What are the typical lab findings in a patient with Immune Thrombocytopenic Purpura (ITP)?

Laboratory Findings in Immune Thrombocytopenic Purpura (ITP)

The essential lab findings in ITP are isolated thrombocytopenia on complete blood count with a peripheral blood smear showing normal-sized or slightly enlarged platelets, normal red blood cell morphology, and normal white blood cell morphology—all other cell lines must be normal. 1

Essential Initial Laboratory Tests

Complete Blood Count (CBC):

• Isolated thrombocytopenia (platelet count <100 × 10⁹/L) is the defining feature 1
• Hemoglobin and hematocrit are normal unless there has been significant bleeding 1, 2
• White blood cell count and differential are normal (though atypical lymphocytes and eosinophilia may occur in children) 1
• The absence of anemia and leukocytopenia strongly predicts ITP diagnosis 2

Peripheral Blood Smear (Mandatory):

• Platelets appear normal in size or slightly larger than normal—consistently giant platelets approaching the size of red blood cells should be absent 1
• Normal red blood cell morphology without poikilocytosis or schistocytes (unless polychromatophilia from response to bleeding) 1
• Normal white blood cell morphology without immature or abnormal cells 1
• The smear must exclude pseudothrombocytopenia from EDTA-dependent platelet clumping 1, 3

Additional Laboratory Findings That Support ITP Diagnosis

Markers of Increased Platelet Turnover:

• Elevated immature platelet fraction (IPF >9.4%) indicates increased platelet production in response to destruction 3, 4
• Increased percentage of reticulated platelets predicts ITP diagnosis 2
• However, severe ITP can present with low IPF, requiring bone marrow examination 4, 3

Platelet Antibody Testing (Limited Utility):

• Increased frequency of anti-GPIIb/IIIa antibody-producing B cells supports ITP 2
• Increased platelet-associated anti-GPIIb/IIIa antibodies favor ITP 2
• However, the American Society of Hematology guidelines state insufficient evidence exists for routine use of platelet antibody testing 1, 3

Thrombopoietin Levels:

• Normal or slightly increased plasma thrombopoietin levels suggest ITP rather than bone marrow failure 2

Mandatory Testing to Exclude Secondary Causes

Infectious Disease Screening:

• HIV testing is mandatory in all adults with suspected ITP, regardless of risk factors 1, 3
• Hepatitis C virus testing is mandatory in all adults 1, 3
• H. pylori testing should be considered, as eradication can resolve thrombocytopenia 3

Other Tests of Uncertain Routine Necessity:

• ANA, direct antiglobulin test, lupus anticoagulant/APLA have uncertain appropriateness for routine testing at presentation 1
• Thyroid function testing may be useful, as 8-14% of ITP patients develop thyroid disease 3

When Bone Marrow Examination IS Required

Bone marrow examination is NOT necessary in typical ITP presentations 1, 3, but IS mandatory when:

• Age ≥60 years (to exclude myelodysplastic syndromes, leukemias, malignancies) 3
• Systemic symptoms present (fever, weight loss, bone pain) 1, 3
• Abnormal blood count parameters beyond isolated thrombocytopenia (anemia, leukopenia, leukocytosis) 1, 3
• Atypical peripheral smear findings (schistocytes, poikilocytosis, immature cells, leukocyte inclusion bodies) 1, 3
• Low or borderline IPF in severe thrombocytopenia 3, 4
• Failure to respond to first-line therapies (IVIg, corticosteroids, anti-D) 3

Laboratory Findings NOT Consistent with ITP

Red Blood Cell Abnormalities:

• Poikilocytosis, schistocytes, or polychromatophilia (unless from bleeding response) suggest alternative diagnoses like thrombotic microangiopathy 1, 5

White Blood Cell Abnormalities:

• Leukocytosis or leukopenia with immature or abnormal cells suggests bone marrow disorders 1

Platelet Abnormalities:

• Predominantly giant platelets approaching red blood cell size suggest inherited thrombocytopenias 1

Coagulation Studies:

• PT, aPTT, fibrinogen, and D-dimer should be normal in ITP 3
• Abnormal coagulation studies suggest DIC or other consumptive coagulopathies 3, 5

Critical Diagnostic Pitfalls to Avoid

• Never diagnose ITP without reviewing the peripheral blood smear personally—automated counts can miss pseudothrombocytopenia, giant platelets, or schistocytes 1, 3
• Do not assume elevated IPF definitively confirms ITP—severe ITP can present with low IPF requiring bone marrow examination 3, 4
• Never skip HIV and HCV testing in adults—these infections can be clinically indistinguishable from primary ITP and may precede other symptoms by years 1, 3
• Presence of splenomegaly, hepatomegaly, or lymphadenopathy excludes primary ITP—pursue alternative diagnoses aggressively 1, 3, 6
• In patients >60 years, always perform bone marrow examination regardless of how "typical" the presentation appears 3