What is the next step to evaluate thrombocytosis (high platelet count) in a 16-year-old teenage boy?

Evaluation of Thrombocytosis in a 16-Year-Old Boy

A complete blood count with peripheral blood smear examination, iron studies, and inflammatory markers should be the next step in evaluating thrombocytosis (platelet count of 470) in a 16-year-old boy. 1

Initial Diagnostic Approach

Step 1: Confirm Thrombocytosis and Assess Severity

• The platelet count of 470 × 10^9/L is only mildly elevated (normal range typically 150-450 × 10^9/L)
• This level falls into the "mild thrombocytosis" category (500-700 × 10^9/L) 2
• Verify the count is persistent with a repeat CBC if this is the first finding

Step 2: Peripheral Blood Smear Examination

• Essential to evaluate platelet morphology and to rule out other hematologic abnormalities
• Look for:
  • Platelet size and granularity
  • Red blood cell morphology (to rule out concurrent anemia)
  • White blood cell morphology and differential 1

Step 3: Laboratory Tests to Determine Etiology

1. Iron studies (ferritin, iron, TIBC, transferrin saturation)

   • Iron deficiency is a common cause of reactive thrombocytosis in adolescents 2, 3
   • Present in up to 24% of pediatric thrombocytosis cases 3

2. Inflammatory markers

   • ESR, CRP to identify inflammatory or infectious causes
   • Infection is the most common cause (48%) of thrombocytosis in children 3

3. Consider additional tests based on clinical suspicion:

   • Liver function tests
   • Renal function tests
   • Thyroid function tests if symptoms suggest thyroid disease

Diagnostic Reasoning

The diagnostic approach is guided by the understanding that:

1. Secondary (reactive) thrombocytosis is vastly more common in adolescents than primary thrombocytosis

   • Primary thrombocytosis incidence is approximately 1 per million children 2
   • Secondary causes account for 83.1% of all thrombocytosis cases 4

2. Most common causes of secondary thrombocytosis in children:

   • Infections (48%)
   • Iron deficiency (24%)
   • Combined infection and iron deficiency (15%) 3

3. Bone marrow examination is not necessary initially

   • The ASH guidelines clearly state that bone marrow examination is not necessary in children and adolescents with typical features of thrombocytopenia/cytosis 1
   • Only indicated if other abnormalities are present in the CBC or if primary thrombocytosis is strongly suspected

Management Considerations

• Mild thrombocytosis (as in this case) generally requires monitoring rather than immediate intervention 2
• No antiplatelet therapy is necessary for secondary thrombocytosis 1
• Treatment should focus on the underlying cause:
  • Iron supplementation if deficient
  • Appropriate antimicrobials if infection is identified
  • Anti-inflammatory treatment if inflammatory condition is diagnosed

When to Consider Referral to Hematology

• If platelet count exceeds 1,000 × 10^9/L (extreme thrombocytosis)
• If thrombocytosis persists without identified secondary cause
• If there are symptoms of bleeding or thrombosis
• If family history suggests hereditary thrombocytosis 1, 2

Important Caveats

• Avoid premature diagnosis of primary thrombocythemia without thorough investigation of secondary causes
• Remember that adolescents with thrombocytosis are more likely than younger children to develop persistent elevation 1
• Secondary thrombocytosis rarely causes clinical complications, while primary thrombocythemia can lead to both bleeding and thrombotic events 5