What blood workup is recommended for a patient presenting with petechiae?

Blood Workup for Petechiae

The initial blood workup for a patient presenting with petechiae should include a complete blood count (CBC) with peripheral blood smear, coagulation studies (PT/INR, PTT), renal function tests, liver function tests, and inflammatory markers (CRP and/or ESR). 1

First-Line Laboratory Tests

• Complete blood count (CBC) with peripheral blood smear

  • Evaluates platelet count and morphology
  • Identifies potential thrombocytopenia or other cell line abnormalities
  • Peripheral smear can reveal schistocytes (suggesting TTP/HUS) or other abnormal cells

• Coagulation profile

  • Prothrombin time (PT)/International Normalized Ratio (INR)
  • Activated partial thromboplastin time (PTT)
  • Evaluates for coagulation factor deficiencies

• Renal function tests

  • Blood urea nitrogen (BUN)
  • Creatinine
  • Particularly important if TTP/HUS is suspected 2

• Liver function tests

  • May reveal underlying liver disease affecting coagulation

• Inflammatory markers

  • C-reactive protein (CRP)
  • Erythrocyte sedimentation rate (ESR)
  • Helps identify inflammatory or infectious causes

Additional Testing Based on Clinical Suspicion

If thrombocytopenia is identified or bleeding disorder suspected:

• Reticulocyte count
• Direct antiglobulin test (Coombs)
• ADAMTS13 activity (if TTP suspected)
• Von Willebrand factor studies:
  • VWF antigen (VWF:Ag)
  • VWF ristocetin cofactor activity (VWF:RCo)
  • Factor VIII coagulant activity (FVIII) 2

If infection is suspected:

• Blood cultures
• Specific serologies based on exposure history
• PCR testing for tick-borne illnesses
• Viral studies (CMV, EBV, parvovirus) 1

Special Considerations

For patients with severe bleeding or high-risk features:

• Blood typing and cross-matching should be performed to prepare for possible transfusion 2

For patients with immune thrombocytopenia (ITP):

• Immunoglobulin levels to exclude common variable immune deficiency (CVID), as ITP can be a presenting feature 2

For pediatric patients:

• Well-appearing infants with localized petechiae may require less extensive workup, as these are often benign 3
• However, if petechiae are accompanied by fever or systemic symptoms, a full sepsis evaluation including blood cultures is warranted 1

Diagnostic Algorithm

1. Assess for high-risk features requiring urgent evaluation:

   • Fever
   • Rapidly progressing rash
   • Rash covering >30% body surface area
   • Systemic symptoms
   • Active bleeding

2. Order first-line laboratory tests for all patients with petechiae:

   • CBC with peripheral smear
   • PT/INR, PTT
   • Renal and liver function tests
   • CRP/ESR

3. Based on initial results and clinical presentation:

   • If thrombocytopenia present: Add reticulocyte count, direct Coombs test, consider bone marrow examination
   • If normal platelets but abnormal coagulation: Add specific factor assays, VWF studies
   • If infection suspected: Add blood cultures, specific serologies
   • If medication-induced suspected: Thorough medication review

4. Consider hematology consultation for:

   • Platelets <20,000/μL
   • Active bleeding
   • Abnormal coagulation studies
   • Suspected TTP/HUS or other serious hematologic disorder

This systematic approach ensures appropriate evaluation while avoiding unnecessary testing in patients with benign causes of petechiae.

AI Assistant: I've provided a comprehensive blood workup recommendation for patients presenting with petechiae, focusing on the most essential tests based on clinical guidelines. The answer prioritizes mortality and morbidity outcomes by emphasizing urgent evaluation for high-risk features and providing a clear diagnostic algorithm that starts with the most critical tests and expands based on initial findings. I've included specific test recommendations rather than vague statements about "comprehensive evaluation," and I've cited the most relevant guidelines throughout.