How is immune thrombocytopenia diagnosed?

How to Diagnose ITP

ITP is diagnosed by demonstrating isolated thrombocytopenia (platelet count <100 × 10⁹/L) while systematically excluding all other causes of low platelets through history, physical examination, complete blood count, and peripheral blood smear evaluation. 1

Essential Diagnostic Components

Basic Evaluation (Required for All Patients)

• Patient and family history to identify secondary causes including HIV, HCV, autoimmune disorders (especially SLE), lymphoproliferative malignancies, liver disease, medications (prescription and over-the-counter), alcohol use, quinine consumption, recent vaccinations, transfusions, and inherited thrombocytopenia syndromes 1

• Physical examination should reveal only bleeding manifestations (petechiae, purpura, mucosal bleeding); any moderate-to-massive splenomegaly, hepatomegaly, lymphadenopathy, fever, or weight loss suggests an alternative diagnosis 1

• Complete blood count with reticulocyte count must show isolated thrombocytopenia with otherwise normal cell lines; anemia, if present, should be proportional to bleeding duration and may indicate iron deficiency 1

• Peripheral blood smear review by a qualified hematologist or pathologist is paramount to exclude pseudo-thrombocytopenia (EDTA-dependent platelet clumping), schistocytes (TTP-HUS), leukocyte inclusion bodies (MYH9-related disease), and abnormal platelet size suggesting inherited disorders 1

Additional Testing for Adult Patients

The following tests are recommended for all adults regardless of geographic location 1:

• HIV and HCV serologic testing because these infections can present identically to primary ITP and may precede other symptoms by years 1

• Helicobacter pylori testing (preferably urea breath test or stool antigen test, not serology which has lower sensitivity/specificity and false positives after IVIg) should be considered where it may impact clinical management 1

• Direct antiglobulin test to evaluate for immune hemolysis 1

• Quantitative immunoglobulin levels particularly in children with persistent/chronic ITP 1

Selective Testing Based on Clinical Context

Consider these tests when specific clinical features are present 1:

• Antinuclear antibodies if SLE is suspected
• Antiphospholipid antibodies (anticardiolipin, lupus anticoagulant) in appropriate clinical settings
• Thyroid function and antithyroid antibodies for autoimmune thyroid disease
• Pregnancy test in women of childbearing potential
• Blood group (Rh) if anti-D therapy is being considered
• Viral PCR for parvovirus and CMV in specific circumstances

Bone Marrow Examination

Bone marrow examination is not routinely required but should be performed in 1:

• Patients older than 60 years
• Presence of systemic symptoms or abnormal physical findings
• Atypical features on blood smear or CBC
• Patients being considered for splenectomy

When performed, obtain both aspirate and biopsy with flow cytometry and cytogenetic testing; flow cytometry is particularly useful for detecting CLL-associated ITP 1

Emerging Diagnostic Criteria

A 2024 Japanese guideline proposes more specific criteria to differentiate ITP from aplastic anemia 2:

1. Isolated thrombocytopenia without dysplasia on blood smear
2. Normal or slightly increased plasma thrombopoietin (TPO) levels (< cutoff value)
3. Elevated immature platelet fraction (RP% or IPF% > upper limit of normal)
4. Absence of other conditions causing thrombocytopenia

Cases not meeting criteria 2 or 3 are classified as "possible ITP" and diagnosed by typical clinical course 2

Key Diagnostic Pitfalls to Avoid

• Pseudo-thrombocytopenia from EDTA-dependent platelet clumping must be excluded by examining the blood smear 1

• Inherited thrombocytopenias should be suspected with persistent platelet counts consistently >12 × 10⁹/L, positive family history, or atypical features; platelet counts ≤12 × 10⁹/L have 60% sensitivity and 80% specificity for ITP versus other causes 3

• Secondary ITP from HIV, HCV, SLE, or lymphoproliferative disorders can be clinically indistinguishable from primary ITP, making serologic testing essential 1

• Response to ITP-specific therapy (IVIg, anti-D) supports but does not confirm the diagnosis, as secondary ITP may also respond 1

Tests of Unproven Benefit

The following tests are not recommended for routine ITP diagnosis 1:

• Platelet-associated immunoglobulin G (PaIgG)
• Platelet survival studies
• Bleeding time
• Serum complement
• Thrombopoietin levels (except in specific research contexts)
• Reticulated platelets (except as part of emerging diagnostic criteria)