Interpretation and Recommended Work-Up
This peripheral blood smear shows a dimorphic picture with combined microcytosis and macrocytosis, suggesting coexisting nutritional deficiencies—most likely combined iron deficiency and vitamin B12/folate deficiency. The moderate anisocytosis and slight polychromasia support this interpretation, while the few ovalocytes may indicate early megaloblastic changes 1.
Key Morphologic Findings and Their Significance
The simultaneous presence of moderate microcytosis and slight macrocytosis is diagnostically important. When microcytosis and macrocytosis coexist, they can neutralize each other resulting in a falsely normal MCV 1. The moderate anisocytosis (high RDW) is a critical clue in this situation, as elevated RDW is an indicator of iron deficiency and helps identify this mixed picture 1.
Morphologic Interpretation:
- Moderate microcytosis → Iron deficiency anemia, thalassemia trait, or anemia of chronic disease
- Slight macrocytosis → Vitamin B12 or folate deficiency, medications (thiopurines, azathioprine), alcohol, hypothyroidism, or reticulocytosis 1
- Slight polychromasia → Increased reticulocyte response (regenerative anemia)
- Few ovalocytes → May suggest early megaloblastic changes
- Moderate anisocytosis → Mixed RBC populations, strongly suggests combined deficiencies
Recommended Laboratory Work-Up
Minimum Initial Testing (Priority Order):
1. Complete Blood Count with RBC Indices:
- Confirm MCV (may be falsely normal due to dimorphic population)
- RDW (Red Cell Distribution Width) - expect this to be elevated 1
- Reticulocyte count - determines bone marrow response 1
2. Iron Studies:
- Serum ferritin (most important initial test) 2
- Transferrin saturation (TfS)
- Serum iron
- Total iron-binding capacity (TIBC)
3. Inflammatory Markers:
- CRP - to distinguish iron deficiency from anemia of chronic disease 1
4. Vitamin Levels:
- Vitamin B12
- Folate 1
Interpretation Thresholds:
For Iron Deficiency:
- Without inflammation: serum ferritin <30 μg/L indicates iron deficiency 1
- With inflammation (elevated CRP): ferritin up to 100 μg/L may still indicate iron deficiency 1
Reticulocyte Count Interpretation:
- Low or normal reticulocytes → deficiency states or bone marrow failure (inappropriate response)
- Elevated reticulocytes → excludes deficiencies; consider hemolysis 1
Extended Work-Up (If Initial Testing Inconclusive):
- Haptoglobin, LDH, bilirubin (if reticulocytes elevated - to evaluate for hemolysis) 1
- Hemoglobin electrophoresis (if thalassemia trait suspected) 2
- Soluble transferrin receptor
- Percentage of hypochromic red cells
- Differential white blood cell count 1
Clinical Pitfalls and Caveats
Critical Warning: The coexistence of microcytosis and macrocytosis can mask each individual abnormality, resulting in a deceptively normal MCV. Always check the RDW in addition to MCV - a high RDW with "normal" MCV should trigger investigation for combined deficiencies 1.
Common Causes of Combined Deficiency:
- Chronic gastrointestinal blood loss (iron) with malabsorption (B12/folate)
- Inflammatory bowel disease
- Celiac disease
- Chronic alcoholism
- Post-gastrectomy states
- Medications (especially thiopurines causing macrocytosis while patient has concurrent iron deficiency)
When to Consult Hematology
Hematology consultation is appropriate if:
- The cause of anemia remains unclear after extended workup 1
- Bone marrow examination is being considered
- Concern for myelodysplastic syndrome (especially with persistent unexplained cytopenias)
Source Investigation
Once deficiencies are confirmed, identify the underlying cause. For iron deficiency in adults, presume gastrointestinal blood loss until proven otherwise and evaluate for potential malignancy 2. For B12/folate deficiency, consider malabsorption syndromes, dietary insufficiency, pernicious anemia, or medication effects.