transglutaminase antibodies, especially if malabsorption suspected 2
• Bidirectional endoscopy (EGD and colonoscopy) for adults with unexplained iron deficiency, as GI malignancy must be excluded 2, 3
• Consider video capsule endoscopy or deep enteroscopy if upper and lower endoscopy are negative and bleeding persists 2

Genetic Causes (When Standard Workup is Negative)

Suspect genetic disorders of iron metabolism or heme synthesis when 1:

• Elevated ferritin AND/OR elevated TSAT with microcytic anemia
• Low TSAT with low-normal ferritin (>20 μg/L)
• Anemia refractory to iron supplementation
• Family history of unexplained anemia
• Associated neurologic symptoms or photosensitivity

These include:

• Iron-refractory iron deficiency anemia (TMPRSS6 mutations)
• Sideroblastic anemias (multiple genetic causes)
• Hypotransferrinemia

Critical warning: Sideroblastic anemias cause iron overload despite anemia - iron supplementation is harmful and can lead to severe morbidity/mortality 1. These patients need iron chelation, not supplementation.

Common Diagnostic Errors to Avoid

1. Stopping at ferritin alone - 34% of microcytic anemia cases receive no follow-up testing 4
2. Missing inflammatory states - 26% of patients with elevated CRP and inconclusive ferritin don't receive confirmatory iron studies 4
3. Proceeding to colonoscopy without iron studies - documented in 18 patients in one series 5
4. Failing to test for thalassemia when MCV is severely depressed 4
5. Giving iron supplementation to sideroblastic anemia patients - worsens iron overload 1

Algorithm Summary

1. MCV <80 fL confirmed → Order serum ferritin
2. Ferritin <45 μg/dL → Iron deficiency confirmed → Find source (GI evaluation, dietary, menstrual)
3. Ferritin 20-100 μg/dL or elevated CRP → Add TSAT, TIBC, serum iron ± reticulocyte hemoglobin
4. MCV <65 fL or normal ferritin with microcytosis → Add hemoglobin electrophoresis
5. Refractory to treatment or elevated ferritin/TSAT → Consider genetic causes, avoid iron supplementation until diagnosis clear