Complete Blood Count (CBC) is the Most Appropriate Initial Test
A Complete Blood Count (CBC) with platelet count is the essential first step to confirm the diagnosis in a child presenting with ecchymosis 2 weeks after an infection, as this clinical presentation strongly suggests immune thrombocytopenia (ITP). 1
Clinical Context and Differential Diagnosis
The presentation of ecchymosis (bruising) occurring 2 weeks after an infection in a child is a classic pattern for post-infectious immune thrombocytopenia (ITP), which is a transient, often self-resolving bleeding disorder characterized by low platelet count. 1 This temporal relationship between infection and bleeding manifestations is highly characteristic of this condition. 2
Why CBC is the Correct Initial Test
The CBC with platelet count is the fundamental screening test that will immediately identify thrombocytopenia, which is the most likely cause in this clinical scenario. 3, 1
Physical examination findings of ecchymoses, along with CBC results, provide the diagnostic foundation for bleeding disorders in children. 3
The CBC is an essential diagnostic component in numerous clinical situations involving hematologic problems in children and serves as a powerful diagnostic tool when used appropriately. 4, 2
Why Other Options Are Less Appropriate
Blood Film (Option A)
- While blood film examination can provide additional morphologic information, it is a secondary step after the CBC confirms abnormal cell counts. 5
- The blood film does not quantify platelet numbers as precisely as automated CBC analysis. 5
Coagulation Time (Option C)
- Coagulation studies (PT/aPTT) do not detect thrombocytopenia or platelet disorders, which are the most likely causes in this scenario. 1
- These tests are useful for detecting factor deficiencies but would be normal in isolated ITP. 1
- Coagulation testing should be considered as part of a broader evaluation if the CBC shows normal platelets, but this is not the initial priority. 3
Bone Marrow Aspiration (Option D)
- Bone marrow examination is not an initial diagnostic test and should only be performed if the etiology remains unclear after initial studies. 5
- This invasive procedure is reserved for cases where the diagnosis cannot be established through less invasive means or when there are atypical features suggesting malignancy or bone marrow failure. 5
Diagnostic Algorithm After CBC
Once the CBC confirms thrombocytopenia:
If platelet count is below 100,000/mm³, further evaluation should include maternal history, physical examination for signs of intrauterine infection or congenital syndromes, and consideration of bone marrow examination only if the etiology remains unclear. 5
Additional testing may include coagulation profile if there are concerns beyond isolated thrombocytopenia, though this is typically normal in ITP. 6
Well-appearing children with localized ecchymosis and normal CBC likely have a benign etiology and may only require observation. 6
Critical Pitfall to Avoid
Do not assume that normal PT/aPTT rules out all bleeding disorders, as these tests miss von Willebrand disease, Factor XIII deficiency, and platelet function disorders. 1 However, in the specific context of post-infectious ecchymosis in a child, thrombocytopenia from ITP is the primary concern, making CBC the definitive initial test.