Blood Film (Peripheral Blood Smear) is the Confirmatory Test
A blood film (peripheral blood smear) is the confirmatory diagnostic test for a baby presenting with multiple ecchymoses approximately one week after influenza infection, as it directly visualizes and quantifies platelets to confirm thrombocytopenia and excludes other serious hematologic conditions. 1
Clinical Context: Post-Influenza Immune Thrombocytopenic Purpura
- Influenza is a well-established trigger for immune thrombocytopenic purpura (ITP) in children, with thrombocytopenia documented in 5-7% of influenza A cases 1
- The timing of approximately one week post-infection aligns with the typical latency period for immune-mediated platelet destruction following viral illness 1
- Multiple ecchymoses in an otherwise well infant after recent viral infection is pathognomonic for ITP 1
Why Blood Film is the Confirmatory Test
The blood film directly visualizes and quantifies platelets, confirming thrombocytopenia (typically platelet count <100,000/μL in influenza-associated cases) and simultaneously excludes other serious hematologic conditions such as leukemia or aplastic anemia. 1
Key Diagnostic Features on Blood Film:
- Platelet morphology assessment: Platelets in ITP are typically large and morphologically normal, distinguishing it from other causes of thrombocytopenia 1
- Evaluation of other cell lines: Confirms isolated thrombocytopenia rather than pancytopenia, which would suggest bone marrow failure or malignancy 1
- Complete blood count with differential: Should be performed in all severely ill children with suspected influenza complications, as leukopenia and lymphopenia are common in influenza, particularly in severe cases 1
Why Other Options Are Incorrect
Coagulation Profile (Option B):
- Coagulation studies (PT/PTT) assess clotting factor function, not platelet number or function 1
- In ITP, coagulation studies are typically normal because the problem is platelet quantity, not clotting cascade dysfunction 1
- The American Academy of Pediatrics specifically recommends not delaying blood film examination while waiting for coagulation studies in a child with petechiae/ecchymoses 1
Bone Marrow Biopsy (Option C):
- Bone marrow biopsy is invasive and not the initial confirmatory test 1
- The World Health Organization advises against proceeding to bone marrow biopsy without first obtaining and reviewing a peripheral blood smear 1
- Bone marrow examination is reserved for atypical presentations, failure to respond to treatment, or when the blood film suggests alternative diagnoses 1
Critical Clinical Pitfalls to Avoid
- Do not assume bleeding is due to coagulopathy without first confirming platelet count, as recommended by the Centers for Disease Control and Prevention 1
- Do not delay blood film examination while pursuing other diagnostic tests in a child with petechiae or ecchymoses 1
- Do not proceed directly to invasive testing (bone marrow biopsy) without first performing the simple, rapid, and highly informative blood film 1