Microcytic Anemia Definition
Microcytic anemia is defined as anemia with a mean corpuscular volume (MCV) less than 80 fL in adults, characterized by the presence of small, often hypochromic red blood cells. 1, 2, 3
Laboratory Criteria
MCV threshold: The defining feature is MCV <80 fL (or <80 μm³), which distinguishes microcytic from normocytic (80-100 fL) and macrocytic (>100 fL) anemias 1, 2, 3
Hemoglobin levels: Anemia itself is defined by hemoglobin below the lower limit of normal for the laboratory performing the test, typically <13 g/dL for men and <12 g/dL for women 4, 2
Associated findings: Microcytic anemia is often accompanied by hypochromia (low mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration) 2, 5
Age considerations: For patients younger than 17 years, age-specific MCV parameters should be used rather than the adult threshold of 80 fL 3
Common Causes
Iron deficiency is the most common cause of microcytic anemia, accounting for the majority of cases 1, 2, 5, 3, 6
Other important causes include:
Thalassemia: Should be suspected when MCV is particularly low with normal or elevated red cell count, especially in certain ethnic groups 4, 1, 2
Anemia of chronic disease: Can present with microcytosis due to inability to utilize iron stores, though ferritin is typically elevated (>100 μg/dL) 4, 1, 2
Sideroblastic anemia: A less common inherited or acquired cause 1, 2, 3
Key Diagnostic Features
Red cell distribution width (RDW): An elevated RDW (>14%) with low MCV strongly suggests iron deficiency anemia, while RDW ≤14% with low MCV suggests thalassemia minor 1, 2, 7
Serum ferritin: The most powerful single test for iron deficiency, with levels <12-15 μg/L diagnostic of iron deficiency (though <30 μg/L is often used as the threshold in the absence of inflammation) 4, 1, 2
Transferrin saturation: Values <16-30% suggest iron deficiency 4, 1
Important Clinical Pitfalls
Mixed deficiencies: Microcytosis may be absent when iron deficiency coexists with folate or B12 deficiency, as macrocytosis and microcytosis can neutralize each other, resulting in a falsely normal MCV 4, 1
Inflammatory states: Serum ferritin can be falsely elevated in patients with concurrent chronic inflammation, malignancy, or hepatic disease, potentially masking iron deficiency 4, 2
Severity misconception: Mild anemia should not be dismissed as less clinically significant than severe anemia, as both may indicate important underlying disease such as gastrointestinal malignancy 4