What is the confirmatory test for a baby who had influenza followed by multiple ecchymoses (bruises)?

Confirmatory Test for Post-Influenza Ecchymosis in an Infant

A blood film (peripheral blood smear) is the confirmatory test for this infant presenting with multiple ecchymoses following influenza infection, as this clinical picture strongly suggests immune thrombocytopenic purpura (ITP), and the blood film will confirm thrombocytopenia while excluding other hematologic abnormalities.

Clinical Context and Diagnosis

This presentation is classic for post-viral ITP, one of the most common acquired bleeding disorders in children:

• Influenza is a known trigger for ITP in children, with thrombocytopenia documented in 5-7% of influenza A cases 1
• The timing (approximately 1 week post-infection) fits the typical latency period for immune-mediated platelet destruction following viral illness
• Multiple ecchymoses (bruising) in an otherwise well infant after recent viral infection is pathognomonic for ITP

Diagnostic Approach

Initial Test: Blood Film (Option A)

The blood film serves as the confirmatory test because it:

• Directly visualizes and quantifies platelets, confirming thrombocytopenia (typically platelet count <100,000/μL in influenza-associated cases) 1
• Excludes other serious hematologic conditions such as leukemia, aplastic anemia, or microangiopathic hemolytic anemia that could present similarly
• Assesses platelet morphology - in ITP, platelets are typically large and morphologically normal, just reduced in number
• Evaluates other cell lines to ensure isolated thrombocytopenia rather than pancytopenia

Supporting Laboratory Tests

While the blood film is confirmatory, guidelines recommend comprehensive evaluation in severely ill children:

• Complete blood count with differential should be performed in all severely ill children with suspected influenza complications 1
• Leukopenia (WBC <4.0) and lymphopenia are common in influenza, particularly in severe cases 1

Role of Other Options

Option B (Coagulation Profile):

• Not the confirmatory test because ITP is a disorder of platelet destruction, not coagulation factor deficiency
• Coagulation studies (PT/aPTT) are typically normal in ITP, which helps distinguish it from coagulopathies
• May be ordered as part of the workup but does not confirm the diagnosis

Option C (Bone Marrow Biopsy):

• Not indicated as initial confirmatory test in typical post-viral ITP
• Reserved for atypical presentations: failure to respond to treatment, abnormal blood film findings suggesting other pathology, or concern for malignancy
• Invasive and unnecessary when clinical picture and blood film are consistent with ITP

Common Pitfalls to Avoid

• Do not delay blood film examination while waiting for coagulation studies in a child with petechiae/ecchymoses
• Do not assume bleeding is due to coagulopathy without first confirming platelet count
• Do not proceed to bone marrow biopsy without first obtaining and reviewing a peripheral blood smear
• Remember that thrombocytopenia was documented in 4 of 7 Vietnamese children with H5N1 influenza 1, highlighting the importance of checking platelet counts in post-influenza complications

Clinical Significance

The identification of thrombocytopenia impacts immediate management decisions regarding bleeding precautions, activity restrictions, and potential need for platelet transfusion or immunoglobulin therapy if severe bleeding occurs. Most cases of post-viral ITP in children resolve spontaneously within weeks to months, but severe thrombocytopenia (<20,000/μL) may require intervention to prevent life-threatening hemorrhage.