What is the interpretation of a CBC showing elevated RBC count, normal MCV, low MCH, elevated RDW, and thrombocytopenia, with the rest of the CBC being within normal limits?

Interpretation of CBC Results

Primary Diagnosis: Iron Deficiency Anemia with Reactive Erythrocytosis

This CBC pattern indicates iron deficiency anemia with a compensatory increase in red blood cell production, characterized by elevated RBC count (6.58), microcytic hypochromic indices (MCV 76.6, MCH 25.5), markedly elevated RDW (18%), and mild thrombocytopenia (113). 1, 2

Laboratory Pattern Analysis

The combination of findings creates a distinctive diagnostic signature:

• Elevated RBC count with microcytosis: The body is producing more red cells to compensate for their reduced oxygen-carrying capacity due to iron deficiency 2
• Low MCH (25.5): Indicates hypochromic red cells with reduced hemoglobin content per cell, characteristic of iron deficiency 2
• Markedly elevated RDW (18%): Reflects heterogeneous red cell populations as iron stores become progressively depleted; this high RDW strongly distinguishes iron deficiency from thalassemia trait, which typically presents with normal or only slightly elevated RDW 1, 2
• Mild thrombocytopenia (113): May indicate concurrent bone marrow stress, chronic disease, or early myelodysplastic changes requiring further evaluation 3

Immediate Diagnostic Workup Required

You must obtain the following tests immediately:

• Serum ferritin (most powerful single test): <30 μg/L confirms iron deficiency in the absence of inflammation; <100 μg/L may still indicate iron deficiency if inflammation is present 1, 4
• Transferrin saturation: <30% supports iron deficiency 2, 4
• C-reactive protein (CRP): Essential to interpret ferritin correctly in the context of inflammation 1, 2
• Reticulocyte count: Assesses bone marrow response and helps distinguish decreased production from increased destruction 3, 2

Mandatory Investigation for Underlying Cause

All adult men and postmenopausal women with confirmed iron deficiency require complete gastrointestinal evaluation, regardless of symptom presence. 1, 4

This includes:

• Upper endoscopy with mandatory small bowel biopsies to evaluate for celiac disease and other malabsorptive disorders 1
• Colonoscopy or CT colonography to investigate for malignancy, as gastrointestinal blood loss is the most common source in these populations 1

For premenopausal women, evaluate for:

• Menstrual blood loss (menorrhagia)
• Pregnancy-related iron demands
• Dietary insufficiency 4

Critical Differential Considerations

Thalassemia trait must be excluded, though less likely given the markedly elevated RDW:

• Thalassemia typically shows RDW ≤14% despite microcytosis 4
• If ferritin is normal and RDW remains elevated, consider hemoglobin electrophoresis 3

Combined deficiency states (iron plus folate/B12):

• Can present with normal MCV but elevated RDW due to mixed cell populations 2, 4
• Check vitamin B12 and folate levels if clinical suspicion exists 3

Anemia of chronic disease:

• Typically shows ferritin >100 μg/L with transferrin saturation <20% 4
• Distinguished from iron deficiency by elevated inflammatory markers 3

Thrombocytopenia Considerations

The mild thrombocytopenia (113) requires additional evaluation:

• Rule out spurious platelet count: Platelet clumping in EDTA tubes can cause falsely low counts; request citrated sample if suspected 5
• Evaluate for concurrent conditions: Chronic disease, bone marrow infiltration, or early myelodysplastic syndrome 3
• Monitor trend: If persistent after iron repletion, consider bone marrow biopsy 3

Treatment Algorithm

Once iron deficiency is confirmed (ferritin <30 μg/L):

1. Initiate oral iron supplementation as first-line treatment (ferrous sulfate 325 mg daily) 4
2. Continue for 3-6 months after hemoglobin normalizes to replete iron stores 1, 4
3. Consider parenteral iron if malabsorption is present, losses exceed maximal oral replacement capacity, or true intolerance to oral iron occurs 4

Treatment of the underlying cause will cure the anemia, making identification of the source critical 1

Common Pitfalls to Avoid

• Do not assume dietary insufficiency alone in adult men or postmenopausal women without completing GI evaluation—malignancy must be excluded 1, 4
• Serum ferritin can be falsely elevated in inflammatory conditions, chronic disease, malignancy, or liver disease, potentially masking iron deficiency 2
• Do not stop investigation if initial ferritin is 30-100 μg/L—this may still represent iron deficiency in the presence of inflammation 1, 2
• Microcytosis and macrocytosis can coexist, resulting in a normal MCV but elevated RDW, masking combined deficiencies 2