Diagnostic Criteria for Immune Thrombocytopenia (ITP)
ITP is primarily a diagnosis of exclusion, defined as a platelet count less than 100 × 10⁹/L in the absence of other causes or disorders that may be associated with thrombocytopenia. 1
Essential Diagnostic Components
History and Physical Examination
- Assess for:
- Recent viral illnesses
- Medication exposure
- Alcohol consumption
- Symptoms of underlying systemic diseases
- Family history of thrombocytopenia or bleeding disorders
- Physical findings:
- Petechiae, bruising, or mucosal bleeding may be present
- Absence of lymphadenopathy or hepatosplenomegaly (splenomegaly may be present in up to 12% of children with ITP) 1
- Normal physical examination except for bleeding manifestations
Laboratory Testing
Required tests:
- Complete blood count with differential
- Peripheral blood smear examination
- Reticulocyte count 1
Peripheral blood smear findings consistent with ITP: 1
- Thrombocytopenia
- Normal-sized or slightly larger platelets (but not giant platelets)
- Normal red blood cell morphology
- Normal white blood cell morphology
Classification by Duration
ITP is classified as: 1
- Newly diagnosed: diagnosis to 3 months
- Persistent: 3 to 12 months from diagnosis
- Chronic: lasting more than 12 months
Secondary Causes to Exclude
Common secondary causes of ITP include: 1
- Antiphospholipid syndrome
- Autoimmune disorders (e.g., Evans syndrome, SLE)
- Common variable immune deficiency
- Drug-induced thrombocytopenia
- Infections (CMV, H. pylori, HCV, HIV, varicella zoster)
- Lymphoproliferative disorders
Additional Testing Based on Clinical Context
Tests of Potential Utility
- HIV testing (recommended for all adult patients) 1
- Hepatitis C virus testing 1
- H. pylori testing 1
- Quantitative immunoglobulin levels (especially in children with persistent/chronic ITP) 1
- Direct antiglobulin test 1
- Blood group (Rh) 1
Bone Marrow Examination
- Not routinely necessary in children and adolescents with typical features of ITP 1
- Not necessary in children who fail IVIg therapy 1
- Not necessary before initiation of corticosteroid treatment or splenectomy 1
- Indicated in patients with:
Atypical Features Warranting Further Investigation
Features that are not typical of ITP and should prompt additional testing: 1
- Fever
- Bone or joint pain
- Family history of low platelets or easy bruising
- Risk factors for HIV infection
- Skeletal or soft-tissue morphologic abnormalities
- Non-petechial rash
- Lymphadenopathy
- Abnormal hemoglobin level
- Abnormal white blood cell count or morphology
Tests Generally Not Recommended
- Antinuclear antibody testing is not necessary in the evaluation of children and adolescents with suspected ITP 1
- Platelet antibody tests are not recommended for routine diagnosis due to low specificity 2
- Thrombopoietin levels 1
- Reticulated platelets 1
- Platelet-associated immunoglobulin G 1
- Platelet survival study 1
- Bleeding time 1
- Serum complement 1
Special Considerations
In Pregnancy
- Blood pressure measurement to rule out preeclampsia
- Liver function testing 2
In Children
- Higher likelihood of spontaneous remission
- Similar diagnostic approach as adults 2
- Bone marrow examination generally not required with typical presentation
Diagnostic Pitfalls to Avoid
- Relying solely on platelet count without peripheral smear examination
- Failing to exclude secondary causes of thrombocytopenia
- Performing unnecessary bone marrow examination in typical cases
- Overlooking medication-induced thrombocytopenia
- Misdiagnosing other forms of thrombocytopenia (particularly TTP) as ITP
By following these diagnostic criteria and considering the appropriate additional testing based on clinical presentation, clinicians can accurately diagnose ITP while avoiding unnecessary procedures and tests.