In an 11‑month‑old infant with a platelet count of 626 × 10⁹/L, what is the significance and what evaluation and management steps are recommended?

Thrombocytosis in an 11-Month-Old Infant

A platelet count of 626 × 10⁹/L in an 11-month-old infant represents mild reactive (secondary) thrombocytosis that requires no specific treatment but warrants investigation for the underlying cause.

Significance of This Platelet Count

This platelet count falls into the mild thrombocytosis category (defined as >500,000/μL but <700,000/μL) and is almost certainly reactive rather than primary in nature 1. Secondary thrombocytosis occurs in 3-13% of hospitalized children and is particularly common in infants, with a median age of 9 months in large pediatric series 2, 3.

Key reassuring features:

• Primary (essential) thrombocytosis is extraordinarily rare in children, occurring in only 1 per million children—60 times less common than in adults 1
• Primary thrombocytosis typically presents with platelet counts >1,000/μL, not in the 600,000 range 1
• At this platelet level, thrombotic or hemorrhagic complications are practically nonexistent in the absence of other risk factors 2

Recommended Evaluation

Focused history and physical examination should identify:

• Recent or current infections (the most common cause, accounting for approximately 50% of cases, especially respiratory, gastrointestinal, or central nervous system infections) 4, 3
• Recent surgical procedures or trauma (second most common cause) 4
• Iron deficiency (check for dietary history, pallor, or signs of chronic blood loss) 1, 2
• Chronic inflammatory conditions (Kawasaki disease, inflammatory bowel disease, juvenile arthritis) 1
• Medications (corticosteroids, epinephrine, all-trans retinoic acid) 1
• Splenomegaly or history of splenectomy (functional or surgical asplenia) 1

Laboratory workup should include:

• Complete blood count with differential to assess for anemia, leukocytosis, or eosinophilia 5
• Peripheral blood smear to evaluate red cell morphology and platelet morphology 5
• Serum ferritin or iron studies (iron deficiency is a common reversible cause) 1, 5
• C-reactive protein or erythrocyte sedimentation rate if inflammatory condition suspected 1

Additional testing is NOT routinely needed:

• Bone marrow examination is not indicated for isolated thrombocytosis at this level 5
• JAK2 mutation testing is not warranted unless platelet count exceeds 1,000/μL or other features suggest primary thrombocytosis 1

Management Approach

No specific treatment is required for this degree of thrombocytosis 1, 2, 3. The management strategy should focus on:

1. Treat the underlying condition (e.g., antibiotics for infection, iron supplementation for deficiency) 1, 2

2. Monitor platelet count with repeat CBC in 2-4 weeks to document resolution 1

   • Reactive thrombocytosis typically begins to decline within 3 days of treating the underlying cause 3
   • Complete resolution usually occurs within weeks to months 4

3. No antithrombotic prophylaxis is needed unless additional thrombotic risk factors are present (vessel damage, immobilization, hyperviscosity, persistent iron deficiency) 2

When to Refer to Pediatric Hematology

Consultation is indicated if: 1

• Platelet count remains elevated (>500,000/μL) beyond 4-6 weeks without identified cause
• Platelet count exceeds 1,000/μL (extreme thrombocytosis)
• Other cytopenias or abnormalities are present on CBC
• Splenomegaly is detected on examination
• Bleeding or thrombotic symptoms develop

Common Pitfalls to Avoid

• Do not assume malignancy based solely on thrombocytosis—malignant diseases alone rarely cause platelet counts in this range 3
• Do not initiate antiplatelet therapy (aspirin, clopidogrel) for isolated reactive thrombocytosis at this level, as thrombotic risk is negligible 2
• Do not overlook iron deficiency as a treatable cause, even if hemoglobin is normal initially 1, 2
• Do not perform invasive testing (bone marrow biopsy) without clear indication, as this is almost certainly reactive thrombocytosis 5