Diagnostic Criteria for Immune Thrombocytopenic Purpura (ITP)
ITP is diagnosed primarily through history, physical examination, complete blood count, and peripheral blood smear examination that excludes other causes of thrombocytopenia—it remains a diagnosis of exclusion with a platelet count threshold of <100 × 10⁹/L. 1
Essential Diagnostic Components
Core Laboratory Requirements
- Complete blood count (CBC) is the fundamental first test, with the diagnosis requiring a platelet count <100 × 10⁹/L 1, 2
- Peripheral blood smear examination must demonstrate:
- Thrombocytopenia with platelets that are normal in size or slightly larger (but not giant platelets approaching red blood cell size) 1
- Normal red blood cell morphology (no schistocytes or poikilocytosis unless related to bleeding response) 1
- Normal white blood cell morphology (though atypical lymphocytes and eosinophilia may occur in children) 1
Clinical Assessment Requirements
- History must exclude other causes of thrombocytopenia and document bleeding symptoms or incidental discovery 1
- Physical examination should confirm the absence of splenomegaly—the spleen is typically not palpable in adults with ITP, though it may be palpable in approximately 12% of children, particularly infants 1, 3
- The presence of splenomegaly suggests alternative diagnoses such as Gaucher's disease, thalassemia, schistosomiasis, or myelodysplasia 3
Disease Classification by Duration
The International Working Group defines ITP temporally as 1:
- Newly diagnosed: diagnosis to 3 months
- Persistent: 3 to 12 months from diagnosis
- Chronic: lasting more than 12 months
When Additional Testing Is Required
Mandatory Additional Tests in Specific Circumstances
- HIV antibody testing is required for patients with risk factors for HIV infection 1, 2
- Abdominal CT scan or ultrasound is appropriate only when splenomegaly is suspected on physical examination 1, 2
- Bone marrow aspiration should be performed in:
Special Population Considerations
For pregnant women with suspected ITP 1:
- Blood pressure measurement to rule out preeclampsia
- Liver function tests to exclude HELLP syndrome
- HIV antibody testing if risk factors present
Tests That Are Generally NOT Indicated
The following tests are generally unnecessary for routine diagnosis 1:
- ANA (antinuclear antibody)
- Direct antiglobulin test
- Lupus anticoagulant/antiphospholipid antibodies
- Platelet antigen-specific antibody
- Platelet-associated IgG
- Chemistry profile
- Coagulation studies
- Chest x-ray
- Serum immunoglobulins
These tests have uncertain appropriateness and should only be considered in atypical presentations or treatment failures 1
Emerging Diagnostic Markers
Recent research suggests potential utility of 4:
- Plasma thrombopoietin (TPO) levels: normal or slightly increased (below cutoff) in ITP
- Immature platelet fraction (IPF% or RP%): elevated above upper limit of normal in ITP, indicating peripheral destruction rather than production failure
- These markers help differentiate ITP from hypoproliferative thrombocytopenia such as aplastic anemia 4
However, a low IPF does not exclude ITP, as atypical presentations can occur requiring bone marrow biopsy for definitive diagnosis 5
Critical Diagnostic Pitfalls
- Do not confuse gestational thrombocytopenia with ITP in pregnancy—gestational thrombocytopenia typically presents with platelet counts >70,000 × 10⁹/L in late gestation without prior history of thrombocytopenia 1
- Atypical findings that suggest alternative diagnoses include: schistocytes, predominant giant platelets, red blood cell poikilocytosis, leukocytosis/leukopenia with immature cells, or any splenomegaly 1
- In children, only 4% had alternative diagnoses on bone marrow examination, and all had atypical presenting features 1