What lab tests are most helpful to evaluate bruising in a 7-month-old boy with no prior history of trauma or bleeding?

Laboratory Evaluation for Bruising in a 7-Month-Old Boy

Prothrombin time, activated partial thromboplastin time, von Willebrand antigen, ristocetin cofactor, and factor VIII and IX levels are the most appropriate laboratory tests to evaluate bruising in this 7-month-old boy with no prior history of trauma or bleeding. 1

Rationale for Laboratory Selection

When evaluating unexplained bruising in a non-mobile infant, it's critical to rule out both potential child abuse and bleeding disorders. The pattern of bruising on the ear, chin, buttocks, and pretibial surfaces without a history of trauma raises significant concerns.

Why This Panel is Most Appropriate:

1. Comprehensive Coagulation Screening: PT and aPTT are essential first-line tests that can detect most factor deficiencies 1

   • PT evaluates the extrinsic and common pathways (factors VII, X, V, II, and fibrinogen)
   • aPTT evaluates the intrinsic and common pathways (factors XII, XI, IX, VIII, X, V, II, and fibrinogen)

2. Detection of von Willebrand Disease: VWD is the most common inherited bleeding disorder (prevalence up to 1/1000) 1, 2

   • Von Willebrand antigen and ristocetin cofactor activity are required to diagnose VWD
   • Standard PT/aPTT may be normal in VWD despite clinically significant bleeding tendency

3. Hemophilia Assessment: Factor VIII and IX levels specifically evaluate for hemophilia A and B 1

   • Mild deficiencies in factor VIII or IX might not cause abnormalities in aPTT but could still result in significant bleeding

Why Other Test Panels Are Less Appropriate:

• Liver function tests (AST, ALT, LDH) with uric acid: These primarily assess liver function and tumor lysis, not primary bleeding disorders 1

• D-dimer, fibrin degradation products, PT/INR, and fibrinogen: This panel focuses on disseminated intravascular coagulation (DIC) or fibrinolytic disorders, which are extremely rare and typically present with more severe, acute bleeding 1

• Direct antiglobulin test and immunoglobulins: These evaluate for immune-mediated processes rather than primary hemostatic disorders 1

Clinical Approach to Bruising in Infants

Red Flags in This Case:

• Multiple bruises in unusual locations (ear, chin)
• Non-mobile 7-month-old (bruising in non-mobile infants is concerning)
• No history of trauma to explain the bruising
• No prior bleeding history

Interpretation of Test Results:

If the PT/aPTT panel is abnormal:

• Prolonged PT suggests deficiencies in factors VII, X, V, II, or fibrinogen
• Prolonged aPTT suggests deficiencies in factors XII, XI, IX, VIII, X, V, II, or fibrinogen
• Both prolonged suggests common pathway defects (factors X, V, II, or fibrinogen)

For von Willebrand testing:

• A core VWF panel should include VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), factor VIII coagulant activity (FVIII:C), and VWF:RCo/VWF:Ag ratio 2
• A ratio of <0.5-0.7 typically indicates qualitative VWF defects 2

Important Considerations

• False positives: Coagulation tests are very sensitive to specimen handling; inappropriate handling commonly leads to false-positive results 1

• Child abuse evaluation: The presence of a bleeding disorder does not exclude the possibility of abuse 1

• Follow-up: If initial testing suggests a bleeding disorder, consultation with a pediatric hematologist is recommended for expanded testing and interpretation 1

• Platelet count: As mentioned in the question, a platelet count should be included to evaluate for immune thrombocytopenia (ITP), which is a transient, often self-resolving bleeding disorder 1

By selecting this comprehensive panel of coagulation tests, you can effectively evaluate for the most common and clinically significant bleeding disorders that might explain this infant's bruising pattern, while maintaining vigilance for potential non-accidental trauma.