# RPGN Diagnostic Algorithm

## Overview

Rapidly progressive glomerulonephritis (RPGN) is a clinical-pathologic syndrome characterized by rapidly worsening kidney function with suspected glomerular inflammation, often with crescent formation on kidney biopsy; a fixed percentage GFR loss is not a prerequisite for urgent assessment \[1\]. Suspected RPGN is a kidney emergency requiring immediate nephrology evaluation and cause-directed treatment. Infection-related disease requires infection control; immunosuppression is selected according to the cause, activity, and treatment risks. This guide provides a stepwise diagnostic and treatment approach based on immunofluorescence (IF) and serologic findings.

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## Key Clinical Pearl

**“RPGN is a medical emergency. If clinical suspicion is high (rapid SCr rise + active urinary sediment + absence of other explanations), initiate empiric immunosuppression BEFORE biopsy results return; waiting may result in irreversible renal failure.”** The three types are distinguished by IF pattern on biopsy, but serologic workup can guide presumptive therapy.

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## Part 1: RPGN Definition and Classification

### RPGN Clinicopathologic Definition

**Clinical Criteria:** - Rapid creatinine rise or declining filtration with a compatible presentation; do not wait for a fixed percentage change - Active urinary sediment (dysmorphic RBCs, RBC casts, proteinuria) - Absence of non-glomerular causes (prerenal, postrenal, ATN)

**Pathologic Criteria:** - **Crescent formation** (\>50% of glomeruli involved in RPGN; 10–50% = “crescentic GN”) - Light microscopy (LM) shows fibrocellular or fibrinoid crescents with glomerular necrosis - Immunofluorescence (IF) determines type (see below)

### Three Types of RPGN (IF-Based Classification)

| Type | IF Pattern | Serology | Frequency | Prognosis |
|----|----|----|----|----|
| **Type I: Anti-GBM** | **Linear IgG + C3** along GBM | Anti-GBM+ serum antibodies | 10–15% | Poor if Cr \>4 at presentation |
| **Type II: Immune Complex (IC)** | **Granular** IgG ± IgA ± IgM + C3 | Varies (ANA+, anti-dsDNA, ASO titer, etc.) | 20–30% | Depends on underlying disease |
| **Type III: Pauci-Immune (ANCA)** | **Pauci-immune** (minimal IF) ± ANCA on immunoelectron microscopy | ANCA+ (c-ANCA/PR3 or p-ANCA/MPO) in 90%; ANCA-negative in 10% | 50–60% | Best if treated early; Cr \>4 at presentation = poor |

### Key Distinction: “Crescent” vs “RPGN” Terminology

- **RPGN** = clinical syndrome of rapid GFR decline + biopsy findings (crescent formation)
- **Crescentic GN** = pathologic finding of crescent without necessarily rapid presentation
- **Not all crescents = RPGN** (e.g., IgA nephropathy with 10% crescents but stable Cr = not RPGN)

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## Part 2: Clinical Presentation and Initial Workup

### Presenting Features of RPGN

**Systemic symptoms (40–60%):** - Malaise, fever, weight loss - Arthralgia, myalgia (especially in ANCA-associated)

**Renal symptoms:** - Hematuria (tea-colored or smoky urine; RBC casts on UA) - Oliguria or anuria (if severe) - Rapid rise in Cr over days/weeks (accelerated from baseline)

**Pulmonary hemorrhage (10–15% of RPGN):** - Dyspnea, hemoptysis - Bilateral alveolar infiltrates on CXR - Diffusion capacity reduced (DLCO↓); A-a gradient widened - Associated with Type I (anti-GBM; Goodpasture syndrome) or Type III (ANCA; GPA with pulmonary involvement)

**Systemic vasculitis features (if ANCA-associated):** - Palpable purpura (legs, buttocks) - Sinusitis, nasal ulcers (GPA) - Polyarthritis, constitutional symptoms

### Urgent Baseline Laboratory Assessment

**Renal function & urinalysis:** - SCr, BUN, GFR (estimate baseline if not known) - Urinalysis: RBC casts (strong clue to glomerular bleeding; may be absent in GN), dysmorphic RBCs, granular casts, proteinuria

**Electrolytes & metabolic:** - Sodium, K, Cl, HCO3, phosphate (assess AKI severity) - Ca (hypocalcemia suggests rhabdomyolysis or sepsis, not RPGN)

**Coagulation & anemia:** - PT/INR, aPTT (assess bleeding risk if biopsy planned) - CBC (anemia from hematuria; thrombocytopenia rare in RPGN)

**Serologic panel (send immediately; do NOT delay for biopsy):** - ANCA + anti-PR3 + anti-MPO (ANCA-associated vasculitis) - Anti-GBM (Type I) - ANA + anti-dsDNA + anti-C1q (lupus) - Complement: C3, C4, CH50 - Cryoglobulins, SPEP/UPEP (monoclonal protein) - Blood cultures (if fever; exclude infection-associated GN) - Hepatitis serology (HBV, HCV; associated with vasculitis) - ASO titer, anti-hyaluronidase (post-streptococcal)

**Imaging:** - Chest X-ray (evaluate for pulmonary hemorrhage) - Renal ultrasound (assess kidney size, echogenicity, rule out obstruction)

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## Part 3: RPGN Diagnostic Algorithm

    SUSPECTED RPGN (rapid Cr rise + active sediment)
    │
    ├─→ RAPID ASSESSMENT
    │   ├─ Calculate Cr rise: Is it >50% over days/weeks? YES → Proceed
    │   ├─ Check UA: RBC casts present? YES → Strongly suggestive of RPGN
    │   ├─ Rule out AKI mimics (NSAID overdose, ACE inhibitor-induced, volume depletion)
    │   └─ If diagnostic doubt: TREAT EMPIRICALLY while awaiting biopsy
    │
    ├─→ SEROLOGIC WORKUP (STAT — send immediately)
    │   │
    │   ├─ ANCA + anti-PR3/anti-MPO
    │   │   ├─ c-ANCA/PR3+ → Type III pauci-immune (GPA likely)
    │   │   ├─ p-ANCA/MPO+ → Type III pauci-immune (MPA/EGPA likely)
    │   │   └─ ANCA negative (10%) → Type III pauci-immune (still possible) OR Type I/II
    │   │
    │   ├─ Anti-GBM serum antibodies
    │   │   ├─ Positive → Type I (linear IgG IF on biopsy expected)
    │   │   └─ Negative → Type II or Type III; consider pauci-immune ANCA-negative RPGN
    │   │
    │   ├─ ANA + anti-dsDNA + anti-C1q + Complement (C3, C4)
    │   │   ├─ ANA+/anti-dsDNA+ + low C3/C4 → Type II (lupus nephritis, likely Class IV)
    │   │   ├─ ANCA-negative/anti-GBM-negative + normal complement → Type III (pauci-immune)
    │   │   └─ Normal serologies overall → consider ANCA-negative RPGN
    │   │
    │   └─ Cryoglobulins, SPEP/UPEP, Hepatitis serology, ASO titer
    │       (Refine diagnosis if workup equivocal)
    │
    ├─→ RENAL BIOPSY (URGENT)
    │   │
    │   ├─ Light Microscopy:
    │   │   └─ Fibrocellular crescents with segmental necrosis in >50% of glomeruli
    │   │       (Fibrinoid necrosis indicates active disease)
    │   │
    │   ├─ Immunofluorescence:
    │   │   ├─ LINEAR IgG + C3 along GBM → TYPE I (Anti-GBM disease)
    │   │   ├─ GRANULAR IgG/IgA/IgM + C3 → TYPE II (Immune complex; proceed to EM)
    │   │   └─ PAUCI-IMMUNE (minimal IF, ANCA+ on immunoelectron) → TYPE III (ANCA-associated)
    │   │
    │   └─ Electron Microscopy:
    │       ├─ Subendothelial deposits → IC-RPGN (lupus, cryoglobulinemia, etc.)
    │       ├─ Subepithelial deposits → post-streptococcal RPGN (rare)
    │       └─ No deposits → Type I or Type III
    │
    └─→ TREATMENT INITIATION
        (Based on serology and/or biopsy; see Part 4)

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## Part 4: Serologic Interpretation for Immediate Therapy Guidance

### If ANCA+ (c-ANCA/PR3 or p-ANCA/MPO): Type III Presumptive Diagnosis

**Presumed diagnosis:** ANCA-associated vasculitis (AAV) with RPGN.

**ANCA subtypes:** - **c-ANCA/anti-PR3 (60% of AAV-RPGN):** Granulomatosis with polyangiitis (GPA) - Upper respiratory involvement common (sinusitis, nasal ulcers, subglottic stenosis) - Lower respiratory involvement (pulmonary nodules, infiltrates, hemorrhage) - **p-ANCA/anti-MPO (40% of AAV-RPGN):** Microscopic polyangiitis (MPA) or eosinophilia-associated - Pulmonary hemorrhage more common - Skin/GI involvement frequent - Pulmonary granulomas rare

**Immediate therapy (induction):** - High-dose corticosteroids (methylprednisolone 500–1000 mg IV daily × 3 days, then oral prednisone taper) - **Immunosuppression induction:** Rituximab 375 mg/m² IV weekly × 4 weeks (preferred) OR cyclophosphamide 15 mg/kg IV monthly × 3–6 months - If pulmonary hemorrhage: Add plasmapheresis (controversial; consider if hemoptysis, diffuse alveolar hemorrhage with Hgb drop, or anti-GBM coexistence)

**Maintenance (18–24 months):** - Rituximab 500 mg IV at 6 months, then every 6–9 months × 18 months - OR azathioprine 1–2 mg/kg/day + low-dose prednisone

### If Anti-GBM+: Type I Presumptive Diagnosis

**Presumed diagnosis:** Anti-glomerular basement membrane (anti-GBM) disease ± pulmonary hemorrhage (Goodpasture syndrome).

**Clinical pearl:** Anti-GBM disease is rare but often fulminant; pulmonary hemorrhage in 40–60%; dual-positive (anti-GBM + ANCA) in 10% (worse prognosis).

**Urgent therapy (EMERGENCY induction):** - **Plasmapheresis:** 50 mL/kg/day × 7–14 days (remove circulating anti-GBM antibodies IMMEDIATELY) - Continue daily until serum anti-GBM titer negative - Reduces morbidity/mortality significantly if initiated early - **High-dose steroids + cyclophosphamide:** Methylprednisolone 500–1000 mg IV daily × 3 days, then oral prednisone; cyclophosphamide 2 mg/kg/day (or 15 mg/kg IV monthly) - Continue until anti-GBM serology persistently negative

**Prognosis:** Creatinine \>4.0 mg/dL at presentation = 80% progress to ESRD; Cr \<3.0 with early plasmapheresis = 50–70% renal recovery.

### If ANA+/Anti-dsDNA+/Low C3: Type II Presumptive Diagnosis

**Presumed diagnosis:** Lupus nephritis (Class III focal or Class IV diffuse crescentic).

**Immediate therapy (induction):** - High-dose corticosteroids (methylprednisolone 500 mg IV daily × 3 days, then oral prednisone) - Immunosuppression induction: **Mycophenolate mofetil (MMF) 2–3 g/day** OR **cyclophosphamide 500 mg IV monthly × 3–6 months** (cyclophosphamide preferred if Class IV with crescents)

**Maintenance:** - Azathioprine 1–2 mg/kg/day OR MMF 2 g/day + low-dose prednisone - Hydroxychloroquine 200–400 mg/day (retinopathy monitoring)

**Specific for RPGN presentation:** Consider plasmapheresis if severe (Cr \>4 or oliguria) or double-seronegative (\<50% IgG deposition on biopsy).

### If ANCA-Negative, Anti-GBM-Negative: Type III (ANCA-Negative) Presumptive Diagnosis

**Presumed diagnosis:** ANCA-negative pauci-immune RPGN (10% of RPGN; poor prognosis).

**Challenge:** Serologies negative, but biopsy pauci-immune with crescents. Diagnosis confirmed only on biopsy.

**Treat identically to ANCA-positive RPGN:** - High-dose steroids + induction immunosuppression (rituximab or cyclophosphamide) - Outcomes: ANCA-negative patients have higher progression rates to ESRD (50–70% vs 40% in ANCA+)

**Special consideration:** May harbor complement-mediated RPGN or rare glomerulonephritis types; if no response to standard therapy, consider repeat biopsy or genetic testing (C3, CFH, CFHR mutations).

### If Serologies Largely Negative: Atypical RPGN Types

**Consider:** 1. **Immune-complex RPGN without SLE:** Cryoglobulinemia (HCV+), post-streptococcal, drug-induced (propylthiouracil, hydralazine, levamisole) 2. **Monoclonal immunoglobulin-associated RPGN:** SPEP/UPEP shows M-spike; free light chain assay elevated 3. **Atypical complement-mediated RPGN:** Genetic mutations (CFH, CFI, C3); may respond poorly to standard therapy 4. **Infection-associated RPGN:** Endocarditis, HBV, HIV; treat underlying infection ± brief immunosuppression

**Action:** Obtain biopsy urgently; interpret EM findings carefully.

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## Part 5: Timeline and Treatment Decisions

### Presentation to Therapy Initiation (Hours to Days)

| Time | Action |
|----|----|
| **Hour 0–2** | Recognize RPGN (rapid Cr + active sediment); STAT serologic workup |
| **Hour 2–6** | Renal biopsy scheduled (urgent); imaging (CXR, renal ultrasound) |
| **Hour 6–24** | Biopsy performed ± preliminary LM/IF results available |
| **Day 1–2** | Full biopsy interpretation available (LM, IF, EM); begin induction therapy if RPGN confirmed |
| **Day 2–7** | **Do NOT wait for biopsy if high clinical suspicion; empiric therapy justified** |

**Risk of delay:** Each week of untreated RPGN increases risk of permanent renal failure. A patient with Cr rising to 4.0 who waits 2 weeks for treatment is likely to progress to dialysis dependence.

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## Part 6: Special Clinical Scenarios

### Dual-Positive Disease (ANCA+ AND Anti-GBM+)

**Frequency:** 5–10% of RPGN patients (mixed Type I/Type III).

**Serologic clue:** Both ANCA+ (usually p-ANCA/MPO) AND anti-GBM+ on same serum sample.

**Prognosis:** Worse than either disease alone; higher progression to ESRD.

**Treatment:** - Aggressive induction: Plasmapheresis (for anti-GBM) + high-dose steroids + cyclophosphamide - Continue until BOTH ANCA and anti-GBM titers negative

### ANCA-Negative Pauci-Immune RPGN with Poor Response

**Challenge:** ANCA-negative RPGN treated with standard therapy (steroids + rituximab/cyclophosphamide) but no improvement in Cr; worsening renal function despite immunosuppression.

**Diagnostic considerations:** 1. **Complement-mediated RPGN:** C3GN; Factor H deficiency; C3 nephritic factor; genetic mutations 2. **Double-seronegative lupus nephritis:** Very rare; diagnosis of exclusion; confirm with kidney biopsy EM showing subendothelial deposits 3. **Anti-phospholipase A2 receptor (PLA2R) membranous nephropathy with crescent:** Rare; IF shows IgG + C3 in granular pattern but with subepithelial deposits on EM

**Next steps:** - Repeat kidney biopsy if diagnosis remains unclear - Genetic testing (CFH, CFI, C3) if atypical complement-mediated RPGN suspected - Complement-blocking therapies: Consider C5a complement inhibitor (iptacopan), C3 complement inhibitor (pegcetacoplan) in research settings

### RPGN in Pregnancy

**Challenge:** Rapid GFR decline in pregnant/postpartum woman; RPGN diagnosis complicated by eclampsia/HELLP syndrome mimicry.

**Key distinguishing features:** - **RPGN:** RBC casts on UA (preeclampsia typically has proteinuria without casts); ANCA/anti-GBM positive - **Preeclampsia:** Hypertension, proteinuria, thrombocytopenia, elevated LFTs; no RBC casts; serologies negative

**Treatment:** - If RPGN confirmed: Corticosteroids + induction therapy (rituximab preferred over cyclophosphamide in pregnancy; teratogenic risk) - If timing allows: Delay induction therapy until post-partum (except if severe/life-threatening RPGN)

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## Part 7: Remission and Relapse Monitoring

### Definitions

**Complete remission:** Renal function stabilization (Cr \<1.5× baseline) + urinary sediment normalization (no RBC casts, \<5 RBC/hpf) + absence of systemic vasculitis.

**Relapse:** New-onset hematuria, RBC casts, or rising Cr in remission (common in ANCA-associated; 50% at 5 years).

### Maintenance Monitoring (Post-Induction)

| Parameter | Frequency | Threshold for Action |
|----|----|----|
| **Serum creatinine** | Monthly × 3 months, then 3-monthly | Rise \>20% suggests relapse |
| **Urinary sediment** | Monthly × 3 months, then 3-monthly | Appearance of RBC casts → relapse |
| **ANCA titers** | 3-monthly in first year, then 6-monthly | Rising titer predicts relapse (predictive in 50% of cases) |
| **CBC** | 6-monthly (if on azathioprine/MMF) | Monitor for cytopenias; adjust dose if Hgb \<10 or WBC \<3500 |
| **Transaminases** | 6-monthly (if on azathioprine/MMF) | Monitor for hepatotoxicity |

### Relapse Management

**If rising ANCA titer but stable renal function and UA:** - Intensify monitoring; consider pulse corticosteroids or rituximab booster (if \>1 year since last dose)

**If new-onset hematuria/RBC casts + rising Cr:** - Treat aggressively with rituximab booster or cyclophosphamide pulse - Rule out alternative diagnoses (infection, drug toxicity, secondary GN)

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## Part 8: Key Warnings

### Warning 1: Never Assume “Overlap” Without Biopsy

A patient with both clinical ANCA-associated vasculitis AND a positive anti-GBM test may have true dual-positive disease (5–10% of RPGN) OR may have one disease with an incidental antibody. **Biopsy is essential** to determine IF pattern (linear vs pauci-immune) and guide therapy.

### Warning 2: Pulmonary Hemorrhage Mandates Aggressive Therapy

Hemoptysis + alveolar infiltrates + anemia in a patient with RPGN = **Goodpasture syndrome or GPA with pulmonary hemorrhage**. This mandates: - Plasmapheresis (if anti-GBM or if type uncertain) - High-dose immunosuppression - ICU-level monitoring (airway protection if massive hemorrhage)

Failure to treat aggressively = rapid demise; ARDS from pulmonary hemorrhage is a leading cause of death in RPGN.

### Warning 3: Cyclophosphamide Toxicity

Long-term cyclophosphamide (\>6 months of pulse therapy) increases risk of: - **Hemorrhagic cystitis:** Monitor UAs; ensure aggressive hydration - **Malignancy:** Bladder cancer, lymphoma (5–10% at 10 years) - **Infection:** Opportunistic (PCP, CMV) from prolonged immunosuppression

**Mitigation:** Switch to maintenance azathioprine/MMF after induction; mesna in high-risk cyclophosphamide regimens; PCP prophylaxis if CD4 \<200.

### Warning 4: ANCA-Negative RPGN Has Worse Prognosis

10% of RPGN cases are ANCA-negative pauci-immune. These patients have: - Slower initial response to therapy (delayed Cr improvement) - Higher rates of ESRD progression (50–70% vs 40% in ANCA-positive) - Need for longer, more intensive induction therapy

**Implication:** Do not de-escalate therapy based on ANCA negativity; treat aggressively.

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## Related Resources

- ← Back to RPGN Hub
- ← Back to Glomerular Diseases Hub
- ← Back to Nephrology Hub
- Back to Medical Education Dashboard

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## References

\[1\] Jennette JC, Falk RJ, Bacon PA, et al. 2012 revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides. *Arthritis Rheum*. 2013;65:1–11. [PubMed](https://pubmed.ncbi.nlm.nih.gov/23045170/)

\[2\] McAdoo SP, Pusey CD. Anti-glomerular basement membrane disease. *Clin J Am Soc Nephrol*. 2017;12:1162–1172. [PubMed](https://pubmed.ncbi.nlm.nih.gov/28515156/)

Kidney Disease: Improving Global Outcomes (KDIGO) Glomerular Diseases Work Group. KDIGO 2021 Clinical Practice Guideline for the Management of Glomerular Diseases. Kidney Int. 2021;100(4S):S1-S276. [PubMed](https://pubmed.ncbi.nlm.nih.gov/34556256/)

\[4\] Berden AE, Ferrario F, Hagen EC, et al. Histopathologic classification of ANCA-associated glomerulonephritis. *J Am Soc Nephrol*. 2010. [PubMed PMID 20616173](https://pubmed.ncbi.nlm.nih.gov/20616173/)

\[5\] Jennette JC, Nachman PH. ANCA Glomerulonephritis and Vasculitis. *Clin J Am Soc Nephrol*. 2017. [PubMed PMID 28842398](https://pubmed.ncbi.nlm.nih.gov/28842398/)

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*Last updated: 2026-02-28* *Board-review quality curriculum for nephrology education.*


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