Microcytic Anemia with Elevated RDW: Iron Deficiency vs. Thalassemia
This patient most likely has iron deficiency anemia (IDA), and the next steps should include serum ferritin, transferrin saturation, and a peripheral blood smear to confirm the diagnosis and exclude thalassemia trait.
Laboratory Findings Analysis
The key abnormalities in this patient are:
- Elevated RBC count (5.99 × 10⁶/L) - above the normal range of 4.14-5.80 1
- Low MCH (26.2 pg) - below the normal range of 26.6-33.0 1
- Elevated RDW (16.6%) - above the normal range of 11.6-15.4 1
- Normal MCV (82 fL) - within the normal range of 79-97 1
- Normal hemoglobin (15.7 g/dL) and hematocrit (48.8%) 1
Diagnostic Interpretation
Iron Deficiency as the Primary Diagnosis
The elevated RDW is the most critical finding here, as it indicates anisocytosis (variation in red cell size) which is characteristic of early iron deficiency. 2, 3
- RDW increases in the latent stage of iron deficiency when MCV may still be normal, because a microcytic population of cells begins appearing in the blood alongside normocytic cells 3
- In one study, RDW had 82.3% sensitivity and 97.4% specificity for detecting iron deficiency, significantly outperforming MCV (29.2% sensitivity), MCH (68.1% sensitivity), and MCHC (15% sensitivity) in early stages 3
- The low MCH (26.2 pg) further supports iron deficiency, as MCH is slightly more accurate than MCV in diagnosing empty iron stores 4
Why This is Likely NOT Thalassemia Trait
The elevated RBC count might initially suggest alpha-thalassemia trait, but several features argue against this:
- Thalassemia trait typically presents with normal or low-normal RDW (usually <15%), whereas this patient has significantly elevated RDW at 16.6% 5
- In isolated alpha-thalassemia, MCV, MCH, and MCHC values are typically higher than in iron deficiency or combined iron deficiency with thalassemia 5
- The normal hemoglobin and hematocrit make significant thalassemia less likely, as thalassemia trait usually causes mild anemia 1
Recommended Next Steps
Essential Initial Testing
Serum ferritin - the gold standard for assessing tissue iron stores 1
- Values <15 μg/L confirm iron deficiency 4
- This is the most important single test to order
Transferrin saturation (TSAT) - represents iron available to bone marrow for erythropoiesis 1
- Low TSAT (<20%) supports iron deficiency
Peripheral blood smear - to evaluate red cell morphology and confirm microcytosis/hypochromia 1
- Look for pencil cells, target cells, and variation in cell size
Additional Considerations
Complete iron panel including serum iron and total iron-binding capacity if ferritin and TSAT are equivocal 1
Hemoglobin electrophoresis - only if iron studies are normal and thalassemia trait remains in the differential 2, 5
- Alpha-thalassemia requires molecular genetic testing (PCR multiplex for common deletions) as HbA2 levels are normal 5
Assessment for blood loss - particularly gastrointestinal bleeding in non-menstruating patients with confirmed iron deficiency 1
Clinical Pitfalls to Avoid
- Do not assume normal MCV excludes iron deficiency - RDW elevation precedes MCV reduction in early iron deficiency 3
- Do not order hemoglobin electrophoresis before checking iron studies - iron deficiency is far more common and must be excluded first 1, 2
- Do not rely on MCHC alone - it has poor sensitivity (15%) for detecting iron deficiency 3
- Normal hemoglobin does not exclude iron deficiency - this patient may have latent iron deficiency (depleted stores without anemia) 3
If Iron Deficiency is Confirmed
- Investigate the underlying cause, particularly in females of childbearing age (menstrual losses, dietary insufficiency, malabsorption) 1
- In patients without obvious menstrual losses, carefully assess for gastrointestinal bleeding 1
- Initiate iron supplementation and monitor response with repeat CBC in 4-8 weeks 1