How is secondary thrombocytosis diagnosed in children?

Diagnosing secondary (reactive) thrombocytosis in children requires a systematic, stepwise approach to distinguish it from primary (clonal) disorders such as essential thrombocythemia. The evaluation begins with a thorough history and physical examination—focusing on signs of infection, inflammation, iron deficiency, recent surgery or trauma, malignancy, or chronic inflammatory conditions (e.g., juvenile idiopathic arthritis, inflammatory bowel disease). Laboratory testing is central: a complete blood count (CBC) with peripheral blood smear review confirms elevated platelet count (typically >450 × 10⁹/L) and assesses for abnormal platelet morphology, leukocytosis, anemia, or red cell microcytosis suggestive of iron deficiency. Additional initial tests include serum ferritin (to evaluate iron stores), C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) (to assess inflammation), and renal and liver function tests. If clinical suspicion remains high for an underlying condition, targeted investigations may follow—such as blood cultures, stool studies, imaging, or autoimmune serologies. Importantly, bone marrow aspiration and biopsy are not routinely indicated in children with isolated, mild-to-moderate thrombocytosis and no concerning features (e.g., splenomegaly, abnormal blood counts, or clonal markers); they are reserved for cases with atypical findings or suspected myeloproliferative neoplasm. Genetic testing for *JAK2*, *CALR*, and *MPL* mutations is generally unnecessary in pediatric reactive thrombocytosis but may be considered if clinical or laboratory features raise concern for a primary disorder.