What are the diagnostic criteria for Immune Thrombocytopenic Purpura (ITP)?

Diagnostic Criteria for Immune Thrombocytopenia (ITP)

The diagnosis of ITP is based principally on the history, physical examination, complete blood count, and examination of the peripheral blood smear, which should exclude other causes of thrombocytopenia. 1

Primary Diagnostic Criteria

1. Platelet count less than 100 × 10⁹/L 1
2. Exclusion of other causes of thrombocytopenia
3. Normal peripheral blood smear except for thrombocytopenia

Peripheral Blood Smear Findings in ITP

Consistent with ITP diagnosis:

• Thrombocytopenia
• Platelets normal in size or slightly larger than normal
• Normal red blood cell morphology
• Normal white blood cell morphology 1

Not consistent with ITP diagnosis:

• Red blood cell poikilocytosis or schistocytes
• Predominant giant platelets (approaching the size of red cells)
• Polychromatophilia (unless in response to bleeding)
• Leukocytosis or leukopenia with immature or abnormal cells 1

Bone Marrow Examination

• Not necessary in children and adolescents with typical features of ITP (Grade 1B) 1
• Not necessary in children who fail IVIg therapy (Grade 1B) 1
• Not necessary before initiation of corticosteroid treatment or splenectomy (Grade 2C) 1
• Should be performed in patients with persistent thrombocytopenia (lasting more than 6-12 months) 1
• Should be performed in patients unresponsive to IVIg 1

Additional Testing

Tests that should be performed in specific situations:

• HIV antibody testing in patients with risk factors for HIV infection 1
• Abdominal CT scan or ultrasound in patients with suspected splenomegaly on physical examination 1
• Liver function tests in pregnant women with thrombocytopenia to rule out preeclampsia 2

Tests of uncertain necessity/appropriateness:

• Antinuclear antibodies (ANA)
• Direct antiglobulin test
• Lupus anticoagulant/antiphospholipid antibodies
• Chemistry profile
• Coagulation studies
• Chest X-ray 1, 2

Clinical Classification of ITP

• Newly diagnosed: Diagnosis to 3 months
• Persistent: 3 to 12 months from diagnosis
• Chronic: Lasting for more than 12 months 1

Response Criteria

• Complete response (CR): Platelet count ≥100 × 10⁹/L measured on 2 occasions 7 days apart and absence of bleeding
• Response (R): Platelet count ≥30 × 10⁹/L and >2-fold increase from baseline measured on 2 occasions 7 days apart and absence of bleeding
• No response (NR): Platelet count <30 × 10⁹/L or <2-fold increase from baseline or presence of bleeding 1

Common Pitfalls in ITP Diagnosis

1. Failure to exclude secondary causes of thrombocytopenia such as:

   • Autoimmune diseases (particularly antiphospholipid antibody syndrome)
   • Viral infections (HCV, HIV)
   • Drug-induced thrombocytopenia
   • Lymphoproliferative disorders 1

2. Over-reliance on platelet count alone without considering clinical presentation and peripheral blood smear findings

3. Unnecessary bone marrow examination in typical cases, particularly in children 1

4. Misinterpreting immature platelet fraction (IPF) - typically elevated in ITP but can be low in some cases 3

5. Overlooking liver disease as a cause of thrombocytopenia, which can present with normal or only mildly abnormal liver function tests in 19% of cases 2

ITP remains a diagnosis of exclusion, and clinicians should maintain a high index of suspicion for alternative diagnoses, particularly in patients with atypical presentations or those who do not respond to standard therapies.