What labs should be ordered for a patient with unexplained bruising?

Laboratory Tests for Unexplained Bruising

For patients with unexplained bruising, initial laboratory screening should include complete blood count (CBC) with platelet count, prothrombin time (PT), activated partial thromboplastin time (aPTT), and von Willebrand factor (VWF) screening tests. 1

Initial Screening Tests

First-line Laboratory Tests:

• Complete blood count (CBC) with platelet count
• Peripheral blood smear examination
• Prothrombin time (PT)
• Activated partial thromboplastin time (aPTT)

These initial tests help determine whether coagulation factor deficiency or thrombocytopenia might be potential causes of clinical bleeding 1, 2. The results will guide further diagnostic workup:

• Normal PT and aPTT: Suggests platelet disorder, most commonly von Willebrand disease 2
• Normal PT and prolonged aPTT: Indicates deficit in the intrinsic pathway (factors VIII, IX, XI) 2
• Abnormal PT and normal aPTT: Consider vitamin K deficiency 2
• Prolonged PT and aPTT: Consider liver failure workup 2

Second-line Testing Based on Initial Results

If von Willebrand disease is suspected:

• VWF antigen (VWF:Ag)
• VWF ristocetin cofactor activity (VWF:RCo)
• Factor VIII coagulant activity (FVIII) 1

If coagulation factor deficiency is suspected:

• Specific factor assays (Factor II, V, VII, X, VIII, IX, XI, XIII)
• Fibrinogen (Clauss method) 1

If platelet dysfunction is suspected:

• Light Transmission Aggregometry (LTA) with multiple agonists (epinephrine, ADP, collagen, arachidonic acid, ristocetin)
• Platelet granule release assessment 1

Special Considerations

For persistent unexplained bruising despite normal initial tests:

• Consider referral to a hematologist for specialized testing 2
• Advanced tests may include VWF multimer analysis, expanded flow cytometry, thrombin generation assay, and genetic testing 1

Important testing pitfalls to avoid:

1. Medication effects: Ensure patient has discontinued medications affecting platelet function 7-10 days before testing 1
2. Sample handling: Blood samples for VWF assays should be transported at room temperature to prevent cryoprecipitation 3
3. Patient factors: Stress, exercise, inflammation, pregnancy, or estrogen use can elevate VWF and FVIII levels, potentially masking disorders 3
4. Blood group influence: Individuals with blood group O have VWF levels approximately 25% lower than other ABO blood groups 3

Testing Algorithm

1. Perform initial screening tests (CBC, PT, aPTT)
2. If normal results but strong clinical suspicion remains, proceed with VWF screening
3. Based on abnormal results, pursue targeted testing for specific disorders
4. If all tests are normal but clinical suspicion remains high, refer to hematology for specialized testing

Remember that the PFA-100 and skin bleeding time tests are no longer recommended due to insufficient specificity and sensitivity 1.

In cases where bleeding disorders cannot be identified despite thorough testing, patients may have a bleeding disorder of unknown cause (BDUC), which affects up to 60-70% of patients investigated for mild-moderate bleeding disorders 3.