What blood workup is recommended for a patient suspected of having Henoch-Schönlein Purpura (HSP)?

Blood Workup for Henoch-Schönlein Purpura (HSP)

The essential blood workup for suspected HSP includes a complete blood count with peripheral smear to confirm normal platelet count (distinguishing HSP from thrombocytopenic purpura), along with basic metabolic panel, urinalysis, and renal function tests, as the diagnosis is primarily clinical but these tests exclude mimics and assess for renal involvement. 1

Core Laboratory Tests

Complete Blood Count with Peripheral Smear

• A CBC with peripheral smear is essential and typically shows normal platelet count, normal red and white cell morphology 1
• This distinguishes HSP from immune thrombocytopenic purpura, which presents with low platelets 1
• The palpable purpura in HSP represents small vessel vasculitis, not thrombocytopenia 1

Renal Function Assessment

• Blood urea nitrogen (BUN) and serum creatinine to detect renal dysfunction, which can be present at diagnosis 2
• Serum albumin level, as nephrotic syndrome can occur with low albumin 2
• Urinalysis with microscopy is mandatory to detect hematuria, proteinuria, and red blood cell casts 2, 3
• Renal involvement leads to chronic renal failure in up to 20% of pediatric patients after 20 years 4, 5

Additional Blood Tests

• Basic metabolic panel to assess electrolytes and overall metabolic status 2
• Blood pressure measurement should accompany laboratory workup 3

Confirmatory Tissue Diagnosis

Skin Biopsy - Gold Standard

• Skin biopsy remains the gold standard for diagnosis, showing IgA deposition within walls of superficial dermal vessels and demonstrating leukocytoclastic vasculitis with neutrophil infiltration 1
• Both H&E staining and direct immunofluorescence are required 1
• A skin biopsy should be performed for histological and immunofluorescence studies in cases of clinical suspicion of HSP to avoid missed or incorrect diagnoses 4
• The finding of IgA deposits in vessel walls associated with characteristic signs of small-vessel vasculitis is sine qua non for diagnosis 4

Renal Biopsy Indications

• Reserved for patients with significant renal involvement (nephrotic/nephritic syndrome, persistent proteinuria, elevated creatinine) 2
• Shows IgA deposition in glomeruli and can reveal crescentic glomerulonephritis in severe cases 2
• The percentage of glomeruli with epithelial crescents correlates with prognosis 5

Clinical Context and Pitfalls

Key Diagnostic Principle

• HSP diagnosis should not be excluded based on a normal platelet count - this is what distinguishes it from thrombocytopenic purpura 1
• The hallmark is palpable purpura (non-blanchable, violaceous lesions) affecting bilateral lower extremities, particularly below the waist 1

Common Pitfalls to Avoid

• Failing to perform urinalysis at presentation and during follow-up, as renal signs can become manifest years after initial presentation 4, 3
• Relying solely on clinical diagnosis without tissue confirmation in atypical cases, risking misdiagnosis of other forms of leukocytoclastic vasculitis 4, 6
• Inadequate follow-up duration - monitoring should continue for at least 6 months with regular urine testing for proteinuria/hematuria and blood pressure measurements 3

Special Monitoring Considerations

• Women with childhood HSP history require close monitoring during pregnancy for proteinuria and hypertension 3
• Patients can develop silent chronic renal failure decades later without appropriate follow-up 4
• The clinical picture is often incomplete initially, and renal signs may appear years after initial symptoms 4

Laboratory Tests NOT Routinely Needed

Blood workup does not require specialized immunologic panels, complement levels, or ANCA testing for typical HSP presentations, as the diagnosis is clinical combined with the basic tests outlined above 1, 3, 6. These may be considered only when the presentation is atypical and other vasculitides need exclusion 6.