What is the most likely diagnosis and recommended evaluation and management for a complete blood count showing elevated red blood cell count, low mean corpuscular volume (MCV), low mean corpuscular hemoglobin (MCH), high red cell distribution width (RDW), neutrophilia, and lymphopenia?

Microcytic Polycythemia with Neutrophilia: Iron Deficiency with Secondary Polycythemia

The most likely diagnosis is iron deficiency causing microcytic red blood cells in the setting of secondary polycythemia (elevated RBC count), and you should immediately obtain serum ferritin, transferrin saturation, and iron studies to confirm iron deficiency, followed by investigation for the underlying cause of both the iron deficiency and the polycythemia.

Interpretation of the CBC Pattern

Your patient presents with a distinctive pattern:

• Microcytic hypochromic anemia pattern (MCV 72.2, MCH 23.1) with elevated RBC count (6.81)
• High RDW (19.4) indicating significant red cell size heterogeneity
• Neutrophilia with lymphopenia suggesting possible chronic inflammation or stress response

This combination of microcytosis with polycythemia is uncommon but well-described 1.

Differential Diagnosis Priority

The key differential includes:

1. Iron deficiency with secondary polycythemia (most likely given high RDW)
2. Thalassemia minor (less likely given the high RDW)
3. Polycythemia vera with concurrent iron deficiency (must exclude)

The elevated RDW strongly favors iron deficiency over thalassemia, as thalassemia typically presents with low MCV disproportionate to anemia but normal or only slightly elevated RDW 1, 2. In contrast, iron deficiency characteristically produces high RDW due to heterogeneous red cell populations 3.

Recommended Diagnostic Workup

Immediate Laboratory Tests

Confirm iron deficiency 4:

• Serum ferritin (most specific test; <15 μg/L confirms absent iron stores, <30 μg/L indicates low stores)
• Transferrin saturation (<16% supports iron deficiency)
• Total iron and TIBC (low iron, elevated TIBC in iron deficiency)

Important caveat: Ferritin is an acute phase reactant. If inflammation is present (suggested by your neutrophilia), ferritin may be falsely normal. In inflammatory states, ferritin <45 μg/L still suggests iron deficiency, and levels >150 μg/L essentially exclude absolute iron deficiency 4.

Exclude Polycythemia Vera

Given the elevated RBC count, you must exclude a myeloproliferative neoplasm 5:

• JAK2 V617F mutation testing (present in >95% of polycythemia vera cases)
• Consider TRAKJAK score to predict JAK2 mutation probability before testing 5

Exclude Thalassemia

If iron studies are normal or equivocal 4:

• Hemoglobin electrophoresis (especially if patient has Mediterranean, African, or Southeast Asian ancestry)
• In thalassemia, MCV is typically reduced out of proportion to hemoglobin level with normal RDW

Identify Secondary Causes

For the polycythemia 1:

• Pulse oximetry and arterial blood gas (exclude hypoxia-driven erythrocytosis)
• Sleep study if clinical suspicion for sleep apnea
• Renal imaging if other features suggest renal pathology (hypernephroma can produce erythropoietin)

For the iron deficiency 4:

• GI evaluation: Upper and lower endoscopy for occult GI bleeding (most common cause in adults)
• Celiac serology (tissue transglutaminase antibodies)
• Gynecologic evaluation in menstruating women

Management Algorithm

If Iron Deficiency Confirmed

1. Initiate iron replacement therapy 4:

   • Oral ferrous sulfate 200mg 2-3 times daily (or equivalent elemental iron)
   • Recheck hemoglobin in 2 weeks: Rise ≥10 g/L confirms iron deficiency even if initial studies were equivocal 4

2. Monitor response:

   • After iron repletion, if RBC count remains elevated and MCV normalizes, this confirms secondary polycythemia with incidental iron deficiency 1
   • Continue investigating the cause of polycythemia

3. Investigate source of iron loss (mandatory in all adults) 4:

   • GI investigation should be considered at any level of anemia with iron deficiency
   • Men and non-menstruating women warrant more aggressive investigation

If Polycythemia Vera Confirmed

• Refer to hematology for management
• Iron deficiency in polycythemia vera can occur from chronic phlebotomy or increased iron utilization

Clinical Pitfalls to Avoid

• Don't assume thalassemia based solely on microcytosis: The high RDW (19.4) makes iron deficiency much more likely 2
• Don't miss polycythemia vera: The elevated RBC count mandates JAK2 testing regardless of iron status 5
• Don't rely on MCV/MCH alone: In iron deficiency with inflammation, these may be less sensitive 4
• Don't stop at diagnosing iron deficiency: You must find the source—GI malignancy is a critical consideration 4

Addressing the Neutrophilia and Lymphopenia

The neutrophil predominance (76.1%) with lymphopenia (15.8%) suggests:

• Acute stress response or chronic inflammation
• Possible infection (though absolute counts needed)
• Steroid effect if patient on corticosteroids
• Smoking-related changes if applicable

This pattern supports investigating for chronic inflammatory conditions that could cause both iron deficiency and reactive erythrocytosis.