What initial bloodwork is recommended for a pediatric patient presenting with petechiae?

Initial Bloodwork for Pediatric Petechiae

The essential initial bloodwork for a pediatric patient presenting with petechiae consists of a complete blood count (CBC) with differential and peripheral blood smear review 1, 2.

Core Laboratory Evaluation

Tier 1 Screening Tests

The following tests constitute the fundamental workup:

• Complete Blood Count (CBC) with differential - This identifies isolated thrombocytopenia versus other cytopenias and evaluates hemoglobin, white blood cell count, and platelet count 1, 2
• Peripheral blood smear examination - Critical for assessing platelet size, confirming true thrombocytopenia, evaluating red and white cell morphology, and identifying schistocytes 1, 2

When Additional Testing is NOT Needed

If the history, physical examination, CBC, and peripheral blood smear are typical of immune thrombocytopenia (ITP), no further testing is required 1. The American Society of Hematology specifically recommends against routine testing for antiplatelet antibodies, antiphospholipid antibodies, antinuclear antibodies, thrombopoietin levels, or platelet parameters from automated analyzers in children with suspected ITP 1.

Bone marrow examination is not necessary in children with typical features of ITP 1.

Red Flags Requiring Expanded Workup

Additional testing beyond CBC and smear should be pursued when atypical features are present:

• Fever - suggests infectious etiology requiring broader evaluation 1
• Bone or joint pain 1
• Family history of thrombocytopenia or easy bruising 1
• Lymphadenopathy or hepatosplenomegaly 1
• Abnormal hemoglobin level, white blood cell count, or white cell morphology 1, 2
• Non-petechial rash 1
• Skeletal or soft-tissue morphologic abnormalities 1

When these features are present, bone marrow evaluation and additional testing should be performed to rule out leukemia, aplastic anemia, or other serious disorders 1.

Context-Specific Considerations

For MIS-C Evaluation (if epidemiologic link to SARS-CoV-2)

If multisystem inflammatory syndrome in children (MIS-C) is suspected, tier 1 evaluation includes:

• CBC, comprehensive metabolic panel (CMP), ESR, CRP, and SARS-CoV-2 PCR and/or serologies 1
• Proceed to tier 2 testing if platelet count <150,000/µL, absolute lymphocyte count <1,000/µL, or sodium <135 mmol/L 1

For Well-Appearing Infants with Localized Petechiae

In well-appearing infants under 8 months with localized petechiae (particularly lower extremities), no fever, and no systemic signs, the etiology is typically benign (often mechanical/tourniquet phenomenon) 3. A CBC may be sufficient, with observation for 4 hours to assess for progression 3.

Common Pitfalls to Avoid

• Do not delay diagnosis waiting for antibody testing - ITP is a clinical diagnosis based on history, examination, CBC, and smear 1
• Do not routinely perform bone marrow examination - A retrospective study of 332 children with typical ITP features found no cases of acute leukemia and only one case of bone marrow aplasia 1
• Do not assume geographic immunity from tickborne illness - Rocky Mountain spotted fever should be considered throughout the contiguous United States, not just endemic regions 1
• Recognize that broad-spectrum antibiotics (penicillins, cephalosporins, aminoglycosides) are ineffective against rickettsial diseases if this etiology is suspected 1