What are the likely differential diagnoses for a patient with acute kidney injury and stroke whose renal biopsy shows microangiopathy?

Differential Diagnosis: AKI + Stroke + Renal Microangiopathy

The most likely differential diagnoses are ANCA-associated vasculitis (particularly microscopic polyangiitis or granulomatosis with polyangiitis), antiphospholipid syndrome, thrombotic thrombocytopenic purpura, and systemic lupus erythematosus with thrombotic microangiopathy—all of which can present with simultaneous neurologic and renal involvement with microangiopathic changes on biopsy.

Primary Diagnostic Considerations

1. ANCA-Associated Vasculitis (AAV)

This should be at the top of your differential when you see combined kidney injury and stroke with microangiopathy 1.

Key diagnostic features:

• Pauci-immune necrotizing crescentic glomerulonephritis on biopsy
• Little to no immune complex deposition
• CNS involvement occurs in AAV and can manifest as stroke
• ~90% are ANCA-positive (MPO or PR3), but ANCA negativity does not exclude the diagnosis 1

Immediate workup:

• MPO-ANCA and PR3-ANCA (antigen-specific immunoassays are preferred screening) 1
• Do not delay immunosuppressive therapy while waiting for biopsy results if clinical presentation is compatible and ANCA is positive 1

2. Antiphospholipid Syndrome (APS)

This is critical to consider given the stroke presentation.

Key features:

• Thrombotic microangiopathy on renal biopsy
• Arterial thrombosis (stroke) is a hallmark manifestation
• Can occur as primary APS or secondary to SLE 2

Immediate workup:

• Antiphospholipid antibodies (anticardiolipin, anti-β2-glycoprotein I, lupus anticoagulant) 2
• If positive with normal ADAMTS13, this points toward APS nephropathy requiring anticoagulation 2

3. Thrombotic Thrombocytopenic Purpura (TTP)

Key features:

• Thrombocytopenia, hemolytic anemia, neurologic symptoms (including stroke), renal involvement 3
• The classic "pentad" is rarely complete—don't wait for all features 3

Immediate workup:

• ADAMTS13 activity and antibodies 2
• Calculate PLASMIC score (if >5 points = moderate/high risk for TTP) 2
• Start plasma exchange and glucocorticoids immediately while awaiting ADAMTS13 results if TTP is suspected 2

4. Systemic Lupus Erythematosus (SLE) with TMA

Key features:

• Can present with lupus nephritis plus superimposed TMA 2
• May have SLE-associated TTP (low ADAMTS13) or complement-mediated TMA 2
• Neurologic manifestations including stroke are common in SLE

Immediate workup:

• ANA, anti-dsDNA, complement levels (C3, C4)
• ADAMTS13 activity
• Antiphospholipid antibodies 2

5. Complement-Mediated TMA (Atypical HUS)

Key features:

• Can present with renal-limited or systemic TMA 4
• Important: Absence of schistocytes does not exclude TMA—renal-limited TMA may lack systemic hematologic features 4
• Genetic variants in complement regulatory proteins (CD46, CFH, CFI, etc.) 4

Workup:

• Complement levels (C3, C4, CH50)
• Genetic testing for complement abnormalities 3
• Consider eculizumab if complement-mediated TMA is suspected 2

Algorithmic Approach to Diagnosis

Within 24-48 hours of presentation, obtain:

1. Hematologic panel: CBC with peripheral smear (schistocytes, thrombocytopenia), LDH, haptoglobin, reticulocyte count

2. Serologic testing (send simultaneously):

   • ADAMTS13 activity and antibodies
   • MPO-ANCA and PR3-ANCA
   • Antiphospholipid antibodies (full panel)
   • ANA, anti-dsDNA, complement levels (C3, C4)
   • Anti-GBM antibodies (if pulmonary hemorrhage present)

3. Imaging: Brain MRI to characterize stroke pattern (large vessel vs. small vessel vs. embolic)

Treatment initiation timeline:

• If ANCA-positive with compatible presentation: Start immunosuppression immediately, don't wait for biopsy confirmation 1
• If PLASMIC score >5 or high TTP suspicion: Start plasma exchange + glucocorticoids immediately 2
• If antiphospholipid antibodies positive: Initiate anticoagulation 2

Critical Pitfalls to Avoid

1. Don't wait for complete hematologic triad: Renal-limited TMA may lack schistocytes or thrombocytopenia 4. The biopsy showing microangiopathy is sufficient to pursue TMA workup.

2. Don't delay treatment for biopsy results: In rapidly progressive disease with positive serology (ANCA or low ADAMTS13), treatment should begin within 1-4 days 1, 3

3. Don't assume single etiology: Patients can have dual positivity (e.g., ANCA + anti-GBM, or SLE + APS) 1, 2

4. Don't overlook drug-induced causes: Recent data shows drug-induced TMA (especially anti-VEGF agents, calcineurin inhibitors) is increasingly common 5, 6. Take detailed medication history.

5. Consider age and sex patterns: Younger males with severe hypertension and acute vascular lesions suggest severe vascular injury-type TMA 5. Older patients with monoclonal gammopathy may have paraprotein-related TMA 5.

Additional Considerations Based on Biopsy Patterns

If biopsy shows:

• Pauci-immune with crescents: AAV is most likely 1
• Immune complex deposition (IgM, C3, κ, λ): Consider SLE or immune-mediated TMA 5
• Marked endothelial damage with mesangiolysis, minimal immune staining: Suggests toxic/drug-induced TMA 5
• Acute vascular lesions (fibrin thrombi, onion-skinning, mucoid intimal edema): Severe vascular injury pattern, consider malignant hypertension, scleroderma renal crisis, or complement-mediated TMA 5

The diagnosis requires integration of clinical presentation (stroke + AKI), serologic markers, and histopathologic patterns—no single test is definitive, and multidisciplinary collaboration between nephrology, hematology, and rheumatology is essential for optimal outcomes 3, 4.