In a 17-year-old patient with 2 days of abdominal and back pain, proteinuria, elevated serum creatinine (~3.2 mg/dL) and increased C-reactive protein, what is the differential diagnosis?

Differential Diagnosis: Rapidly Progressive Glomerulonephritis Until Proven Otherwise

In a 17-year-old with acute abdominal/back pain, proteinuria, creatinine 3.2 mg/dL, and elevated CRP, the primary differential diagnosis is rapidly progressive glomerulonephritis (RPGN), specifically ANCA-associated vasculitis (AAV), followed by lupus nephritis, post-infectious glomerulonephritis, and anti-GBM disease.

Immediate Diagnostic Priorities

The constellation of acute kidney injury (AKI) with proteinuria and elevated inflammatory markers in an adolescent demands urgent evaluation for RPGN 1. This presentation represents a medical emergency requiring same-day workup.

Most Likely Diagnoses (in order of priority):

1. ANCA-Associated Vasculitis (AAV)

• Most critical to rule out given the acute presentation with rapidly declining kidney function 1
• Approximately 90% of patients with small-vessel vasculitis have ANCA positivity (MPO or PR3) 1
• Can present with abdominal pain from mesenteric vasculitis
• Back pain may indicate retroperitoneal involvement
• The elevated CRP is consistent with active systemic inflammation 1
• Key point: ANCA negativity does NOT exclude this diagnosis—10% are seronegative 1

2. Lupus Nephritis (Class IV)

• Common in adolescents and young adults, particularly females
• Presents with proteinuria, hematuria, and rising creatinine 2
• Elevated CRP can occur despite lupus typically being associated with normal CRP
• Abdominal pain may reflect serositis
• Look for: malar rash, oral ulcers, photosensitivity, arthritis, serositis 2

3. Post-Streptococcal Glomerulonephritis

• Relevant in this age group with acute presentation 3
• Can present as pulmonary-renal syndrome with elevated inflammatory markers 3
• Typically follows pharyngitis or skin infection by 1-3 weeks
• Associated with low complement C3 and positive anti-streptolysin O titers 3

4. Anti-GBM Disease (Goodpasture's Syndrome)

• Less common but life-threatening
• Presents with pulmonary-renal syndrome in severe cases 1
• Requires urgent plasma exchange if positive 1
• Must be ruled out emergently

5. IgA Nephropathy

• Can present with gross hematuria following upper respiratory infection
• Usually more indolent but can have acute presentations 2
• Less likely given the severity and rapidity of presentation

Critical Immediate Workup

Urinalysis with Microscopy (STAT)

• Look for: Dysmorphic RBCs, RBC casts (glomerular), proteinuria quantification 1
• Presence of glomerular hematuria with proteinuria confirms glomerulonephritis 1
• Pyuria without infection supports RPGN 1

Serologic Testing (STAT - Do Not Wait for Results to Treat)

• MPO-ANCA and PR3-ANCA (antigen-specific immunoassays preferred) 1
• Anti-GBM antibodies (most urgent—positive requires immediate plasma exchange) 1
• ANA, anti-dsDNA, complement C3/C4 (for lupus) 1, 2
• Anti-streptolysin O titer (for post-streptococcal GN) 3
• Complete blood count, comprehensive metabolic panel

Imaging

• Chest X-ray: Rule out pulmonary hemorrhage (pulmonary-renal syndrome) 1, 3
• Renal ultrasound: Assess kidney size, rule out obstruction
• Abdominal CT if indicated: Evaluate source of abdominal pain, rule out vasculitis complications

Kidney Biopsy

• Should be pursued urgently but NOT delay treatment 1
• In AAV with positive ANCA and compatible clinical picture, start immunosuppression immediately without waiting for biopsy results 1
• Biopsy confirms diagnosis and guides long-term management 1

Critical Clinical Pitfalls

Do NOT delay immunosuppressive therapy waiting for kidney biopsy if clinical presentation is compatible with small-vessel vasculitis and ANCA is positive, especially in rapidly deteriorating patients 1. This is a common and dangerous error.

Assess for pulmonary involvement immediately: Ask about hemoptysis, dyspnea, and obtain chest imaging 1, 3. Pulmonary-renal syndrome requires urgent plasma exchange consideration 1.

The elevated CRP is significant: While CRP elevation is nonspecific, it correlates with active inflammation in glomerulonephritis and predicts proteinuria development 4, 5, 6. In this acute setting with AKI, it supports active inflammatory kidney disease rather than chronic processes.

Age-Specific Considerations

At 17 years, this patient is in a transitional age where both pediatric and adult-onset diseases overlap 7. Adolescents may underreport symptoms and have reduced compliance, making early aggressive diagnosis critical 7. Lupus nephritis and AAV are both relevant in this age group, while post-infectious GN remains possible 3.

Initial Management Approach

If ANCA-positive with compatible presentation: Start glucocorticoids combined with rituximab or cyclophosphamide immediately 1. For severe kidney disease (creatinine >4 mg/dL), limited data exist for rituximab, but both agents are acceptable 1.

If anti-GBM positive: Urgent plasma exchange plus immunosuppression 1.

If lupus suspected: High-dose corticosteroids plus cyclophosphamide or mycophenolate mofetil 2.

The abdominal and back pain warrant specific attention—these may represent vasculitic involvement of mesenteric vessels, retroperitoneal inflammation, or serositis, all of which influence urgency and treatment intensity.