In a patient with leukocytosis, microcytic anemia, thrombocytosis, elevated red cell distribution width, and neutrophil predominance, what is the most likely diagnosis and what evaluation and treatment should be pursued?

Most Likely Diagnosis: Iron Deficiency Anemia with Reactive Thrombocytosis and Leukocytosis

This patient's laboratory findings—microcytic anemia (MCV 79.8), severe hypochromia (MCHC 28.5), elevated RDW (19.9), thrombocytosis (479), and leukocytosis (12.9) with neutrophilia—are most consistent with iron deficiency anemia (IDA) causing reactive thrombocytosis and leukocytosis.

Diagnostic Reasoning

The constellation of findings strongly points toward IDA:

• Microcytic anemia (MCV 79.8, MCH 22.8, MCHC 28.5) indicates defective hemoglobin synthesis 1
• Markedly elevated RDW (19.9) is characteristic of IDA, reflecting anisocytosis from mixed populations of iron-deficient cells 2, 3
• Reactive thrombocytosis (479) commonly accompanies IDA; iron deficiency can cause thrombocytosis ranging from moderate to extreme levels (even >2500 × 10⁹/L) 4
• Leukocytosis with neutrophilia (WBC 12.9,73.7% neutrophils) can occur with severe IDA, though this is less common than thrombocytosis 4

The severely low MCHC (28.5) and MCH (22.8) indicate profound hypochromia, which is pathognomonic for iron deficiency when this severe 1.

Critical Evaluation Steps

Immediate Laboratory Assessment

Order the following tests to confirm IDA:

• Serum ferritin (expect <30 ng/mL in IDA)
• Transferrin saturation (TSAT) (expect <20% in IDA)
• Serum iron and total iron-binding capacity
• Reticulocyte count
• Peripheral blood smear review

Rule Out Alternative Diagnoses

While IDA is most likely, you must exclude:

1. Thalassemia trait: Would show microcytosis but typically normal or elevated RDW, higher RBC count relative to degree of anemia, and target cells on smear 2, 3. The markedly elevated RDW (19.9) argues against thalassemia.

2. Essential thrombocythemia (ET): Would require platelets ≥450 × 10⁹/L with JAK2/CALR/MPL mutations, but the microcytic anemia with low MCHC makes this unlikely 5. Secondary thrombocytosis from IDA is far more common 6.

3. Anemia of chronic disease: Would show elevated or normal ferritin with low TSAT 1.

If ferritin is low-normal (>20 mg/L) with low TSAT, consider genetic iron metabolism disorders like IRIDA (iron-refractory iron deficiency anemia) due to TMPRSS6 defects 1. However, this is rare and typically presents with refractoriness to oral iron.

Source Identification

Investigate the cause of iron deficiency:

• Gastrointestinal blood loss: Most common in adults, especially men and postmenopausal women. Order fecal occult blood testing and consider endoscopy.
• Menstrual blood loss: In premenopausal women, assess menstrual history for menorrhagia 4.
• Dietary insufficiency: Particularly in vegetarians or those with poor nutritional intake 4.
• Malabsorption: Consider celiac disease, H. pylori infection, or inflammatory bowel disease.

Treatment Approach

Initial Management

Start oral iron supplementation immediately:

• Ferrous sulfate 325 mg (65 mg elemental iron) three times daily, or
• Ferrous sulfate 325 mg once daily if tolerability is an issue
• Consider adding vitamin C (ascorbic acid) to enhance absorption 1

Monitoring Response

• Recheck CBC in 2-4 weeks; expect reticulocytosis within 7-10 days and hemoglobin rise of 1-2 g/dL by 3-4 weeks
• The thrombocytosis and leukocytosis should resolve rapidly with iron repletion 4
• Continue iron for 3-6 months after normalization to replenish stores

When to Use Intravenous Iron

Consider IV iron if:

• Intolerance to oral iron
• Malabsorption
• Ongoing blood loss exceeding replacement capacity
• Need for rapid correction (e.g., preoperative)
• Lack of response to oral iron after 4-6 weeks 1

Critical Pitfalls to Avoid

1. Do not assume thrombocytosis is primary (ET) without first excluding IDA. Secondary thrombocytosis from IDA is far more common and resolves with iron treatment 6.

2. Do not order expensive molecular testing (JAK2, CALR, MPL) before confirming iron status. Active malignancy, chronic inflammatory disease, splenectomy, and iron deficiency are all associated with secondary thrombocytosis 6.

3. Do not overlook the source of iron loss. Failure to identify GI malignancy or other bleeding sources can lead to significant morbidity and mortality 1.

4. Monitor ferritin during treatment; keep levels <500 mg/L to avoid iron overload toxicity, especially in patients who may have unrecognized genetic disorders 1.

5. If anemia persists despite adequate iron repletion and no ongoing losses, consider hemoglobin electrophoresis to evaluate for thalassemia or hemoglobinopathies 1.