Initial Investigation for Suspected ITP Following Flu-Like Illness
The first and essential investigation is a complete blood count (CBC) with peripheral blood smear examination by a qualified hematologist or pathologist 1, 2, 3. This is the cornerstone of ITP diagnosis and must be performed before any other testing.
Why CBC is the Correct Answer
A CBC is the only test among the options that is universally recommended as the initial essential investigation for suspected ITP 1, 2, 4. The diagnosis of ITP requires:
- Confirmation of isolated thrombocytopenia (platelet count <100 × 10⁹/L) without anemia or leukopenia 4, 5
- Exclusion of pseudothrombocytopenia through peripheral blood smear review 1, 2, 3
- Verification that red blood cell and white blood cell morphology are normal 1, 3
The peripheral smear should demonstrate a small number of large platelets with normal morphology, without schistocytes, leukocyte inclusion bodies, or other abnormalities that would suggest alternative diagnoses 1, 4.
Why the Other Options Are Incorrect
Coagulation Profile (Option B)
Coagulation studies are NOT routinely recommended for typical ITP presentation 1. While coagulation testing (PT, aPTT, fibrinogen, D-dimer) may be useful to evaluate for disseminated intravascular coagulation (DIC) in patients with severe thrombocytopenia and atypical features, it is not part of the initial diagnostic workup for a clear case of ITP 3. ITP is characterized by isolated thrombocytopenia with normal coagulation parameters.
Anti-Platelet Antibody Testing (Option C)
Glycoprotein-specific antiplatelet antibody assays are NOT routinely recommended because they lack diagnostic utility 1, 2. Platelet-associated IgG is elevated in both immune and non-immune thrombocytopenia, making these tests non-specific 1. ITP remains a diagnosis of exclusion without a single "gold standard" test, and antiplatelet antibody testing does not change management 3.
Algorithmic Approach After Initial CBC
Once the CBC confirms isolated thrombocytopenia:
Verify the peripheral blood smear shows findings consistent with ITP (normal-sized or large platelets, normal RBC and WBC morphology) 1, 2
Assess for red flags that suggest alternative diagnoses 3:
- Splenomegaly, hepatomegaly, or lymphadenopathy (suggests secondary causes)
- Constitutional symptoms like fever or weight loss (suggests underlying disorders)
- Abnormal hemoglobin, WBC count, or cell morphology
Consider additional targeted testing based on clinical context 1, 2:
- HIV and HCV testing in all adults regardless of risk factors
- Helicobacter pylori testing in adults where it may have clinical impact
- Pregnancy test in women of childbearing potential
- Blood group Rh(D) typing if anti-D immunoglobulin therapy is considered
Bone marrow examination is NOT necessary in typical ITP but should be considered in patients >60 years, those with systemic symptoms, or when splenectomy is being considered 1, 2, 3
Critical Pitfall to Avoid
The most important pitfall is failing to exclude pseudothrombocytopenia due to EDTA-dependent platelet agglutination 1, 2. If pseudothrombocytopenia is suspected, collect blood in a tube containing heparin or sodium citrate and repeat the platelet count 2. This prevents unnecessary treatment and anxiety from a false diagnosis.