What are the key laboratory findings in a patient with Immune Thrombocytopenic Purpura (ITP)?

Laboratory Findings in Immune Thrombocytopenic Purpura (ITP)

Essential Laboratory Features

The diagnosis of ITP requires isolated thrombocytopenia (platelet count <100 × 10⁹/L) with a normal or slightly enlarged platelet size on peripheral blood smear, accompanied by normal red blood cell and white blood cell morphology. 1

Complete Blood Count Findings

• Isolated thrombocytopenia is the defining laboratory feature, with platelet counts typically below 100 × 10⁹/L 1
• Hemoglobin and hematocrit are normal unless there has been significant bleeding, in which case microcytic anemia from chronic blood loss may be present 1
• White blood cell count and differential are normal, though atypical lymphocytes and eosinophilia may occasionally occur in children with ITP 2, 1
• The presence of any cytopenias beyond isolated thrombocytopenia argues against ITP and mandates investigation for alternative diagnoses 1

Peripheral Blood Smear Characteristics

A peripheral blood smear examination is essential and must show specific features consistent with ITP: 2, 1

• Platelets appear normal in size or slightly larger than normal; consistently giant platelets approaching the size of red blood cells should be absent and suggest inherited thrombocytopenias 2, 1
• Red blood cell morphology is normal without poikilocytosis or schistocytes (unless polychromatophilia is present from response to bleeding) 2, 1
• White blood cell morphology is normal without immature or abnormal cells 2, 1
• The smear must exclude pseudothrombocytopenia caused by EDTA-dependent platelet clumping, which can falsely lower automated platelet counts 1

Laboratory Findings NOT Consistent with ITP

The following findings exclude the diagnosis of ITP and require alternative investigation: 2, 1

• Red blood cell poikilocytosis, schistocytes, or predominant giant platelets suggest thrombotic microangiopathy or inherited thrombocytopenias 2, 1
• Leukocytosis or leukopenia with immature or abnormal cells suggests bone marrow disorders such as leukemia or myelodysplastic syndrome 2, 1
• Abnormal coagulation studies (PT, aPTT, fibrinogen, D-dimer) suggest disseminated intravascular coagulation rather than ITP 1

Mandatory Testing to Exclude Secondary Causes

All adults with suspected ITP must undergo testing for HIV and hepatitis C virus, regardless of risk factors, as these infections can be clinically indistinguishable from primary ITP and may precede other symptoms by years. 1

• HIV testing is mandatory in all adults with suspected ITP 1
• Hepatitis C virus testing is mandatory in all adults with suspected ITP 1
• H. pylori screening should be considered, as eradication therapy can resolve thrombocytopenia in some patients 1

Bone Marrow Examination Indications

Bone marrow examination is NOT routinely necessary in patients with typical ITP features. 2, 1

However, bone marrow examination is mandatory in the following situations: 1

• Age ≥60 years to exclude myelodysplastic syndromes, leukemias, or other malignancies 1
• Presence of systemic symptoms such as fever, weight loss, or bone pain 1
• Abnormal blood count parameters beyond isolated thrombocytopenia (anemia, leukopenia, leukocytosis) 1
• Atypical peripheral smear findings such as schistocytes or leukocyte inclusion bodies 1
• Minimal or no response to first-line ITP therapies (IVIg, corticosteroids, anti-D immunoglobulin) 1

In children with typical ITP features, bone marrow examination identified alternative diagnoses in only 4% of cases, all of whom had atypical presenting features 2

Immature Platelet Fraction (IPF)

• IPF is typically elevated in ITP, reflecting increased platelet production in response to peripheral destruction 1, 3
• However, severe ITP can present with low IPF, which complicates diagnosis and may necessitate bone marrow biopsy to exclude bone marrow failure 1, 3
• IPF should never be used as the sole diagnostic criterion, and patients with low IPF require bone marrow examination if other atypical features are present 1

Tests of Uncertain or No Diagnostic Value

The following tests are NOT routinely recommended for ITP diagnosis: 2, 1

• Platelet antigen-specific antibody testing has insufficient evidence for routine diagnostic use and does not change management 2, 1
• Platelet-associated IgG testing lacks specificity and is not recommended 2
• ANA, direct antiglobulin test, and lupus anticoagulant/antiphospholipid antibodies are of uncertain appropriateness at initial presentation but may be considered in patients who fail to respond to primary treatment 2
• Coagulation studies (PT, aPTT) are not indicated in typical ITP, as it is a disorder of platelet destruction, not coagulation factor deficiency 1

Critical Diagnostic Pitfalls to Avoid

• Never diagnose ITP without personally reviewing the peripheral blood smear, as automated counts can miss pseudothrombocytopenia, giant platelets, or schistocytes 1
• Never skip HIV and HCV testing in adults, as these infections may be clinically indistinguishable from primary ITP 1
• Presence of splenomegaly, hepatomegaly, or lymphadenopathy excludes primary ITP and mandates aggressive pursuit of alternative diagnoses such as lymphoproliferative disorders 1, 4
• Do not overlook drug-induced thrombocytopenia; obtain a detailed medication history including over-the-counter medications and herbal supplements 1
• Constitutional symptoms (fever, weight loss) suggest underlying disorders rather than primary ITP and require bone marrow examination 1