ITP can develop at all ages in children. In newborns, thrombocytopenia developing from antiplatelet antibodies transferred through the placenta from a mother with ITP to the fetus is called passive ITP [22]. ITP in children mainly occurs after viral or bacterial infection or vaccination [23, 24].

Bleeding symptoms are mainly bruising and mucocutaneous bleedings such as petechiae (diameter of 0.5–3 mm, no blanching under pressure, not palpable) or ecchymosis (flat, rounded, or irregular red, blue, purplish, or yellow-green patch larger than petechiae), oral or nasal bleeding, hematuria, or hypermenorrhea. Rarely, organ bleeding such as gastrointestinal or intracranial bleeding occurs [25]. Patients are often noticed incidentally during laboratory examinations performed for unrelated diseases. ITP in children is more frequent in males, whereas adult ITP more involves females. Severe bleeding is rare and is found in children with a platelet count less than 20 × 10⁹/L [4, 26, 27]. Predictors of bleeding due to ITP in children include female sex, older age at presentation (age ≥11 years), absence of preceding infection or vaccination, insidious onset, higher platelet count at presentation, presence of antinuclear antibodies, and treatment with a combination of methylprednisolone and intravenous immunoglobulin. Furthermore, children with mucosal bleeding at diagnosis or treatment with intravenous immunoglobulin alone develop chronic ITP less often [7].

The bleeding risk and severity of ITP have usually been evaluated based on peripheral platelet counts and bleeding symptoms. However, objective evaluation of the degree or severity of bleeding symptoms is not easy. Various BATs have been proposed for ITP in children and adults [25, 28–32]. More recently, a new standard in bleeding assessment for ITP in children and adults (ITP-BAT bleeding scale, version 1.0) has been reported from the IWG.

The ITP-BAT bleeding scale is made up of three major bleeding domains of the skin (S), visible mucosae (M), and organs (O), with a gradation of severity (SMOG index). The gradation of severity for each domain is further classified on the site of bleeding with gradings ranging from 0 to 3 or 4 according to the spread of the hemorrhagic area, need for medical intervention and red blood cell transfusion, and reductions in levels of blood hemoglobin. For example, a SMOG index of S2 M2 O3 could represent findings of subcutaneous hematoma (S2), epistaxis (M2), and menorrhagia (O3). Considering the lifelong potential functional impairment caused by intracranial bleeding, the IWG recommends that all cases of intracranial bleeding be reported, irrespective of grade, such as S grade 2 M grade 2 O grade 3 (intracranial grade 2; post trauma, requiring hospitalization). However, the value of summing up the worst grades for all manifestations in each domain is currently uncertain, and interpretation remains ambiguous [25].

Information about the family and medical history before onset are important. The characteristics of bleeding symptoms and physical and laboratory examinations are crucial for differential diagnosis. The Japanese Society of Pediatric Hematology/Oncology proposed diagnostic criteria for pediatric ITP (Table 2) [33]. However, no gold standards exist for ITP diagnosis, and exclusion of other thrombocytopenic disorders remains central to the diagnosis. Bone marrow examination is usually unnecessary in children and adolescents with typical features of ITP, but when abnormal physical symptoms (such as abnormal rash, recurrent fever, or joint pain), lymphadenopathy, splenomegaly, abnormal cell counts, abnormal morphology of red or white blood cells, and poor response to medical treatments such as immunoglobulin and corticosteroids are present, bone marrow evaluation may be needed to rule out other disorders. In recent years, measurement of plasma levels of TPO, TPO receptor protein, and immature platelets (reticulo-platelets), which may indicate platelet production in the bone marrow, has been developed as complementary inspection methods [8].

A diagnosis of ITP is reached by excluding underlying disorders with thrombocytopenia caused by platelet destruction and/or production and congenital thrombocytopenia. Diagnosis of congenital thrombocytopenia is not easy and is usually achieved based on platelet size and morphology in peripheral blood specimens, clinical manifestations of hemorrhagic and nonhemorrhagic symptoms, and results of platelet-function tests such as platelet aggregation tests using three different inducers adenine diphosphate (ADP), collagen and ristocetin, platelet-membrane glycoprotein analysis, flow-cytometric analysis with specific-labeled antiplatelet antibodies, electron microscopic analysis, or genetic analysis. A recent distinction algorithm using platelet size for congenital thrombocytopenia is shown in Table 3 [34, 35].