Focal segmental glomerulosclerosis, usually called FSGS, is a pattern of kidney damage in which scar tissue forms in some (but not all) of the tiny filtering units of the kidney. It is one of the leading causes of a condition called nephrotic syndrome in adults and a significant cause of kidney failure worldwide. FSGS is not a single disease so much as a final common pathway: several very different triggers can produce the same scarring pattern, and understanding which trigger is at work determines how aggressively and with what drugs a person gets treated.
What Actually Happens Inside the Kidney
Your kidneys contain roughly one million glomeruli, each one a microscopic cluster of blood vessels surrounded by specialized cells. The key players in FSGS are podocytes, the highly branched cells that wrap around each capillary loop and form the last barrier that keeps large proteins like albumin from leaking into your urine. Podocytes are unusual: they are fully mature, terminally differentiated cells that cannot divide. When they are injured or lost, neighboring podocytes cannot replicate to fill the gap. Making matters worse, podocytes cling to the capillary wall only by thin extensions called foot processes while their cell bodies float freely in the surrounding fluid, which means damaged podocytes can simply detach and wash away in the urine.1American Journal of Physiology-Renal Physiology. The podocyte’s response to stress: the enigma of foot process effacement Once enough podocytes are lost in a segment of a glomerulus, scar tissue fills the empty space. That scarring is what pathologists see under the microscope and label “focal segmental glomerulosclerosis,” meaning some glomeruli (focal) are affected in only parts (segmental) with hardened scar tissue (sclerosis).
Primary, Genetic, and Secondary Forms
Doctors classify FSGS into three broad categories, and getting this distinction right is the single most important step in choosing treatment.2PubMed Central. Differentiating Primary, Genetic, and Secondary FSGS in Adults: A Clinicopathologic Approach
Primary (idiopathic) FSGS is thought to be driven by a circulating factor, likely an immune-related molecule, that injures podocytes. It typically hits suddenly with heavy protein loss in the urine (above 3.5 grams per day) alongside low blood albumin levels, a combination defining full-blown nephrotic syndrome.3Clinical Kidney Journal. Clinical and pathological phenotype of genetic causes of focal segmental glomerulosclerosis in adults Because the cause appears to be immune-mediated, primary FSGS is the form most likely to respond to drugs that suppress the immune system.
Genetic FSGS results from mutations in genes encoding podocyte structural proteins or other glomerular components. Children and young adults with a monogenic (single-gene) cause tend to present earlier in life and are less likely to respond to immunosuppressive treatment.4JAMA Network Open. Comparing Kidney Health Outcomes in Children, Adolescents, and Adults With Focal Segmental Glomerulosclerosis Genetic testing has become increasingly important because giving months of steroids or calcineurin inhibitors to someone whose FSGS is caused by a structural gene defect exposes them to drug side effects with little chance of benefit.
Secondary FSGS encompasses everything else: obesity-related overwork of the glomeruli, viral infections such as HIV, drugs like certain chemotherapy agents, and conditions that reduce the total number of working nephrons (a single functioning kidney, for example). In secondary forms, the protein loss tends to creep up gradually rather than arriving all at once, and the classic full nephrotic syndrome with severe edema is less common.3Clinical Kidney Journal. Clinical and pathological phenotype of genetic causes of focal segmental glomerulosclerosis in adults
The APOL1 Gene and Racial Disparities
One of the most significant genetic discoveries in nephrology over the past two decades involves the APOL1 gene on chromosome 22. Variants in APOL1 were originally identified in a region that also contains MYH9 and are found almost exclusively on chromosomes of African descent.5Kidney International. A risk allele for focal segmental glomerulosclerosis in African Americans is located within a region containing APOL1 and MYH9 African Americans who carry two copies of the high-risk APOL1 variants face roughly seventeen-fold higher odds of developing FSGS compared with those who do not, and about twenty-nine-fold higher odds of HIV-associated kidney disease. APOL1-associated FSGS also tends to appear at a younger age and progress to kidney failure more quickly.6PubMed Central. APOL1 genetic variants in focal segmental glomerulosclerosis and HIV-associated nephropathy
These findings help explain a long-observed pattern: FSGS is diagnosed more frequently in Black Americans, and outcomes tend to be worse. A large registry study found that Black adults with glomerular disease missed more work or school due to kidney disease, reported worse physical health, and had greater fatigue compared with other racial and ethnic groups, though those gaps were largely explained by differences in socioeconomic status and disease severity rather than race itself.7PubMed Central. Racial-Ethnic Differences in Health-Related Quality of Life among Adults and Children with Glomerular Disease In other words, the genetic vulnerability is real and measurable, but the downstream consequences are amplified by social and economic factors.
Symptoms and How FSGS Is Recognized
FSGS does not produce a unique symptom. Instead, it causes nephrotic syndrome, a cluster of problems that can arise from many different kidney diseases. The hallmarks are severe swelling (edema), especially around the eyes, ankles, and feet; heavy protein leakage into the urine, often visible as foamy or frothy urine; and low albumin levels in the blood.8PubMed Central. Recurrence and Treatment after Renal Transplantation in Children with FSGS High cholesterol frequently accompanies the picture, as the liver ramps up protein production to compensate for urinary losses and overshoots into lipid overproduction. Some people also develop blood clots because proteins that regulate clotting are among those lost in the urine.
Because several glomerular diseases can look identical in the clinic, a kidney biopsy is almost always needed to confirm FSGS. The pathologist examines tissue under light microscopy, immunofluorescence, and electron microscopy. Electron microscopy is particularly useful for distinguishing primary FSGS (where foot process flattening is widespread) from secondary forms (where it tends to be patchy). In some cases, electron microscopy also catches underlying basement membrane abnormalities that could point toward a genetic collagen disorder masquerading as FSGS.9PubMed Central. The use of electron microscopy in the diagnosis of focal segmental glomerulosclerosis
Researchers are also exploring whether noninvasive alternatives could eventually reduce the need for biopsy in certain situations. A urine peptide-based classifier has shown promise in distinguishing primary from secondary FSGS with very high specificity, though its sensitivity still needs confirmation in larger independent groups of patients before it could be used in routine clinical practice.10Clinical Kidney Journal. Differentiating primary and secondary FSGS using non-invasive urine biomarkers
Histological Variants and What They Mean
Under the microscope, FSGS does not all look the same. The Columbia classification, published in 2004, divides it into five variants based on where and how the scarring appears: collapsing, tip, cellular, perihilar, and not otherwise specified (NOS). This system has been widely used for over a decade as both a diagnostic and prognostic tool.11PubMed Central. Practical Application of Columbia Classification for Focal Segmental Glomerulosclerosis
These variants are not just academic labels. In a prospective clinical trial, patients with the tip variant had the lowest injury scores, the lowest baseline creatinine, and the slowest rate of disease progression, with only about 7% reaching kidney failure within three years. Patients with the collapsing variant fared the worst: roughly 47% progressed to kidney failure in that same window. The collapsing form was also strongly associated with Black race (63% of cases) and carried the highest pathologic injury scores. The NOS variant fell in between, with about 20% reaching kidney failure at three years.12PubMed Central. Association of histologic variants in FSGS clinical trial with presenting features and outcomes Knowing which variant a patient has helps nephrologists set realistic expectations and decide how aggressively to treat.
Causes of Secondary FSGS
When FSGS is secondary, the treatment strategy shifts from immunosuppression to addressing whatever is injuring the kidney. The list of secondary triggers is long, but a few stand out.
Obesity is one of the more common culprits. Excess body mass forces the remaining glomeruli to filter more blood than they were designed to handle. Over time, that hemodynamic stress injures podocytes. The mechanisms also involve overactivation of the renin-angiotensin system, insulin resistance, chronic low-grade inflammation, and abnormal fat deposition around the kidney.13PubMed Central. Obesity-related glomerulopathy: Current approaches and future perspectives Weight loss and medications that lower the pressure inside the glomerulus (particularly drugs that block the renin-angiotensin system) are the primary treatment here, not immunosuppression.
Viral infections represent another important trigger, especially HIV. The collapsing variant of FSGS is the hallmark kidney lesion in HIV-associated nephropathy. Other viruses linked to collapsing FSGS include cytomegalovirus, Epstein-Barr virus, and parvovirus B19.14PubMed Central. Collapsing Focal Segmental Glomerulosclerosis in Viral Infections In the case of HIV, effective antiretroviral therapy is the treatment; for other viruses, managing the infection and supporting kidney function are the priorities.
Drugs and reduced kidney mass round out the common secondary causes. Certain medications (including some used in cancer therapy and even illicit intravenous drug use) can be directly toxic to podocytes. People born with a single kidney, or who have donated a kidney, or who lost kidney tissue to injury, can develop FSGS years later because the remaining nephrons are overworked.
First-Line Treatment for Primary FSGS
For decades, primary FSGS was considered largely unresponsive to treatment. That view has changed. Over the last twenty years it has become clear that a substantial portion of adults with primary FSGS do respond to immunosuppressive therapy, and achieving remission dramatically improves long-term kidney survival.15Journal of the American Society of Nephrology. Treatment of Primary FSGS in Adults
High-dose corticosteroids (typically prednisone) remain the standard first-line treatment. The catch is that response can be slow: courses often last several months, and the side effects of prolonged steroid use, including weight gain, high blood sugar, bone thinning, and mood changes, are significant. Some patients achieve complete remission, meaning their urine protein drops to near-normal. Others achieve partial remission, a meaningful reduction in protein loss that still carries a much better prognosis than no response at all. In one large validation study, the rate of progression to kidney failure was about 7 per 100 patient-years for those reaching complete remission and about 9 per 100 patient-years for partial remission, compared with roughly 20 per 100 patient-years for non-responders.16PubMed Central. An Outcomes-Based Definition of Proteinuria Remission in Focal Segmental Glomerulosclerosis
The relationship between reducing protein in the urine and preserving kidney function is strikingly consistent across studies. Each meaningful reduction in proteinuria corresponds to a slower decline in filtration rate. One analysis found that every tenfold drop in the urine protein-to-creatinine ratio was associated with about a 4 mL/min/1.73 m² per year improvement in the rate of kidney function change, and the risk of a composite kidney outcome fell by about 77% for that same proteinuria reduction.17PubMed Central. Proteinuria Reduction and Kidney Survival in Focal Segmental Glomerulosclerosis This is why nephrologists monitor urine protein so closely: it is the best available real-time indicator of whether a treatment is working.
When Steroids Are Not Enough
Patients who do not respond to steroids (steroid-resistant) or who relapse as soon as steroids are tapered (steroid-dependent) need second-line options. Calcineurin inhibitors, primarily cyclosporine and tacrolimus, are the most commonly used alternatives. These drugs suppress the immune system through a different pathway than steroids and appear to have a direct protective effect on podocytes as well. A systematic review comparing cyclosporine with mycophenolate plus dexamethasone pulses in steroid-resistant primary FSGS found that the mycophenolate group had somewhat lower odds of at least partial remission, though the difference did not reach statistical significance.18PubMed Central. Calcineurin Inhibitors in the Treatment of Primary Focal Segmental Glomerulosclerosis: A Systematic Review and Meta-analysis of the Literature In practice, many nephrologists use calcineurin inhibitors as their go-to second-line agent.
One limitation of calcineurin inhibitors is that they can themselves be toxic to the kidneys over long periods, so patients often cannot stay on them indefinitely. This creates a frustrating cycle in which FSGS relapses after the drug is withdrawn. Alkylating agents like cyclophosphamide and various combination regimens have also been tried, though evidence from randomized trials remains limited.19Kidney Medicine. Systematic Review and Meta-analysis of Immunosuppressive Therapies in Primary Focal Segmental Glomerulosclerosis
Rituximab and Other Newer Approaches
Rituximab, a drug that depletes a type of immune cell called B cells, has attracted considerable interest for refractory FSGS. Case reports and small series describe patients who achieved complete remission after a single infusion. In one well-documented adult case, a 22-year-old woman with refractory FSGS saw her proteinuria drop from 4.5 grams per day to 0.27 grams within months of a single rituximab dose, and she remained in remission for over four years across two treatment episodes.20PubMed Central. Refractory focal segmental glomerulosclerosis in the adult: complete and sustained remissions of two episodes of nephrotic syndrome after a single dose of rituximab Pediatric case reports have shown similar results, with combinations of rituximab, calcineurin inhibitors, and mycophenolate sustaining remission for years in children with steroid-resistant disease.21PubMed Central. Rituximab, Mycophenolic Acid, and Calcineurin Inhibitors Achieve Long-Term Remission in Pediatric Focal Segmental Glomerulosclerosis with Steroid-Resistant and Frequently Relapsing Nephrotic Syndrome
A more recent and larger study of rituximab as an add-on therapy in adults with primary FSGS reported response rates of about 57% at three months, 71% at six months, and 74% at twelve months. Patients with lower baseline proteinuria and the NOS histological variant were the most likely to respond. Conversely, resistance to calcineurin inhibitors and early B-cell repopulation after treatment predicted a poorer response.22Kidney International Reports. Efficacy of Rituximab as Adjunctive Therapy to Immunosuppressive Agents in Adult Primary Focal Segmental Glomerulosclerosis These results are encouraging but still come from observational data; large randomized trials are needed before rituximab becomes a standard-of-care recommendation.
On an entirely different front, sparsentan represents a new class of drug. It is a dual-acting molecule that blocks both the endothelin receptor and the angiotensin receptor, targeting two pathways involved in glomerular damage simultaneously. In a randomized trial comparing sparsentan with irbesartan (a standard angiotensin blocker), about 42% of patients on sparsentan achieved partial remission of proteinuria at 36 weeks compared with 26% on irbesartan, a statistically significant difference. That benefit was sustained through 108 weeks. However, the rate of kidney function decline over two years was not significantly different between the two groups.23New England Journal of Medicine. Sparsentan versus Irbesartan in Focal Segmental Glomerulosclerosis Whether the greater proteinuria reduction from sparsentan will translate into preserved kidney function over a longer follow-up remains an open question, and it is one that the nephrology community is watching closely.
FSGS After Kidney Transplantation
One of the more disheartening aspects of primary FSGS is that it can come back in a transplanted kidney, sometimes within hours of surgery. This recurrence supports the idea that a circulating factor, not something intrinsic to the kidney itself, drives the disease. Recurrent FSGS after transplant leads to accelerated graft loss and is a major concern for transplant teams evaluating candidates with a history of primary FSGS.
Research into predicting who will recur has turned up a panel of seven antibodies targeting glomerular proteins that, when measured in pre-transplant blood samples, predicted recurrence with about 92% accuracy across a multi-center cohort. Among those, antibodies against CD40, a protein expressed on podocytes, showed the strongest individual correlation. Purified anti-CD40 antibodies from patients with recurrent FSGS proved directly harmful to podocytes in laboratory cultures and amplified proteinuria in animal models.24PubMed Central. A circulating antibody panel for pretransplant prediction of FSGS recurrence after kidney transplantation If validated in larger studies, such a panel could help transplant centers identify high-risk patients and prepare preemptive treatment strategies, including plasma exchange to strip out the offending antibodies before or immediately after surgery.
Practical Predictors of Outcome
Beyond histological variant and treatment response, several real-world factors influence how FSGS plays out. A large registry-based study identified the amount of protein in the urine at baseline as the single strongest predictor of major kidney events: patients with above-median proteinuria had about 3.5 times the risk of a composite outcome that included a 40% decline in kidney function, kidney failure, or death. Hospitalization, lower starting kidney function, and non-commercial health insurance also predicted worse outcomes, underscoring how tightly clinical trajectories are linked to access and resources.25PubMed Central. Population risk predictors of major adverse kidney events attributed to focal segmental glomerulosclerosis from the CURE-CKD registry
For children and adolescents, the picture is somewhat more nuanced. Monogenic causes of FSGS cluster in younger patients and carry a different prognosis from immune-mediated disease. Once patients with known genetic mutations are excluded, however, the overall kidney outcomes in children appear comparable to those in adults, suggesting the biology of non-genetic FSGS is similar regardless of the patient’s age.4JAMA Network Open. Comparing Kidney Health Outcomes in Children, Adolescents, and Adults With Focal Segmental Glomerulosclerosis
A Diagnosis That Has Kept Evolving
FSGS has been a moving target since it was first described. The term itself is fundamentally a description of what the pathologist sees under the microscope: scarring in some glomeruli, in some segments. It was not designed to specify a cause. Over the last century, the definitions and morphologic features used to diagnose FSGS have been repeatedly refined, and the clinical diversity of conditions lumped under the FSGS label has made it one of the more problematic diagnoses in kidney pathology.26PubMed. Focal segmental glomerulosclerosis: a morphologic diagnosis in evolution There is active debate in the nephrology community about whether “FSGS” should eventually be retired as a disease name in favor of more precise labels tied to cause, such as “APOL1-associated podocytopathy” or “circulating-factor glomerulopathy.” For now, FSGS remains the working term, but patients should understand that two people carrying the same diagnosis may have fundamentally different diseases requiring fundamentally different treatments.