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Thrombocytosis (platelet count >450×10^9/L) in children is most often secondary (reactive). Primary (essential) thrombocytosis – due to a myeloproliferative neoplasm (MPN) such as essential thrombocythemia (ET) – is rare in pediatrics. By contrast, secondary thrombocytosis arises from an underlying condition and is usually transient. In children, the common triggers are reactive processes:

• Infection and Inflammation. Bacterial or other infections and systemic inflammatory states (e.g. pneumonia, Kawasaki disease) markedly increase cytokines (IL-6, thrombopoietin) that stimulate platelet production. Note: viral infections often cause thrombocytopenia or mild reactive thrombocytosis.
• Autoimmune/Inflammatory Diseases. Chronic inflammatory conditions (juvenile idiopathic arthritis, systemic lupus, vasculitis) and autoimmune diseases (e.g. Kawasaki disease, Henoch–Schönlein purpura, inflammatory bowel disease) can drive platelet counts up as an acute-phase reactant. For example, systemic-onset JIA shows high IL‑6 that correlates with platelet elevation, and Kawasaki disease causes a pronounced thrombocytosis in the second week of illness.
• Tissue Injury/Surgery. Trauma, major surgery, burns or severe hemolysis (non‐iron‐deficiency) can provoke rebound thrombocytosis during recovery. Splenectomy or functional hyposplenism (e.g. from sickle cell infarcts) removes the splenic platelet reservoir, causing a sharp rise in platelets. (Children post-splenectomy routinely develop high platelets, which slowly normalize over weeks.)
• Malignancy. Reactive thrombocytosis can occur in cancer. In children this is uncommon but noted with certain solid tumors (especially liver tumors like hepatoblastoma or hepatocellular carcinoma) and small round-cell tumors (neuroblastoma). Rarely (~3% of cases) children with newly diagnosed acute lymphoblastic leukemia present with platelet counts >400×10^9/L.
• Other Conditions. Certain chronic conditions raise platelets. For instance, congenital nephrotic syndrome causes thrombocytosis via decreased PACAP (a platelet inhibitor). Any state of marked inflammation (e.g. Langerhans cell histiocytosis) or cytokine excess can do so.

Table 1 summarizes primary vs secondary thrombocytosis.

Reactive thrombocytosis is usually transient: platelets normalize once the trigger resolves. Very high counts (even >1,000×10^9/L) can occur, but children seldom have clots or bleeding from reactive causes. By contrast, primary (essential) thrombocytosis is persistent, clonal, and carries thrombosis risk. Key clues favoring primary ET are spleen enlargement, thrombotic/bleeding events, family history of MPN, and extreme or long-lasting thrombocytosis.

Pediatric-Specific Causes (Secondary Thrombocytosis). In practice, the most common pediatric triggers are:

• Inflammatory/Autoimmune: Juvenile idiopathic arthritis, Kawasaki disease, vasculitides, IBD, LCH. (E.g. systemic JIA often shows high platelets – IL‑6 drives megakaryocytes; Kawasaki’s classic platelet rise occurs in week 2.)
• Post-surgical/Tissue injury: Major surgery or burns in children commonly lead to a rebound thrombocytosis in recovery. Splenectomy (e.g. for trauma or hereditary spherocytosis) causes a sharp platelet surge.
• Malignancy: Certain childhood cancers (especially liver tumors, neuroblastoma) may induce thrombocytosis via cytokine/TPO production. Rarely, lymphoid leukemias present with high platelets.
• Medications: Systemic corticosteroids and epinephrine can acutely release stored platelets, causing transient elevations. Some chemotherapy agents (e.g. vinca alkaloids) and other drugs (e.g. miconazole) have also been reported to elevate platelet counts.

Overall, in children secondary/reactive thrombocytosis dominates. Primary (MPN-related) thrombocytosis is very uncommon

When to Monitor Platelet Count in Pediatrics

Platelet count is measured whenever a CBC is obtained, but specific monitoring depends on context. In general pediatrics:

• Routine Screening. There is no routine platelet screening in healthy children beyond standard CBCs. The AAP recommends anemia screening (hemoglobin/hematocrit) at 9–12 months (and in between 1–5 years if at risk); a CBC at those times would also report platelets if ordered. Thus, platelets are incidentally checked with any CBC, but we do not order CBCs solely to screen platelets in well children.
• Indications for CBC (includes platelets): CBC is ordered when clinically indicated – e.g. unexplained fatigue/bruise (to check cytopenias or thrombocytopenia), pallor (anemia), infection (high WBC) or chronic illness (inflammation). Any time a CBC is done for these reasons, the platelet count is automatically obtained. Screening CBCs may also be done before surgery or for anemic symptoms.
• Disease-Specific Monitoring. Many pediatric conditions warrant periodic CBCs (thus platelets) to monitor disease activity or treatment. For example:
  • Inflammatory and Autoimmune Diseases (JIA, IBD, lupus): Platelet count often rises during flares (acute-phase reactant). Providers routinely check CBC at diagnosis and during follow-up to assess inflammation. (E.g., systemic JIA patients commonly have thrombocytosis when IL‑6 is high.) A normal or falling platelet count can indicate therapy response.
  • Kawasaki Disease: Platelets typically surge in the 2nd–3rd week of illness. Serial CBCs (and platelet counts) are obtained to confirm this subacute rise. Lack of expected thrombocytosis does not rule out KD.
  • Post-Splenectomy Patients: Children who have undergone splenectomy (for trauma, hematologic disease) need platelet monitoring because their counts can become markedly elevated. Persistent extreme thrombocytosis post-splenectomy may prompt consideration of antiplatelet prophylaxis.
  • Congenital Heart Disease / ECMO / Critical Care: Critically ill children (e.g. on extracorporeal support) often have frequent CBCs including platelets to monitor for thrombosis or bleeding risk.
  • Hematology/Oncology: Children with malignancies or receiving chemotherapy have routine CBC monitoring (platelets included). For example, following acute leukemia therapy, counts are followed to detect relapse or recovery. In rare MPN cases, pediatric hematologists monitor platelets to manage ET.
  • Medication Effects: Some therapies can affect platelets. For instance, children on high-dose steroids or growth factors (G-CSF) are monitored via CBC to watch for rebound thrombocytosis. Similarly, any drug known to alter platelet production (e.g. certain antivirals/antibiotics) would prompt periodic CBC checks.

Platelets are included in every CBC ordered. In healthy pediatric care, we do not order CBCs just to screen platelets, but we do evaluate platelets whenever CBCs are indicated.

Table 2 outlines common scenarios.