In the presence of an atypical rash other vasculitic conditions should be considered. Microscopic polyarteritis, Wegener's granulomatosis, and systemic lupus erythematosus (SLE) may all be accompanied by a crescentic nephritis. The associated clinical features with the presence of ANCA or ANA (antinuclear antibody) can help to differentiate these conditions. Cytoplasmic ANCA (C-ANCA) is more commonly associated with Wegener's granulomatosis while perinuclear ANCA (P-ANCA) is more often associated with microscopic polyarteritis. Serology is also a distinguishing feature in SLE, although ANA positive HSP has been described.
Sepsis may cause a purpuric rash as may clotting disorders or thrombocytopenia. The clinical picture, particularly the distribution of the rash, with haematological investigations should identify these patients.
or if you just want to go through the differential diagnosis of HSP:
- Arthritis, Rheumatoid
- Disseminated Intravascular Coagulation
- Glomerulonephritis, Acute
- Idiopathic Thrombocytopenic Purpura
- Inflammatory Bowel Disease
- Meningitis
- Mononucleosis
- Orchitis
- Pediatrics, Chicken Pox or Varicella
- Pediatrics, Child Abuse
- Pediatrics, Gastroenteritis
- Pediatrics, Gastrointestinal Bleeding
- Pediatrics, Hand-Foot-and-Mouth Disease
- Pediatrics, Intussusception
- Pediatrics, Kawasaki Disease
- Pediatrics, Meningitis and Encephalitis
- Renal Failure, Acute
- Shock, Septic
- Systemic Lupus Erythematosus
- Testicular Torsion
- Thrombocytopenic Purpura
- Tick-Borne Diseases, Rocky Mountain Spotted Fever