Interpretation of Complete Blood Count Results
Primary Interpretation: Microcytic Anemia with Elevated RDW
This CBC demonstrates microcytic anemia (MCV 73 fL) with elevated red cell distribution width (RDW 16.8%), which strongly suggests iron deficiency anemia as the most likely diagnosis. 1
Key Diagnostic Features
Red Cell Indices Pattern
- MCV of 73 fL (microcytic) combined with RDW >14% (16.8%) is the classic pattern for iron deficiency anemia, as RDW >14% with low MCV indicates iron deficiency rather than thalassemia trait 1, 2
- The MCH of 22.9 pg and MCHC of 31.4 g/dL are both reduced, confirming hypochromic microcytic anemia 1
- Elevated RBC count (6.25 × 10⁶/µL) with microcytosis creates a differential that includes iron deficiency, thalassemia trait, or polycythemia with concurrent iron deficiency 2
RDW as the Critical Discriminator
- RDW 16.8% is significantly elevated (normal <14%), which is the single most important finding to differentiate iron deficiency from thalassemia 1, 2
- In thalassemia minor, RDW would typically be normal or minimally elevated (≤14%), whereas iron deficiency characteristically shows RDW >14% 1, 2
- The elevated RDW reflects anisocytosis (variation in red cell size) that occurs as iron-deficient cells are produced alongside older normal-sized cells 3
Hemoglobin and Hematocrit Assessment
- Hemoglobin 14.3 g/dL and hematocrit 45.6% are within normal range, indicating this patient does not have anemia by standard definitions 4
- However, the microcytic indices with elevated RDW suggest early or compensated iron deficiency where the bone marrow is producing increased numbers of small, iron-deficient red cells to maintain normal hemoglobin 2
- The elevated RBC count (6.25) compensates for the reduced size of individual cells, maintaining near-normal oxygen-carrying capacity 2
Recommended Diagnostic Workup
Immediate Next Steps
Order the following tests immediately to confirm iron deficiency and exclude other causes: 1, 2
- Serum ferritin (most specific for iron stores; <30 ng/mL confirms absolute iron deficiency in absence of inflammation) 1
- Complete iron studies including serum iron, total iron-binding capacity (TIBC), and transferrin saturation (<15% with low ferritin confirms iron deficiency) 1
- Peripheral blood smear to visualize RBC morphology and confirm microcytosis, hypochromia, and anisocytosis 1, 2
- Reticulocyte index to assess bone marrow production capacity 1, 2
- C-reactive protein to identify inflammation that could elevate ferritin despite true iron deficiency 1
Secondary Testing if Iron Deficiency Excluded
- Hemoglobin electrophoresis should be ordered if iron studies are normal, as this would suggest thalassemia trait (though less likely given the elevated RDW) 1, 2
- Lead level if clinical suspicion exists, particularly with occupational exposure 1
Clinical Significance and Pitfalls
Important Considerations
- Multiple causes can coexist: A patient may have both iron deficiency and an underlying hemoglobinopathy, with iron deficiency masking the typical pattern 2
- Ferritin interpretation requires caution: Chronic inflammatory conditions can elevate ferritin despite true iron deficiency, necessitating evaluation of transferrin saturation and CRP 1
- Normal hemoglobin does not exclude iron deficiency: This patient demonstrates that microcytic indices with elevated RDW can occur before frank anemia develops, representing early iron-deficient erythropoiesis 1, 2
Prognostic Implications
- Elevated RDW (16.8%) has broader clinical significance beyond anemia diagnosis, as it independently predicts mortality and adverse outcomes in various populations, though this is more relevant when combined with other abnormalities 5, 6
- In this case with normal hemoglobin, the elevated RDW primarily reflects the diagnostic finding of iron-deficient erythropoiesis rather than prognostic significance 3