What tests are used to diagnose Immune Thrombocytopenic Purpura (ITP)?

Diagnostic Tests for Immune Thrombocytopenic Purpura (ITP)

The diagnosis of ITP is primarily based on history, physical examination, complete blood count, and peripheral blood smear examination, which should exclude other causes of thrombocytopenia. 1 Additional testing is generally unnecessary in patients with typical presentation of ITP.

Essential Diagnostic Tests

First-line Tests (Required for All Patients)

• Complete blood count (CBC): Shows isolated thrombocytopenia with otherwise normal blood counts 1
• Peripheral blood smear examination: Critical for diagnosis and must be reviewed by a qualified hematologist or pathologist 1
  • Should show normal-sized or slightly larger platelets
  • Normal red blood cell morphology
  • Normal white blood cell morphology
  • Absence of schistocytes, blasts, or other abnormal cells 1

Physical Examination Findings

• Typically normal except for bleeding manifestations
• Mild splenomegaly may be present in younger patients
• Moderate or massive splenomegaly suggests alternative diagnosis 1
• Absence of hepatomegaly, lymphadenopathy, or constitutional symptoms 1

Selective Additional Testing

Testing Based on Clinical Presentation

1. HIV testing: Recommended for all adult patients with suspected ITP 1
2. Hepatitis C virus (HCV) testing: Recommended for all adult patients with suspected ITP 1
3. Bone marrow examination: Not routinely needed but indicated in:
   • Patients older than 60 years
   • Those with systemic symptoms or abnormal signs
   • Patients with persistent thrombocytopenia (>6-12 months)
   • Those unresponsive to initial therapy 1

Additional Testing Based on Clinical Suspicion

• Abdominal ultrasound or CT scan: Only if splenomegaly is suspected on physical examination 1, 2
• Helicobacter pylori testing: Consider in adults where it may have clinical impact, preferably with urea breath test or stool antigen test 1
• Blood group Rh(D) typing: Important if anti-D immunoglobulin therapy is being considered 1

Tests of Uncertain Utility (Not Routinely Recommended)

• Antiplatelet antibody assays: Not routinely recommended as platelet-associated IgG is elevated in both immune and non-immune thrombocytopenia 1
• Antiphospholipid antibodies: Not recommended in absence of symptoms of antiphospholipid syndrome 1
• Antinuclear antibodies (ANA): May predict chronicity in childhood ITP but not routinely needed 1
• Antithyroid antibodies: Consider only if symptoms of thyroid disease are present 1

Algorithm for ITP Diagnosis

1. Initial evaluation:

   • CBC showing isolated thrombocytopenia (platelet count <100 × 10^9/L)
   • Peripheral blood smear examination
   • History and physical examination to rule out other causes

2. If typical presentation (isolated thrombocytopenia with normal blood smear):

   • Test for HIV and HCV in all adults
   • No further testing needed in children with typical presentation

3. If atypical features present (abnormal CBC beyond thrombocytopenia, abnormal physical findings):

   • Bone marrow examination
   • Additional testing based on suspected underlying conditions

Common Pitfalls to Avoid

1. Overdiagnosis: Performing unnecessary tests in patients with typical ITP presentation 1
2. Missing secondary causes: Failing to test for HIV or HCV in adults 1
3. Pseudo-thrombocytopenia: Not excluding EDTA-dependent platelet agglutination 1
4. Overlooking inherited thrombocytopenias: Not carefully examining peripheral smear for giant or small platelets 1
5. Routine bone marrow examination: No longer considered necessary at diagnosis regardless of age in patients with typical ITP 1

ITP remains a diagnosis of exclusion, and the diagnostic approach should focus on ruling out other causes of thrombocytopenia rather than confirming ITP directly 3.