Rapidly progressive glomerulonephritis (RPGN), aka crescentic GN, is one of those “drop everything” renal diagnoses for the boards—and real life. It’s a nephritic syndrome that can take a patient from normal kidney function to renal failure in weeks to months, and the key to scoring points on Step 1/2 is recognizing the pattern (nephritic + fast decline) and then sorting it into the three immunopathologic buckets.
What is RPGN (Crescentic GN)?
Definition (high-yield):
RPGN is a clinical syndrome of rapid loss of renal function (usually days to weeks, sometimes up to a few months) with nephritic urine findings and characteristic crescents on renal biopsy.
Nephritic syndrome reminders:
- Hematuria with RBC casts (most testable)
- Proteinuria (usually subnephrotic, but can be significant)
- HTN, periorbital edema
- Azotemia (↑ BUN/Cr)
Classic board phrase: “RBC casts + rapidly rising creatinine.”
Why are there “crescents”? (Pathophysiology)
The crescent is a sign of severe glomerular injury
When the glomerular capillary wall is badly damaged, fibrin leaks into Bowman space, triggering:
- Proliferation of parietal epithelial cells
- Influx of macrophages
- Compression of the glomerular tuft → sharp drop in GFR
Crescents = a morphologic reaction pattern, not a single disease. RPGN is the clinical syndrome; the underlying cause determines the immunofluorescence pattern and systemic clues.
The Big 3 Causes (Know This Table Cold)
RPGN is classified by immunofluorescence (IF) pattern:
| RPGN Type | IF Pattern | Key Antibody/Mechanism | Classic Associations | Buzzwords / Clues |
|---|---|---|---|---|
| Type I (Anti-GBM) | Linear IgG (± C3) | Anti–type IV collagen | Goodpasture syndrome | Hemoptysis + hematuria; “linear” like a paintbrush |
| Type II (Immune complex) | Granular (“lumpy-bumpy”) | Immune complex deposition | Post-strep GN, IgA nephropathy, lupus nephritis, MPGN | Often low complement depending on cause; systemic autoimmune/infectious context |
| Type III (Pauci-immune) | Pauci-immune (little/no IF) | ANCA-associated small vessel vasculitis | GPA (c-ANCA/PR3), MPA (p-ANCA/MPO), EGPA | Pulmonary-renal syndrome possible; systemic vasculitis signs |
Step takeaway: If you’re given RPGN + biopsy/IF, you should instantly label Type I/II/III and name the prototypical disease.
Clinical Presentation (What they’ll give you in a vignette)
Core presentation (nephritic + fast)
- Dark/cola urine (hematuria)
- RBC casts
- Oliguria
- Edema
- Rapidly rising creatinine
“Pulmonary-renal syndrome” pattern
When renal findings + lung involvement appear together, think:
- Goodpasture (Type I anti-GBM): hemoptysis due to pulmonary hemorrhage
- ANCA vasculitis (Type III): pulmonary capillaritis/hemorrhage, sinus/lung disease (esp GPA)
Diagnosis: How to Work It Up (Step-style)
1) UA and basic labs
- UA: hematuria, RBC casts, proteinuria
- BMP: ↑ BUN/Cr, possible hyperkalemia/metabolic acidosis in severe disease
2) Serologies (clue you into Type I vs II vs III)
- Anti-GBM antibodies → Type I (Goodpasture)
- ANCA
- c-ANCA (PR3): GPA
- p-ANCA (MPO): MPA (and EGPA)
- Complement levels
- Often low in many immune complex diseases (Type II), e.g., lupus, post-infectious GN, MPGN
- Often normal in anti-GBM and pauci-immune (not a perfect rule, but high-yield trend)
- Consider disease-specific tests:
- ASO/anti-DNase B (post-strep)
- ANA, anti-dsDNA (SLE)
- Serum IgA (may be elevated in IgA nephropathy)
3) Kidney biopsy (the testable “confirmatory” move)
Biopsy gives three key layers:
Light microscopy (LM):
- Crescents in Bowman space
Immunofluorescence (IF):
- Linear (Type I)
- Granular (Type II)
- Pauci-immune (Type III)
Electron microscopy (EM): depends on the Type II cause
- Post-strep: subepithelial humps
- Lupus: wire-loop (LM) + immune deposits (IF “full house” often)
- IgA nephropathy: mesangial deposits
Treatment (Board-High Yield Principles)
RPGN is treated like a renal emergency because kidneys can scar quickly.
General approach
- High-dose glucocorticoids (often pulse IV methylprednisolone)
- Immunosuppression (commonly cyclophosphamide or rituximab, especially ANCA-associated)
- Supportive care: manage HTN, volume overload, hyperkalemia; dialysis if needed
Cause-specific “must know”
Type I (Anti-GBM/Goodpasture):
- Plasmapheresis (removes circulating anti-GBM antibodies)
- Steroids + cyclophosphamide
- Treat early to prevent irreversible scarring
Type III (Pauci-immune ANCA vasculitis):
- Steroids + cyclophosphamide or rituximab
- Plasmapheresis sometimes used in severe pulmonary hemorrhage (guidelines evolve; know the classic association)
Type II (Immune complex RPGN):
- Treat the underlying cause:
- Post-infectious: supportive ± immunosuppression in select severe cases
- Lupus nephritis: steroids + immunosuppressants (e.g., mycophenolate/cyclophosphamide by class)
- IgA nephropathy: BP control; immunosuppression in select severe/progressive cases
Step exam shortcut:
If they say anti-GBM → think plasmapheresis + immunosuppression.
High-Yield Associations & “Vignette Triggers”
Type I (Anti-GBM / Goodpasture)
- Young adult with hemoptysis + hematuria
- IF: linear IgG
- Antibody: anti–type IV collagen (basement membrane in glomeruli + alveoli)
Type III (ANCA-associated vasculitis)
- Constitutional symptoms (fever, weight loss)
- Upper airway disease (sinusitis, otitis) + lung nodules/cavitation → GPA (c-ANCA/PR3)
- Palpable purpura, neuropathy, rapidly progressive renal failure → small vessel vasculitis picture
- IF: pauci-immune
Type II (Immune complex)
- Recent infection (post-strep), autoimmune disease (SLE), or episodic hematuria after URI/GI infection (IgA)
- IF: granular
- Complement: often low (especially lupus/post-infectious/MPGN)
First Aid Cross-References (Where this lives in your brain)
These are the First Aid “anchor points” RPGN pulls from:
- Nephritic syndromes: hematuria, RBC casts, HTN, mild–moderate proteinuria
- Anti-GBM (Goodpasture): linear IF, pulmonary hemorrhage + GN
- ANCA vasculitides: pauci-immune RPGN, c-ANCA (PR3) vs p-ANCA (MPO)
- Immune complex GN: granular IF; post-strep (subepithelial humps), lupus (full house), IgA (mesangial)
If you’ve memorized FA’s “linear vs granular vs pauci-immune,” RPGN is the clinical frame that tells you why it matters.
Rapid-Fire USMLE Pearls (Memorize These)
- Crescents = fibrin in Bowman space → parietal cell proliferation + macrophages
- RPGN is a nephritic syndrome with rapidly rising creatinine
- IF patterns:
- Linear = anti-GBM (Goodpasture)
- Granular = immune complex (post-strep, lupus, IgA, MPGN)
- Pauci-immune = ANCA vasculitis
- Goodpasture = lung + kidney (hemoptysis + hematuria)
- Treat early: high-dose steroids + immunosuppression; plasmapheresis for anti-GBM