How can I differentiate purpura from bruises and determine the appropriate evaluation?

Differentiating Purpura from Bruises

Purpura does not blanch with pressure while bruises (ecchymoses) do blanch, and purpura is typically palpable when caused by vasculitis, whereas bruises remain flat; this simple bedside test is your first-line discriminator. 1, 2

Key Physical Examination Distinctions

Blanching Test

• Apply firm pressure with a glass slide or your finger directly on the lesion 1
• Purpura remains visible because red blood cells have extravasated into the dermis and cannot be compressed away 1
• Bruises (ecchymoses) will partially blanch because blood remains within vessels that can be temporarily compressed 1

Palpability Assessment

• Palpable purpura indicates cutaneous vasculitis (inflammation and destruction of vessel walls), most commonly from drug reactions, infections, or collagen vascular disease 2
• Non-palpable purpura suggests platelet disorders, coagulation defects, or increased vascular fragility 1
• Bruises are always flat unless there is significant underlying hematoma formation 1

Size and Distribution Patterns

• Petechiae (purpura <3mm) suggest platelet dysfunction or thrombocytopenia 1, 3
• Purpura (3-10mm) indicates more severe platelet or vascular pathology 1
• Ecchymoses/bruises (>10mm) can be traumatic or coagulopathic 1
• Symmetric distribution favors systemic causes (vasculitis, coagulopathy), while asymmetric patterns suggest trauma 1

Associated Features That Guide Diagnosis

• Retiform (net-like) purpura with necrosis indicates vascular occlusion from thrombosis, emboli, or vasculopathy—this is a medical emergency 1
• Mucosal bleeding (gums, nose) accompanying purpura points to platelet disorders rather than simple bruising 3, 4
• Hemarthroses or deep hematomas with bruising suggest coagulation factor deficiencies (hemophilia) 4

When to Pursue Laboratory Evaluation

Mandatory Testing Scenarios

• Any purpura (palpable or non-palpable) requires laboratory workup because it indicates potential systemic disease 5, 6, 3
• Bruising in infants <6 months regardless of explanation 6
• Bruising in high-risk locations (neck, ears, buttocks, genitals, torso) at any age 7, 6
• Personal or family history suggesting bleeding disorder (prolonged bleeding after circumcision, surgery, dental work, or spontaneous joint bleeds) 5, 6, 4
• Mucosal bleeding accompanying skin findings 3, 4

Initial Laboratory Panel for Purpura or Unexplained Bruising

Order all of the following simultaneously 5, 6, 3:

• Complete blood count with platelet count (screens for thrombocytopenia, including immune thrombocytopenic purpura)
• Prothrombin time (PT) (evaluates extrinsic pathway)
• Activated partial thromboplastin time (aPTT) (evaluates intrinsic pathway)
• Fibrinogen level (assesses clotting factor availability)
• Von Willebrand disease testing (VWF antigen, VWF activity, Factor VIII) because VWD affects up to 1% of the population and is not detected by PT/aPTT alone 5, 6

Interpretation Algorithm

Normal PT and aPTT

• Suspect platelet dysfunction or von Willebrand disease 5, 3, 4
• Refer to hematology for platelet function testing 5, 6

Prolonged aPTT with Normal PT

• Indicates intrinsic pathway defect (hemophilia A/B, factor XI deficiency) 6, 3, 4
• Note: Mild hemophilia (factor levels 25-40%) may have normal aPTT yet still cause bleeding 5, 6
• Perform mixing study to distinguish factor deficiency from inhibitor 4

Prolonged PT with Normal aPTT

• Indicates factor VII deficiency or vitamin K deficiency 6, 3
• Consider vitamin K challenge in appropriate clinical context 4

Both PT and aPTT Prolonged

• Consider combined factor deficiencies, vitamin K deficiency, liver disease, or disseminated intravascular coagulation 6, 3, 4

Critical Pitfalls and Limitations

What the Standard Panel Misses

• Factor XIII deficiency causes severe bleeding despite completely normal PT/aPTT (extremely rare, <1 in 500,000) 5, 6
• Mild platelet function disorders 5, 6
• Certain VWD subtypes 5, 6
• Fibrinolytic defects 6

Common False Positives

• Lupus anticoagulant prolongs aPTT but does not cause bleeding—it actually increases thrombosis risk 6
• Factor XII deficiency prolongs aPTT without causing bleeding symptoms 6
• Improper specimen handling frequently yields false results; use experienced laboratories 6

Tests to Avoid

• Bleeding time is not recommended due to poor reproducibility, invasiveness, and low sensitivity for VWD 5
• PFA-100/200 is not sensitive for mild VWD or platelet abnormalities and is influenced by hematocrit and medications 5

Specialist Referral Indications

Immediate Hematology Consultation

• Any abnormal initial coagulation studies 5, 6, 3
• Strong clinical suspicion despite normal initial tests 5, 6, 3
• Palpable purpura with systemic symptoms (fever, arthralgia, renal involvement) suggesting vasculitis 2

Child Abuse Pediatrician Involvement

• Any bruising in infants <9 months 7, 6
• Bruising in high-risk anatomic locations 7, 6
• Patterned bruising or inconsistent history 7, 6
• Consider skeletal survey in children <24 months with concerning bruise patterns 7

Special Clinical Contexts

Palpable Purpura Workup

• Skin biopsy is essential when palpable purpura etiology is unclear from history and physical examination 2
• Most cases are cutaneous vasculitis from drugs, infection, or collagen vascular disease 2
• Many cases are idiopathic and self-limited 2
• Treatment options include dapsone, colchicine, or systemic corticosteroids when necessary 2

Follow-Up Strategy

• Bleeding disorders are permanent (except transient conditions like immune thrombocytopenic purpura) and will not resolve with environmental changes 6
• If bruising resolves after removing a child from a concerning environment, this supports abuse rather than coagulopathy 6
• The presence of a bleeding disorder does not exclude abuse—both can coexist 6