• MPGN accounts for approximately 5–10% of all primary glomerulonephritides in adults and a smaller proportion in children
• It is relatively uncommon compared to IgA nephropathy (the most common primary GN worldwide) and membranous nephropathy
• Bimodal age distribution: can present in children/adolescents (historically more common in paediatrics, especially MPGN Type II/C3 glomerulopathy) and in middle-aged adults (often secondary causes)
• No strong sex predilection overall, though some series suggest slight male predominance

• In Hong Kong, IgA nephropathy is by far the most common primary GN — MPGN is relatively rare
• HBV- and HCV-related MPGN are important secondary causes in Hong Kong given the relatively high prevalence of chronic hepatitis B (~8% of the population are HBsAg carriers historically) and the presence of hepatitis C ^[1][2]
• HCV extrahepatic manifestation: membranoproliferative glomerulonephritis (screening recommended) ^[4]
• Lupus nephritis (another cause of MPGN pattern) is relatively common in young Chinese women

• The incidence of primary (idiopathic) MPGN has declined over the past few decades in developed countries, partly due to:
  • Better recognition and treatment of underlying infections (HCV, HBV)
  • Reclassification: many previously "idiopathic" cases are now recognized as complement-mediated (C3 glomerulopathy) or immune complex-mediated with identifiable causes
• The modern reclassification system (2012 onwards) has shifted the paradigm from a morphology-based to a pathogenesis-based approach

1. Fenestrated endothelium — the innermost layer facing the capillary lumen; has pores (~70–100 nm) that allow passage of plasma but block blood cells
2. Glomerular basement membrane (GBM) — the middle acellular layer; a specialized extracellular matrix composed of type IV collagen, laminin, proteoglycans (heparan sulfate providing charge selectivity), and nidogen. Acts as both a size and charge barrier
3. Podocytes (visceral epithelial cells) — the outermost layer; have interdigitating foot processes connected by slit diaphragms (nephrin, podocin proteins). This is the final size-selective barrier

• Mesangial cells sit between and support the capillary loops within the glomerulus
• They produce mesangial matrix (similar to basement membrane material)
• Functions: structural support, regulation of glomerular blood flow (contractile), phagocytosis and clearance of macromolecules/immune complexes from the GBM
• In MPGN, mesangial cells proliferate and extend into the subendothelial space (mesangial interposition), which is the key pathological event leading to GBM duplication

• The alternative pathway of complement is normally in a state of low-level continuous activation ("tick-over" of C3)
• Regulatory proteins (Factor H, Factor I, Factor B, MCP/CD46) keep this in check
• If regulation fails → uncontrolled complement activation on the GBM → C3 deposition → inflammation → MPGN pattern

The traditional classification (Type I, II, III based on electron microscopy location of deposits) has been superseded by a pathogenesis-based classification since approximately 2012–2013 (Sethi & Fervenza, JASN 2012). This is because the EM-based classification didn't correlate well with clinical outcomes or guide treatment.

The modern approach divides MPGN into two major categories based on immunofluorescence (IF) findings:

IF shows dominant C3 deposition with absent or minimal immunoglobulin staining. This defines the entity of C3 glomerulopathy (C3G), which is subdivided by EM into:

Why does complement dysregulation cause MPGN?

• Uncontrolled C3 activation → C3b deposition on GBM and mesangium → recruitment of inflammatory cells → mesangial proliferation → GBM damage → characteristic MPGN pattern
• Causes of alternative pathway dysregulation:
  • C3 nephritic factor (C3NeF): autoantibody against C3bBb convertase → stabilizes it → persistent C3 cleavage → low serum C3
  • Genetic mutations: Factor H, Factor I, Factor B, C3, MCP/CD46 — loss-of-function in regulatory proteins or gain-of-function in activation proteins
  • Anti-Factor H antibodies: acquired inhibition of complement regulation

Step 1: Initiating Event

• Immune complexes deposit in the subendothelial space and mesangium (immune complex-mediated)
• OR complement fragments (C3b, C3c, C3d) deposit along the GBM (complement-mediated)

Step 2: Mesangial Response

• Mesangial cells are activated → proliferate (hence "proliferative")
• Activated mesangial cells interpose (extend) themselves between the endothelium and the GBM → this creates a new layer of basement membrane-like material on the inner (subendothelial) side
• This "sandwich" of original GBM + new mesangial-derived matrix = "double contour" or "tram-track" appearance on silver stain or PAS stain under light microscopy

Step 3: Inflammatory Amplification

• Complement activation (classical or alternative pathway depending on etiology) → C5a generation → neutrophil and monocyte recruitment
• Release of proteases, reactive oxygen species → further GBM damage
• Matrix expansion → lobular accentuation of glomerular tuft

Step 4: Consequences on the Filtration Barrier

• GBM thickening + duplication → disruption of size and charge selectivity → proteinuria (can reach nephrotic range)
• Endothelial/mesangial proliferation + inflammation → reduced capillary lumen → reduced GFR (nephritic features: oliguria, rising creatinine)
• Inflammatory breach of GBM → red blood cells leak through → haematuria (with dysmorphic RBCs and RBC casts)
• Reduced GFR + sodium/water retention → oedema and hypertension

MPGN characteristically shows: proteinuria (++), nephrotic (++), nephritic (+), haematuria (+), ARF (+), CRF (+) — it is the quintessential "mixed" glomerulonephritis ^[3].

Persistently low C3 > 8 weeks raises the possibility of lupus nephritis or membranoproliferative glomerulonephritis (complement-mediated MPGN) ^[7].

MPGN as a primary proliferative GN alongside IgA nephropathy and crescentic GN ^[1][2].

Secondary MPGN: HBV/HCV-related MPGN, lupus nephritis, cryoglobulinaemia ^[1][2].

Symptoms related to underlying cause:

• Mesangial hypercellularity — increased number of mesangial cells (> 3 per mesangial area)
• GBM thickening and splitting — the "double contour" / "tram-track" appearance (best seen on PAS or silver stain) caused by mesangial interposition between the endothelium and GBM with deposition of new basement membrane material
• Lobular accentuation of the glomerular tuft — exaggeration of the normal lobular architecture due to mesangial expansion
• Endocapillary proliferation — increased cellularity within capillary lumens
• Possible crescent formation if disease is severe/rapidly progressive

• Immune complex-mediated MPGN: granular deposits of IgG, IgM, C3, C1q along capillary walls and mesangium
• C3 Glomerulopathy: dominant C3 staining with absent or scanty immunoglobulin (by definition, C3 staining intensity must be ≥ 2 orders of magnitude greater than any Ig staining)

• Immune complex MPGN (old Type I/III): electron-dense deposits in subendothelial (Type I) ± subepithelial (Type III) locations
• DDD (old Type II): dense, ribbon-like intramembranous deposits within the lamina densa of the GBM — highly characteristic and virtually diagnostic
• C3GN: deposits in varied locations (mesangial, subendothelial, subepithelial, intramembranous) but NOT the characteristic "dense deposit" pattern of DDD

Why it's in the DDx: PSGN is the prototypical acute nephritic syndrome — haematuria, oedema, hypertension, oliguria, with low C3. The acute presentation of MPGN may be indistinguishable from PSGN, but MPGN usually persists beyond 4–6 weeks with further increase in serum creatinine ^[11].

The discriminating principle: Time. PSGN resolves. MPGN does not. If clinical features or low C3 persist beyond 6–8 weeks, suspect MPGN.

Why it's in the DDx: Lupus nephritis is classified as a secondary proliferative GN ^[2] and can produce an MPGN-like histological pattern (especially Class III/IV diffuse proliferative lupus nephritis). It also causes hypocomplementaemia. In Hong Kong, SLE is relatively common in young Chinese women.

The discriminating principle: Systemic features of SLE + positive ANA/anti-dsDNA + "full house" IF. If your MPGN patient has positive ANA and extrarenal features — think lupus nephritis as the underlying driver.

Why it's in the DDx: IgA nephropathy is the most common primary GN globally and in Hong Kong ^[14]. It can present with haematuria and proteinuria, occasionally with nephrotic features. However, the clinical pattern differs.

The discriminating principle: Normal complement + synpharyngitic timing + mesangial IgA on biopsy = IgA nephropathy, not MPGN.

Why it's in the DDx: Essential mixed cryoglobulinaemia (Type II) is a classic cause of immune complex-mediated MPGN. It is strongly associated with HCV ^[4]. The clinical presentation overlaps heavily with MPGN because cryoglobulinaemia IS one of the causes of MPGN.

The discriminating principle: Disproportionately low C4, palpable purpura, positive cryocrit, and HCV seropositivity all point to cryoglobulinaemia.

Why it's in the DDx: ANCA vasculitis (GPA, MPA, EGPA) can cause rapidly progressive GN with haematuria and proteinuria, mimicking MPGN. However, it is a fundamentally different mechanism — pauci-immune (no or minimal immune deposits on IF) ^[16].

The discriminating principle: Normal complement + positive ANCA + pauci-immune IF = vasculitis, not MPGN.

Why it's in the DDx: Presents with RPGN (haematuria, rapidly declining renal function) ± pulmonary haemorrhage. Can be confused with severe MPGN presenting as RPGN.

The discriminating principle: Linear IF staining + positive anti-GBM antibody + pulmonary-renal syndrome.

Why it's in the DDx: Both MPGN and membranous nephropathy cause nephrotic syndrome with GBM thickening. However, membranous nephropathy is non-proliferative ^[2] and presents with a bland sediment.

The discriminating principle: Bland sediment + normal complement + non-proliferative histology = membranous. Active sediment + low complement + proliferative histology = MPGN.

Why they're in the DDx: Both cause nephrotic syndrome, which MPGN can also cause. However:

In an older patient (> 40 years), MPGN pattern on biopsy must prompt screening for monoclonal gammopathy ^[6]. The monoclonal immunoglobulin deposits in the glomerulus and drives the MPGN pattern. This is not really a separate differential — it is an underlying cause of immune complex-mediated MPGN — but you must think of it.

Workup for GN includes malignancy screen (e.g. tumour markers, SIEP) especially in elderly ^[17][18].

The patient has some combination of:

• Haematuria (macro or micro, glomerular type — dysmorphic RBCs, RBC casts)
• Proteinuria (subnephrotic to nephrotic range)
• Renal impairment (rising creatinine/falling GFR)
• Hypertension
• Oedema

Diagnosis of renal diseases for specific treatment begins with clinical history and manifestation: oliguria or anuria, haematuria, proteinuria, oedema, hypertension, loin pain ^[8].

You recognize this is a glomerulonephritis. Now the question is: which one?

Basic laboratory investigations: blood for renal function test (urea and creatinine concentration), urine for microscopy (red cells and white cells), urine for quantification of protein (e.g. 24-hour urine for protein) ^[8].

Serum complement level is important in helping narrow the differential diagnosis ^[20][21].

↓C3/C4 generally indicate immune complex-mediated GN — DDx: MPGN, PSGN, lupus, cryoglobulinaemia, infective endocarditis and shunt nephritis ^[20][21].

Normal C3/C4 generally indicate non-immune-complex-mediated GN (except IgA nephropathy) — DDx: PAN, Goodpasture, HSP/IgAN, ANCA-related renal vasculitis ^[20][21].

This is where MPGN enters the differential — if complement is low.

Work up for glomerulonephritis: autoimmune markers (e.g. ANA, Anti-dsDNA, C3/4, ANCA, Anti-GBM, CRP, cryoglobulins), exclude infective causes (e.g. HBV/HCV/HIV/VDRL/malaria), malignancy screen (e.g. tumour markers, SIEP) especially in elderly ^[17][18].

Renal biopsy is necessary for most cases of nephritic syndrome unless very small kidney on USG (indicating chronic irreversible disease where biopsy won't change management) ^[20][21].

Immunofluorescence pattern is most helpful for diagnosis ^[20][21].

Once biopsy confirms MPGN pattern:

• IF: Ig + C3 → Immune complex-mediated MPGN → search systematically for the underlying cause
• IF: C3-dominant, Ig-negative → C3 Glomerulopathy → proceed to complement studies

This is where MPGN stands out from most other GNs.

GC 057 High Yield: "Total haemolytic complement activity and C3 concentrations are depressed. Persistently low C3 > 8 weeks raises the possibility of lupus nephritis or membranoproliferative glomerulonephritis." ^[23]

Advanced complement studies (for C3 Glomerulopathy workup):

Serology for relevant conditions ^[20][21]:

Renal biopsy is still essential for definitive diagnoses of membranoproliferative GN ^[8].

Why biopsy? Because:

1. You cannot distinguish MPGN from other GNs clinically
2. The IF pattern determines whether it is immune complex-mediated vs complement-mediated
3. The EM pattern determines the subtype (DDD vs C3GN)
4. Histological findings guide treatment and prognosis

Contraindications to percutaneous renal biopsy ^[24]:

• Bleeding diathesis
• Severe uncontrolled hypertension
• Solitary kidney
• Small kidneys indicative of chronic irreversible disease
• Hydronephrosis
• Renal or perirenal infection

How it's done: Percutaneous ultrasound-guided biopsy of the lower pole of the kidney (to avoid major vessels). Typically obtain 2 cores — one for LM/IF, one for EM.

What to look for on biopsy and the expected findings in MPGN:

GC 057 High Yield — Primary kidney diseases that can cause nephrotic syndrome include: focal segmental glomerulosclerosis, membranous glomerulopathy, minimal change disease, membranoproliferative glomerulonephritis, and occasionally IgA nephropathy ^[23].

Monitoring of blood pressure and body weight, home urine dipstick monitoring ^[25].

Why is diuretic use tricky in nephrotic syndrome? In nephrotic patients, the low albumin means oncotic pressure is already reduced, so intravascular volume may be DECREASED even though total body water is INCREASED (oedema). Aggressive diuresis can cause intravascular depletion → pre-renal AKI. Think of it as: the water is in the wrong compartment.

Albumin infusion: Reserved as adjunct to diuretics only for diuretic-resistant oedema with oliguria/uraemia in the absence of severe glomerular damage ^[26]. IV albumin transiently raises oncotic pressure, pulling fluid back into the intravascular space, but is quickly lost in the urine due to the leaky GBM — so the benefit is short-lived.

Anti-proteinuric drugs (ACEI/ARB): for ALL glomerulopathies — ↓glomerular pressure, ↓rate of GFR decline ^[15].

Why do they work? Angiotensin II normally constricts the efferent arteriole more than the afferent. This maintains intraglomerular pressure (useful in dehydration). In glomerular disease, this high pressure is harmful — it pushes more protein across the damaged GBM. By blocking angiotensin II, you dilate the efferent arteriole → drop intraglomerular pressure → less "force" pushing protein through → less proteinuria → slower progression.

ACEI or ARB is a compelling indication in both diabetes and chronic kidney disease ^[28].

Statins: if hyperlipidaemia persists after treatment ^[15].

DVT prophylaxis: compressive stockings ± anticoagulation if high risk ^[15].

Why are nephrotic patients hypercoagulable? Because:

1. Urinary loss of antithrombin III (a natural anticoagulant — small enough to be filtered through damaged GBM)
2. Compensatory hepatic upregulation of procoagulant factors (fibrinogen, factors V, VII, VIII)
3. Thrombocytosis (reactive)
4. Hyperviscosity from haemoconcentration

Nutritious and relatively low-fat diet recommended in patients on steroids ^[25]. Protein restriction is NOT recommended — patients are already losing protein in the urine, so restricting dietary protein would worsen hypoalbuminaemia ^[26].

Pneumococcal vaccination is indicated for ALL patients with nephrotic syndrome because urinary loss of immunoglobulins increases susceptibility to encapsulated organisms (especially Streptococcus pneumoniae) ^[26].

This is one of the most common identifiable causes of immune complex-mediated MPGN, especially in adults. The treatment has been revolutionized by direct-acting antivirals (DAAs).

SLE management depends on severity ^[30]:

Why MMF? Mycophenolate (myco = fungal origin, phenolate = phenolic acid derivative) inhibits inosine monophosphate dehydrogenase → blocks de novo purine synthesis → selectively suppresses lymphocyte proliferation (lymphocytes depend on de novo pathway more than other cells which can use salvage pathway). This makes it relatively lymphocyte-selective with fewer side effects than cyclophosphamide.

Why cyclophosphamide? An alkylating agent that cross-links DNA → prevents cell replication. Potent but toxic: risk of haemorrhagic cystitis (prevent with mesna + hydration), infection (PCP prophylaxis with co-trimoxazole), gonadal toxicity (fertility preservation discussion essential), and malignancy. Reserved for severe or refractory disease.

When the full workup is negative for secondary causes:

Idiopathic MPGN: steroid ± cyclophosphamide (if deteriorating renal function) ^[15].

This can occur due to:

• Crescentic transformation (RPGN)
• Over-diuresis (hypovolaemia)
• Nephrotoxic drugs (NSAIDs, contrast)

Immediate approach to AKI ^[31]:

• Ensure ABC: "A dead person has no renal function"
• Treat hypoxaemia, treat hypotension with fluid resuscitation
• Consider and reverse nephrotoxic drugs (NSAIDs, aminoglycosides, ACEI/ARB)
• Manage life-threatening complications:
  • Fluid overload → IV loop diuretics, dialysis
  • Hyperkalaemia → IV calcium, insulin/dextrose, kayexalate, dialysis
  • Metabolic acidosis → IV bicarbonate, dialysis

If RPGN (crescentic transformation): This is a medical emergency. Treatment follows RPGN protocols based on the IF pattern:

• Usually requires IV pulse methylprednisolone followed by oral prednisolone ± cyclophosphamide or rituximab ^[15][16]
• Can give empirical pulse IV methylprednisolone before renal biopsy if indicated ^[16]

Indications for haemodialysis (mnemonic: AEIOU) ^[31]:

• Acidosis: metabolic acidosis pH < 7.1 refractory to bicarbonate
• Electrolytes: hyperkalaemia > 6.5 or rapidly rising, refractory to medical Rx
• Intoxication: drug removal in overdose
• Overload: fluid overload refractory to diuretics
• Uraemia: features of uraemia (pericarditis, neuropathy, encephalopathy)

MPGN has poor prognosis in general — 50% develop end-stage disease in 10 years after diagnosis ^[13][19].

Therapeutic objectives for CKD ^[22]:

• Delay kidney failure
• Control hypertension
• Reduce albuminuria
• Treat anaemia and MBD (mineral bone disease)
• Treat acidosis, hyperkalaemia
• Control lipids and cardiovascular risk

If progression to stage 5 CKD / ESRD despite all measures:

• Preparation for renal replacement therapy ^[32]
• Options: peritoneal dialysis, haemodialysis, or renal transplantation