Laboratory Findings in Microcytic Anemia
The most powerful laboratory test for diagnosing iron deficiency—the most common cause of microcytic anemia—is serum ferritin, with a level <12 μg/dL being diagnostic of iron deficiency. 1
Key Laboratory Parameters
Mean Corpuscular Volume (MCV)
- Microcytosis is defined as MCV <80 fL (80 μm³) in adults, which is the hallmark laboratory finding in microcytic anemia 2, 3
- Age-specific parameters should be used for patients younger than 17 years 3
- MCV may be particularly low in thalassemia trait, often lower than in iron deficiency alone 3
Serum Ferritin: The Gold Standard Initial Test
- Serum ferritin <12 μg/dL is diagnostic of iron deficiency 1
- Ferritin levels between 12-100 μg/dL may still indicate iron deficiency in the presence of concurrent inflammation, malignancy, or hepatic disease 1
- If ferritin >100 μg/dL, iron deficiency is almost certainly not present 1
- Low serum ferritin is the best single laboratory parameter for diagnosing iron deficiency 4
Additional Iron Studies When Ferritin is Equivocal
- Transferrin saturation <30% supports iron deficiency diagnosis 1
- Total iron-binding capacity (TIBC) is elevated in iron deficiency but decreased in anemia of chronic disease 2
- Serum iron levels are low in both iron deficiency and chronic disease 2
- The combination of low iron with decreased TIBC suggests anemia of chronic disease rather than iron deficiency 2
Red Cell Distribution Width (RDW)
- Elevated RDW is characteristic of iron deficiency and helps distinguish it from thalassemia and chronic disease 1, 5
- RDW may be raised in combined deficiencies (e.g., iron plus folate deficiency) even when MCV appears normal 1
- RDW is elevated to a greater extent in iron deficiency than in chronic disease or thalassemia traits 5
Red Blood Cell Count
- Elevated red cell count with microcytosis suggests thalassemia trait rather than iron deficiency 1
- This is a critical distinguishing feature, as thalassemia patients typically have elevated RBC counts despite low MCV 1
Differential Diagnosis Laboratory Approach
When Ferritin is Normal or Elevated
- Check hemoglobin electrophoresis for thalassemia: elevated hemoglobin A2 suggests beta-thalassemia trait 2
- Measure inflammatory markers (ESR, CRP) to identify anemia of chronic disease 5
- Consider lead toxicity and sideroblastic anemia in appropriate clinical contexts 2
Confirmatory Testing for Iron Deficiency
- Therapeutic trial of oral iron for 3 weeks or bone marrow aspiration are definitive methods to confirm true iron deficiency when doubt remains 1
- Bone marrow examination showing absent iron stores remains the most definitive test, though rarely necessary in practice 6
Common Pitfalls to Avoid
Critical caveat: Ferritin is an acute phase reactant and can be falsely elevated in inflammation, infection, malignancy, or liver disease, potentially masking concurrent iron deficiency 1. In these situations, ferritin levels up to 100 μg/dL may still represent iron deficiency 1.
Do not assume dietary insufficiency or ethnic background explains microcytosis without laboratory confirmation—haemoglobinopathies must be confirmed by laboratory testing, not presumed based on ethnicity alone 1