What is the appropriate workup for an infant presenting with petechiae (small spots due to bleeding under the skin)?

Workup for Petechiae in Infants

Obtain a complete blood count (CBC) with differential and peripheral blood smear as the essential initial bloodwork for any infant presenting with petechiae, while simultaneously assessing for life-threatening conditions that require immediate intervention. 1, 2

Immediate Clinical Assessment

Before ordering any tests, rapidly evaluate for red flags that indicate life-threatening conditions:

• Assess appearance and vital signs: An ill-appearing infant with fever and petechiae requires immediate parenteral antibiotics without waiting for test results, as meningococcal disease can progress to purpura fulminans within hours 2, 3
• Evaluate distribution of petechiae: Petechiae on the trunk, face, or ears (especially in a non-cruising infant) should raise concern for abusive head trauma and warrant careful examination for scalp bruising, bulging fontanel, subconjunctival hemorrhage, or oral injuries 4
• Check for systemic symptoms: Fever, severe headache, altered mental status, hypotension, or rapidly progressive rash indicate potential meningococcemia or Rocky Mountain Spotted Fever requiring emergent treatment 3

Essential Laboratory Workup

Core testing for all infants with petechiae:

• CBC with differential and peripheral blood smear: This is the fundamental test to evaluate hemoglobin, white blood cell count, platelet count, and confirm true thrombocytopenia versus pseudothrombocytopenia 1, 2
• Peripheral smear review: Critical for assessing platelet size, evaluating red and white cell morphology, and identifying schistocytes that may indicate thrombotic microangiopathy 1

Expanded Workup Based on Clinical Findings

Obtain additional testing if ANY of these red flags are present:

• Fever or history of fever
• Bone or joint pain
• Lymphadenopathy or hepatosplenomegaly
• Abnormal hemoglobin, white blood cell count, or white cell morphology on initial CBC
• Non-petechial rash
• Family history of thrombocytopenia or easy bruising
• Skeletal or soft-tissue abnormalities 1, 2

Additional tests for red flag cases:

• Coagulation studies (PT, aPTT, fibrinogen) if bleeding disorder suspected 3
• Blood cultures if febrile or systemically ill 3
• Inflammatory markers (ESR, CRP) if serious bacterial infection suspected, though CRP >6 mg/L alone is poorly correlated with serious illness 5

Tests to AVOID

Do not routinely order:

• Antiplatelet antibodies, antiphospholipid antibodies, antinuclear antibodies, or thrombopoietin levels in suspected immune thrombocytopenia (ITP) 1
• Bone marrow examination in children with typical ITP features (isolated thrombocytopenia with otherwise normal CBC and smear) 1, 2
• H. pylori testing in children with ITP 2

Special Considerations for Well-Appearing Infants

For well-appearing infants without fever and with localized petechiae (particularly lower extremities):

• The majority have benign etiologies such as mechanical causes (tourniquet phenomenon from diapers or clothing) 6
• A 4-hour observation period may be sufficient if initial CBC and coagulation profile are normal and there is no progression of signs 6
• Extensive testing and antibiotic treatment may not be necessary and have potential to cause harm 7

Critical Pitfalls to Avoid

• Never delay antibiotics in ill-appearing infants with fever and petechiae while awaiting test results 2
• Do not miss child abuse: Petechiae on trunk, face, or ears in a non-cruising infant warrant thorough evaluation for abusive head trauma, even if the physical examination is otherwise normal 4
• Do not wait for antibody testing to diagnose ITP—it is a clinical diagnosis based on history, examination, CBC, and smear 1
• Consider geographic and seasonal factors: Rocky Mountain Spotted Fever should be considered throughout the contiguous United States, not just endemic regions, and petechial involvement of palms and soles indicates advanced disease 1, 3