How is immune thrombocytopenia (ITP) diagnosed?

How to Diagnose ITP

ITP is fundamentally a diagnosis of exclusion, established when isolated thrombocytopenia (platelet count <100 × 10⁹/L) is confirmed on peripheral blood smear without other identifiable causes, based on targeted history, physical examination, and selective laboratory testing. 1

Core Diagnostic Criteria

ITP requires:

• Platelet count <100 × 10⁹/L (not <150 × 10⁹/L as in other conditions)
• Isolated thrombocytopenia - all other blood counts must be normal
• No alternative explanation for the low platelet count

Essential Initial Workup

Mandatory Basic Evaluation 1

Every patient requires:

• Complete blood count with reticulocyte count - confirms isolated thrombocytopenia; anemia should only be proportional to bleeding

• Peripheral blood smear review by qualified hematologist/pathologist - this is paramount and non-negotiable; excludes schistocytes (TTP), giant/small platelets (inherited thrombocytopenias), leukocyte inclusions (MYH9 disease), and EDTA-induced pseudothrombocytopenia

• Detailed patient history focusing on:

  • Drug exposures (heparin, quinidine/quinine, sulfonamides, alcohol)
  • HIV/HCV risk factors or known infection
  • Autoimmune disease symptoms (arthralgias, rash, alopecia)
  • Recent infections or vaccinations
  • Family history of thrombocytopenia or bleeding
  • Bleeding with prior surgeries/pregnancies

• Physical examination specifically assessing:

  • Bleeding manifestations only (petechiae, purpura, mucosal bleeding)
  • Absence of splenomegaly (moderate/massive splenomegaly excludes ITP; <3% have mild splenomegaly) 2
  • Absence of lymphadenopathy, hepatomegaly, fever, weight loss (suggests alternative diagnosis)

Selective Additional Testing 1

Tests of potential utility based on clinical context:

• HIV and HCV serology - recommended for all adults regardless of geographic location 1
• Direct antiglobulin test - if hemolytic anemia suspected
• Quantitative immunoglobulin levels - particularly in children with persistent/chronic ITP
• Pregnancy test - all women of childbearing potential
• H. pylori testing (urea breath test or stool antigen preferred over serology) - consider in adults where treatment would be pursued 1
• Antiphospholipid antibodies - if thrombosis history or other autoimmune features
• Thyroid function and antithyroid antibodies - if clinical suspicion

Bone Marrow Examination

When NOT needed (strong recommendations): 3

• Children and adolescents with typical ITP features (Grade 1B)
• Children who fail IVIg therapy (Grade 1B)
• Before corticosteroid initiation in typical cases (Grade 2C)

When to consider: 1

• Age >60 years
• Systemic symptoms or abnormal signs
• Abnormal blood counts beyond isolated thrombocytopenia
• Before splenectomy in some cases
• When performed: obtain both aspirate and biopsy; consider flow cytometry and cytogenetics

Tests That Are NOT Helpful

Do not order: 1, 2

• Platelet-associated IgG (PaIgG) - inappropriate and unnecessary
• Thrombopoietin (TPO) levels - unproven benefit
• Reticulated platelets - unproven benefit
• Platelet survival studies - unnecessary
• Bleeding time - unproven benefit
• Serum complement - unnecessary
• Antinuclear antibodies - not necessary in children/adolescents 3

Glycoprotein-specific platelet antibodies (MAIPA method): High specificity but only ~50% sensitivity; supportive when positive but does not exclude ITP when negative 4, 5, 6

Critical Pitfalls to Avoid

1. Pseudothrombocytopenia: Always confirm on peripheral smear; EDTA-dependent platelet clumping occurs in ~0.1% of adults 2, 1

2. Missing secondary causes:

   • Drug-induced (especially heparin, quinidine, sulfonamides)
   • HIV/HCV infection (may precede other symptoms by years) 1
   • Autoimmune diseases (SLE, antiphospholipid syndrome)
   • Lymphoproliferative disorders (CLL can present as ITP)

3. Splenomegaly presence: Moderate or massive splenomegaly essentially excludes primary ITP 1

4. Abnormal blood counts: Any abnormality beyond isolated thrombocytopenia (and proportional anemia from bleeding) requires alternative explanation 1

5. Atypical smear findings: Schistocytes, abnormal white cells, or platelet size abnormalities mandate further investigation 1

Diagnostic Algorithm Summary

Step 1: Confirm true thrombocytopenia on peripheral smear (exclude pseudothrombocytopenia)

Step 2: Verify isolated thrombocytopenia with normal CBC otherwise (proportional anemia from bleeding acceptable)

Step 3: Expert peripheral smear review - must show no alternative findings

Step 4: Targeted history and physical excluding secondary causes and alternative diagnoses

Step 5: Selective testing: HIV/HCV in adults, pregnancy test in women, consider H. pylori

Step 6: Bone marrow only if: age >60, atypical features, systemic symptoms, or abnormal counts

If all above consistent with ITP and no alternative diagnosis identified → Diagnosis established

The diagnosis remains clinical and based on exclusion; no single confirmatory test exists. Response to ITP-specific therapy (IVIg, anti-D, corticosteroids) supports but does not confirm the diagnosis, as secondary ITP may also respond 1.