What test is most likely to be abnormal in a well-appearing boy with a rash, joint pain, and palpable purpura in his lower extremities?

Urinalysis is Most Likely to Be Abnormal

In a well-appearing boy with palpable purpura on the lower extremities and joint pain, urinalysis is the test most likely to be abnormal, as this clinical presentation is classic for Henoch-Schönlein purpura (IgA vasculitis), which causes renal involvement in 40-50% of cases. 1, 2

Clinical Reasoning

Why Urinalysis is the Answer

• Renal involvement is extremely common in HSP, occurring in 40-50% of patients, manifesting as hematuria and/or proteinuria 2
• Urinalysis abnormalities develop in 37.4% of HSP patients, with 91% of these occurring within the first 6 months of disease 3
• The classic triad of HSP includes palpable purpura (100% of cases), arthritis (75%), and abdominal pain, with renal disease being the fourth major manifestation 2, 4
• Even patients without overt renal symptoms frequently have microscopic hematuria (>5 RBCs/HPF) and/or proteinuria (>1+) on urinalysis 3

Why Other Tests Are Less Likely to Be Abnormal

CBC (Complete Blood Count):

• HSP is characterized by palpable purpura WITHOUT thrombocytopenia 2
• Platelet count remains normal, distinguishing HSP from thrombocytopenic purpura 2
• White blood cell count may be normal or mildly elevated but is not a defining feature 4

INR (International Normalized Ratio):

• Coagulation studies are typically normal in HSP as this is a vasculitic process, not a coagulopathy 2
• INR testing would be abnormal in bleeding disorders or liver disease, which are not suggested by this presentation

Abdominal Ultrasound:

• While 60-65% of HSP patients develop abdominal pain, this patient is described as "well appearing" 2
• Gastrointestinal involvement typically presents with symptomatic abdominal pain requiring clinical attention 4
• Ultrasound would only be indicated if there were concerning abdominal symptoms, which are not mentioned 5

Clinical Pitfalls and Monitoring

Important Caveats

• Normal urinalysis at initial presentation does NOT exclude future renal involvement - 46 of 89 patients (51.7%) with abnormal UA in one cohort developed it during follow-up rather than at diagnosis 3
• Urinalysis monitoring should continue for at least 6 months, as this is when 91% of renal involvement manifests 3
• Older children are at higher risk for renal involvement compared to younger children 3

Prognostic Significance

• The severity of initial urinalysis abnormalities predicts long-term renal outcomes 6
• Patients with normal UA at diagnosis have shorter duration of renal involvement when it does develop 3
• End-stage renal disease occurs in 1-5% of pediatric HSP patients, making urinalysis monitoring critical for early detection 2

Recommended Follow-Up Protocol

• Perform urinalysis at presentation in all suspected HSP cases 3
• Continue monthly urinalysis for 6 months, then every 3 months for an additional 6 months if initially normal 3
• Patients with abnormal initial UA require closer monitoring and possible nephrology referral 3, 6