What is the initial workup for a patient presenting with petechiae and purpura?

Initial Workup for Petechiae and Purpura

The initial workup for petechiae and purpura should include a complete blood count with peripheral smear, coagulation studies, and focused laboratory testing based on suspected etiology, as these skin findings represent a symptom rather than a disease and require identification of the underlying cause for effective management.

Diagnostic Approach

Step 1: Assess Clinical Urgency

• Urgent evaluation required if:
  • Petechiae with fever
  • Rapidly progressing rash
  • Rash covering >30% body surface area
  • Petechial/purpuric rash with systemic symptoms 1
  • Non-blanching petechial or purpuric rash (consider sepsis evaluation) 2

Step 2: Essential Laboratory Tests

• First-line laboratory tests:
  • Complete blood count (CBC) with peripheral blood smear
  • Coagulation profile (PT/INR, PTT)
  • Renal function (BUN, creatinine)
  • Liver function tests
  • C-reactive protein (CRP) and/or erythrocyte sedimentation rate (ESR)

Step 3: Focused Testing Based on Clinical Presentation

For suspected hematologic disorders:

• Reticulocyte count
• Direct antiglobulin test (Coombs)
• ADAMTS13 activity (if TTP suspected)
• Bone marrow examination (if indicated by blood counts) 2

For suspected infectious causes:

• Blood cultures
• Specific serologies based on exposure history
• PCR testing for tick-borne illnesses if indicated 2
• Viral studies (CMV, EBV, parvovirus) if indicated 2

For suspected vasculitis/autoimmune:

• Antinuclear antibody (ANA)
• ANCA panel
• Complement levels (C3, C4, CH50)
• Cryoglobulins if indicated

Management Algorithm Based on Presentation Pattern

Pattern 1: Petechiae with Thrombocytopenia

1. If platelets <20,000/μL with active bleeding:

   • Consider immediate platelet transfusion
   • Hematology consultation
   • Evaluate for ITP, TTP, DIC, or drug-induced thrombocytopenia

2. If platelets 20,000-50,000/μL without active bleeding:

   • Close monitoring
   • Consider treatment based on bleeding risk 2

Pattern 2: Petechiae with Normal Platelets

1. If localized petechiae in well-appearing patient:

   • Consider mechanical causes (pressure, tourniquet phenomenon)
   • Observation may be sufficient 3

2. If generalized petechiae with normal platelets:

   • Evaluate for coagulation disorders
   • Consider vascular fragility disorders
   • Assess medication history for potential drug reactions 1

Pattern 3: Petechiae with Fever

1. Immediate empiric antibiotics if sepsis suspected

   • Do not delay treatment while awaiting laboratory confirmation 2
   • Consider doxycycline if tick-borne illness possible

2. Evaluate for meningococcemia or other bacterial sepsis

   • Blood cultures before antibiotics if possible
   • Lumbar puncture if meningitis suspected 2

Special Considerations

Age-specific Approach

• Children:

  • Well-appearing infants with localized petechiae may have benign causes 3
  • Consider ITP, which may require only observation if no significant bleeding 2

• Adults:

  • Higher suspicion for medication-induced causes
  • Consider autoimmune conditions, malignancy, or liver disease

Common Pitfalls to Avoid

1. Delaying treatment for suspected RMSF while awaiting laboratory confirmation

   • Up to 20% of RMSF cases may have absent or atypical rash 1
   • Early doxycycline treatment is critical for survival

2. Misdiagnosing thrombotic thrombocytopenic purpura (TTP) as idiopathic thrombocytopenic purpura (ITP)

   • Check for neurological symptoms, renal dysfunction, and microangiopathic hemolytic anemia 4
   • ADAMTS13 testing is essential to distinguish between these conditions

3. Overlooking medication-induced causes

   • Obtain thorough medication history, including recent changes
   • Consider immune checkpoint inhibitors in patients receiving cancer treatment 2

4. Missing non-infectious systemic diseases

   • Consider vasculitis, connective tissue diseases when infection is ruled out 5

By following this systematic approach, clinicians can efficiently identify the underlying cause of petechiae and purpura, leading to appropriate treatment and improved patient outcomes.