Peripheral Smear Findings in Iron Deficiency Anaemia
The peripheral blood smear in iron deficiency anaemia characteristically shows microcytic, hypochromic red blood cells with marked anisocytosis (variation in cell size) and poikilocytosis (variation in cell shape), along with an elevated red cell distribution width (RDW >14%). 1
Key Morphological Features
Red Blood Cell Abnormalities
The most prominent findings on peripheral smear include:
- Microcytosis: Small red blood cells reflecting reduced mean corpuscular volume (MCV) 2
- Hypochromia: Pale-appearing red cells with increased central pallor due to reduced hemoglobin content 3, 4
- Anisocytosis: Marked variation in red cell size, which is quantified by an elevated RDW (typically >14%) 1
- Poikilocytosis: Variation in red cell shape, with predominance of small erythrocytes 4, 5
The combination of low MCV and elevated RDW (>14%) is particularly useful diagnostically, as this pattern distinguishes iron deficiency anaemia from thalassemia minor, where the MCV is low but RDW remains normal (≤14%) 1.
Additional Cellular Features
Beyond the classic microcytic hypochromic picture, you may observe:
- Target cells (though less common than in hemoglobinopathies) 6
- Pencil cells (elongated, thin red cells) 3
- Occasional microspherocytes 7
White Blood Cell and Platelet Findings
- Mild leukopenia may accompany the anaemia in approximately one-third of patients 4
- Platelet counts can be normal, increased, or decreased 4
- Neutrophils may show loss of membrane integrity and protrusions in severe cases 7
Diagnostic Algorithm Using Smear Findings
When evaluating a peripheral smear for suspected iron deficiency:
- First, assess red cell size and color: Look for microcytosis and hypochromia as the primary indicators
- Evaluate RDW: An elevated RDW (>14%) combined with low MCV strongly suggests iron deficiency rather than thalassemia 1
- Confirm with laboratory markers: The peripheral smear findings should be correlated with serum ferritin (<15 μg/L diagnostic, <45 μg/L suggestive) 2, 8, 9
- Consider inflammation: In the presence of inflammation, ferritin up to 100 μg/L may still be consistent with iron deficiency 3
Important Clinical Caveats
Microcytosis may be absent in combined deficiencies (e.g., concurrent folate or vitamin B12 deficiency), which can be recognized by the elevated RDW despite normal MCV 2. This represents a critical pitfall where the smear may not show classic microcytic features.
Exclude hemoglobinopathies before attributing microcytosis to iron deficiency, particularly in patients from ethnic backgrounds where thalassemia is prevalent. Hemoglobin electrophoresis is recommended when microcytosis is present with normal iron studies 8. The key distinguishing feature is that thalassemia typically shows microcytosis out of proportion to the degree of anaemia with a normal or low-normal RDW 1.
Anaemia of chronic disease can also present with microcytosis, making the distinction challenging. In these cases, serum ferritin >100 μg/L with transferrin saturation <20% suggests anaemia of chronic disease rather than pure iron deficiency 3.
The peripheral smear remains a valuable diagnostic tool that, when interpreted alongside automated cell counter parameters (particularly MCV, MCH, and RDW) and biochemical markers (ferritin, transferrin saturation), provides a comprehensive picture for diagnosing iron deficiency anaemia 8, 5.