Rare Kidney Disease C3G: Types, Diagnosis, and New Therapies

Treatments and therapies Diseases and conditions Medicines Clinical trials Rare kidney diseases and disorders Kidney diseases and conditions Glomerular diseases C3 glomerulopathy

September 04, 2025

Most people have never heard of C3G kidney disease. Short for complement 3 glomerulopathy, this rare condition can cause inflammation and scarring in the kidneys. C3G can progress to kidney failure if left untreated.

The good news? Researchers are making exciting progress in understanding and treating C3G.

Here’s what you should know.

What is C3G?

C3G stands for complement 3 glomerulopathy.

Let's break it down:

Complement 3: A protein that plays a role in immunity.
Glomerulopathy: Damage to glomeruli, the small filtering units in the kidney

“Glomerulonephritis can happen for many reasons: autoimmune conditions like lupus, infections, or some cancers,” said nephrologist Dr. Andrew Bomback. “C3G is inflammation or swelling caused by an overactive immune response from the complement system.”

What Causes C3G?

A part of the immune system called the complement system usually helps the body fight infections and remove damaged cells. Sometimes this system goes into overdrive and starts targeting the body, causing C3G.

The complement system is an important part of the immune system that helps the body fight off germs and other invaders.

It’s Activated by Three Immune Pathways, or Parts

Classical Pathway

Triggered when antibodies attach to germs or damaged cells.

Lectin Pathway

Activated when certain proteins in the blood spot sugars on the surface of foreign objects.

Alternative Pathway

Always “on” at a low level, ready to fight germs, but can cause problems if it becomes overactive.

“In C3G, the alternative pathway goes from low-level activity to hyperactivity,” said Dr. Bomback. “You essentially lose the brakes on the system.”

When the brakes are gone, the alternative pathway makes too many complement proteins.

“These proteins start to damage the kidneys. This causes inflammation at first,” Dr. Bomback explained, “If left unchecked, that inflammation leads to scarring.”

Types of C3G

C3G is an umbrella term for kidney damage caused by an overactive complement system.

There are Two Types of C3G

Dense Deposit Disease (DDD)

Deposits of complement protein form in the intramembranous region, a specific layer of the filtering barriers in the kidney.

C3 Glomerulonephritis (C3GN)

Deposits of complement protein form in several parts of the kidney.

“We believe DDD is the more aggressive type. It has the fastest rate of kidney failure and often reoccurs quickly after transplant,” said Dr. Bomback. “C3G is also serious, but may progress more slowly.”

Learn more about the types of C3G.

Diagnosing C3G

Kidney disease is diagnosed with simple blood and urine tests. However, these tests cannot confirm C3G.

Since C3G damages specific parts of the kidney, the only way to definitively diagnose it is with a kidney biopsy.

“A kidney biopsy takes a tiny piece of the kidney to review under a microscope,” Dr. Bomback said. “The biopsy should be reviewed by someone with experience in identifying C3G since it is so rare.”

Kidney biopsies are also the only way to determine whether a treatment is working or not. It is normal to receive multiple biopsies if you have C3G.

Learn more about diagnosing C3G.

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C3G Treatments

For a long time, there were no specific treatments for C3 glomerulopathy.

Doctors borrowed medicines from other kidney diseases and suggested lifestyle changes.

These include:

Blood pressure medicines (ACE inhibitors or ARBs): Help control blood pressure and reduce protein loss in urine.
Corticosteroids and immunosuppressive drugs: Calm the immune system so it doesn’t attack the kidney filters (glomeruli). These are also used in people with kidney transplants.
Cholesterol medicines (statins): Help protect the heart.

Healthy habits important to managing C3G include:

Regular exercise: Helps keep your heart and kidneys healthy.
Limiting sodium (salt): Especially important if you have high blood pressure.
Following a kidney-friendly diet: A kidney dietitian can help create a meal plan that meets your needs and protects your kidneys.

Learn more about kidney protective medications.

Thanks to patient participation in clinical trials, targeted treatments for C3G are now available.

Complement-Targeting Therapies

These are drugs that can directly target an overactive complement system, which is the main driver of C3G.

Eculizumab and rituximab are monoclonal antibodies, lab-made proteins, that have been used to slow down the complement system. They may help some people, but they don’t always work and can have major side effects.

Newer complement-targeting therapies include:

Iptacopan: A complement factor B inhibitor that helps regulate C3 breakdown.
Pegcetacoplan: A complement C3 inhibitor that blocks C3 to reduce kidney damage.

Both medicines have been approved by the FDA in 2025 for the treatment of C3G.

Since these medications target the immune system, one of the biggest risks associated with them is infections.

“Your healthcare professional will watch for infections because the complement system helps fight germs,” Dr. Bomback said. “Patients may need vaccines or temporary antibiotics before starting a drug.”

Other possible treatments for C3G are in clinical trials. You can discuss available C3G treatments or clinical trials with a healthcare professional to find out which may be best for you.