Some patients experience recurrent episodes upon re-exposure to the allergen. The extent of renal involvement and its course are critical factors determining prognosis. Allergic purpura can occur at any age but is most common in children and adolescents, particularly preschool and school-aged children. It is rare in infants under one year old, with males affected more frequently than females (approximately 1.4 to 2.1 times).Differential Diagnosis of Allergic Purpura I. Idiopathic Thrombocytopenic Purpura: Distinguishing based on skin purpura morphology (non-elevated, asymmetrical distribution) and thrombocytopenia is relatively straightforward. Allergic purpura is more easily differentiated when accompanied by angioedema, urticaria, or erythema multiforme.
II. Septicemia: Rash from meningococcal septicemia resembles purpura, but this condition presents with severe toxic symptoms, markedly elevated white blood cell counts, and positive bacterial cultures from skin puncture smears.
III. Rheumatic Arthritis: Both conditions may involve joint swelling/pain and low-grade fever, making differentiation difficult before purpura appears. The development of cutaneous purpura aids in distinguishing them.
IV. Intussusception: More common in infants and young children. Suspect this condition if the child exhibits intermittent crying, palpable abdominal masses, and abdominal muscle tension. Barium enema examination can confirm the diagnosis. However, allergic purpura may coexist with intussusception, warranting caution.
V. Appendicitis: Both conditions may present with umbilical and right lower quadrant abdominal pain accompanied by tenderness. However, allergic purpura is distinguished by the absence of abdominal muscle tension and the presence of cutaneous purpura.