What is chronic idiopathic thrombocytopenic purpura (ITP)?

Definition of Chronic Idiopathic Thrombocytopenic Purpura

Chronic idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder characterized by isolated thrombocytopenia (platelet count <100 × 10⁹/L) persisting for ≥12 months after diagnosis, without another identifiable cause. 1, 2

Core Diagnostic Features

ITP is fundamentally a diagnosis of exclusion requiring three essential elements 1, 2:

• Isolated thrombocytopenia with platelet count <100 × 10⁹/L 1, 2
• Normal peripheral blood smear except for reduced platelets 3
• Absence of other obvious causes of thrombocytopenia 1, 2

Temporal Classification

The American Society of Hematology defines ITP by duration 1, 4:

• Newly diagnosed: Initial presentation
• Persistent ITP: Thrombocytopenia lasting 3-12 months 1, 4
• Chronic ITP: Thrombocytopenia persisting ≥12 months after diagnosis 3, 1

Note that older literature sometimes defined chronic ITP as >6 months, but the current standard is 12 months 3.

Pathophysiology

The disease involves dual mechanisms 1, 5:

• Increased platelet destruction: Autoantibodies (typically against platelet glycoproteins IIb-IIIa and/or Ib-IX) coat platelets, leading to premature destruction by the reticuloendothelial system in the spleen and liver 6, 5, 7
• Impaired platelet production: Autoantibodies also bind to megakaryocytes, resulting in decreased thrombopoiesis 1, 5

This represents an evolution from the historical belief that ITP was caused solely by increased platelet destruction 1.

Blood Smear Characteristics

Findings consistent with ITP 3:

• Thrombocytopenia with platelets normal in size or larger than normal
• Giant platelets approaching the size of red blood cells should be absent
• Normal red blood cell morphology
• Normal white blood cell morphology (though atypical lymphocytes and eosinophilia may occur in children) 3

Findings NOT consistent with ITP 3:

• Red blood cell poikilocytosis or schistocytes
• Polychromatophilia (unless responding to bleeding)
• Leukocytosis/leukopenia with immature or abnormal cells
• Predominant giant platelets

Clinical Presentation

The clinical spectrum varies widely 1, 4:

• Minimal symptoms: Many patients present only with bruising and petechiae 1, 4
• Mucosal bleeding: Gastrointestinal hemorrhage and hematuria may occur 1
• Severe bleeding: Reported in 9.5% of adults and 20.2% of children 1
• Intracranial hemorrhage: Rare but serious, occurring in 1.4% of adults and 0.1-0.4% of children 1

Prognosis Differences: Adults vs. Children

Adult chronic ITP has a distinctly different natural history 1, 4:

• Only 20-45% achieve complete remission by 6 months 2, 4
• More likely to become chronic compared to children 1, 4
• 1.3-2.2-fold higher mortality than the general population due to cardiovascular disease, infection, and bleeding 1, 4

Pediatric ITP has more favorable outcomes 1, 2:

• Spontaneous remission rates are age-dependent: 74% in children <1 year, 67% in ages 1-6 years, and 62% in ages 10-20 years 1, 2
• Generally benign course with high spontaneous remission rates 1

Important Caveats

• Bone marrow examination is generally unnecessary in children and adolescents with typical ITP features, though it may be considered before corticosteroid treatment or splenectomy in adults 1
• The condition may be discovered incidentally on routine blood counts in asymptomatic patients 3
• Splenomegaly may be palpable in approximately 12% of children with ITP, particularly infants 3