Lupus Nephritis: Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC
July 20, 2026

Lupus nephritis is inflammation and injury of the kidneys caused by systemic lupus erythematosus (usually just called lupus), an autoimmune disease in which the immune system attacks the body’s own tissues. In lupus nephritis, the immune system forms clumps called immune complexes that lodge in the kidney’s tiny filters and trigger inflammation. It is one of the most serious complications of lupus, because untreated it can lead to permanent kidney damage, and it is common: a large proportion of people with lupus develop some degree of kidney involvement.

A kidney biopsy is central to lupus nephritis, because the way the disease looks under the microscope determines both the treatment and the outlook. The report is organized around a classification system with six classes and two additional scores, and this can be difficult to interpret. This article will help you understand the findings in your pathology report for lupus nephritis, what each term means, and why it matters for your care. The classification section explains the class number and the activity and chronicity scores you are most likely to see on the report.

What do the kidneys do?

The kidneys filter the blood, removing waste and extra water as urine while keeping the body’s salts and fluids in balance. The filtering happens in millions of tiny units called nephrons. At the start of each nephron is a cluster of small blood vessels called a glomerulus (the plural is glomeruli), which works like a sieve. Within each glomerulus, a supporting tissue called the mesangium holds the tiny vessels in place. Lupus nephritis is mainly a disease of the glomerulus, and the classification of lupus nephritis is based largely on where in the glomerulus the immune complexes settle and how much damage they cause, so these terms come up throughout the report.

What causes lupus nephritis?

Lupus nephritis is caused by the underlying autoimmune process of lupus. In lupus, the immune system produces antibodies against the body’s own components, including its DNA. These antibodies bind their targets to form immune complexes, which circulate in the blood and become trapped in the glomeruli of the kidney, or form there directly. Once lodged in the kidney, the immune complexes activate a part of the immune system called the complement system, drawing in inflammatory cells and injuring the filter. Where the complexes settle, in the mesangium, along the inner or outer side of the filter wall, determines what type of injury results and which class of lupus nephritis develops.

Lupus, and therefore lupus nephritis, is far more common in women than in men, and it often develops during the childbearing years. It is also more common, and frequently more severe, in people of African, Hispanic, and Asian ancestry. Kidney involvement most often appears within the first few years after a lupus diagnosis, and in some people the kidney disease is the first sign that leads to the diagnosis of lupus.

What are the symptoms of lupus nephritis?

Lupus nephritis can be present before it causes any symptoms, which is why people with lupus have their urine and blood checked regularly. When symptoms occur, they come from protein and blood leaking into the urine and from declining kidney function, and may include:

Foamy or bubbly urine, from protein leaking into the urine (proteinuria).
Blood in the urine, sometimes visible but often detectable only on a urine test (hematuria).
Swelling of the legs, ankles, feet, or around the eyes, from fluid retention.
High blood pressure.
Fatigue, and in more advanced disease, the effects of reduced kidney function.

Because kidney involvement can be silent, the decision to perform a biopsy is often prompted by abnormal urine or blood tests in a person known to have lupus, rather than by symptoms.

How is the diagnosis made?

Lupus nephritis is diagnosed from a kidney biopsy, in which a thin needle is used to take one or more small cores of kidney tissue that a pathologist examines under the microscope. Blood tests support the diagnosis, including antibodies characteristic of lupus (such as antinuclear antibodies and anti-double-stranded-DNA antibodies) and low levels of complement proteins, which fall as they are consumed by the disease. But these tests cannot show how the kidney itself is affected, and only a biopsy can determine the class of lupus nephritis, which is what guides treatment. The biopsy is examined in three complementary ways, and all three appear in the report.

Light microscopy, using stained slices of tissue under an ordinary microscope, shows the pattern and severity of injury in the glomeruli and the surrounding kidney, and is the basis of the class and the activity and chronicity scores described below. Immunofluorescence, which uses antibodies tagged with a glowing dye to detect deposited proteins, characteristically shows a striking pattern in lupus nephritis called “full house,” meaning the tissue lights up for a full set of immunoglobulins and complement proteins (IgG, IgA, IgM, C3, and C1q) together. This full-house pattern is highly suggestive of lupus and is one of the features that distinguishes it from other kidney diseases. Electron microscopy, which magnifies the tissue enormously, shows the immune complexes directly as dense deposits and reveals their exact location, which refines the class; it may also show tiny structures called tubuloreticular inclusions that point to lupus.

The lupus nephritis classes

The heart of a lupus nephritis report is its class, assigned using a system developed by the International Society of Nephrology and the Renal Pathology Society, usually abbreviated ISN/RPS. It sorts the disease into six classes based on where the immune complexes deposit and how much of the glomerulus is affected. The class largely determines the treatment, so it is the single most important part of the report to understand.

Class I (minimal mesangial) — Immune complexes are present in the mesangium but the glomeruli look normal under an ordinary microscope. This is the mildest form and usually needs no specific kidney treatment.
Class II (mesangial proliferative) — Immune complexes in the mesangium, now with an increase in mesangial cells visible under the microscope. Still relatively mild, and kidney function is usually preserved.
Class III (focal) — Active inflammation involving less than half of the glomeruli. This is one of the more serious, “proliferative” forms and generally requires substantial treatment.
Class IV (diffuse) — Active inflammation involving half or more of the glomeruli. This is the most common and often the most serious form, and it requires the most intensive treatment.
Class V (membranous) — Immune complexes deposit along the outer side of the filter wall, thickening it and causing heavy protein loss, similar to a separate disease called membranous nephropathy. Class V can occur on its own or together with class III or IV.
Class VI (advanced sclerosing) — More than 90% of the glomeruli are permanently scarred, reflecting long-standing, burned-out disease. At this stage, immunosuppressive treatment is unlikely to help the kidney recover.

Two points make the class more informative than it first appears. First, classes III and IV are the “proliferative” forms, marked by active inflammation, and these are the ones that most need strong treatment to prevent permanent damage. Second, more than one class can be present at the same time, most often class V combined with class III or IV, and the report will list both, because the combination affects treatment.

The activity and chronicity scores

Because the class alone does not capture how much of the injury is active and reversible versus permanent, the report for class III, IV, or V lupus nephritis also gives two scores. Understanding the difference between them is key to reading the report, because they point in opposite directions.

Activity index — Measures how much active, ongoing inflammation is present, by scoring features such as increased cells in the glomeruli, inflammatory cell infiltration, cell death, crescents (cellular clusters that form with severe injury), and deposits called wire loops. A higher activity index means more inflammation that treatment can potentially reverse. This score runs from 0 to 24.
Chronicity index — Measures how much permanent scarring has already occurred, by scoring scarred glomeruli (glomerulosclerosis), scarring of the supporting tissue (fibrosis), and shrinkage of the tubules. A higher chronicity index means more damage that cannot be reversed. This score runs from 0 to 12.

Together these scores tell the treatment team how much of the disease is worth treating intensively (high activity, low chronicity favors intensive treatment aimed at recovery) versus how much is already permanent (high chronicity signals damage that treatment cannot undo). Two people with the same class can have very different activity and chronicity scores, and therefore different treatment and outlook.

Other findings described in the report

Alongside the class and scores, the report often describes additional findings that help complete the picture:

Wire-loop lesions — Thickened, stiff-looking glomerular capillary walls caused by large immune-complex deposits, a classic sign of active lupus nephritis.
Crescents — Cellular clusters that form in the space around the glomerulus when injury is severe. Their presence indicates active, severe disease and contributes to the activity index.
Tubulointerstitial disease — Inflammation and scarring in the tissue around the tubules. When extensive, it is an important predictor of long-term kidney function, sometimes more so than the glomerular changes.
Vascular lesions — Changes in the kidney’s blood vessels, which can occur in lupus and may affect the outlook.

What is the outlook?

The outlook for lupus nephritis has improved substantially with modern treatment, and many people achieve remission and keep good long-term kidney function. The outlook depends heavily on the biopsy findings and on the response to treatment. The proliferative forms (classes III and IV) carry the greatest risk of progressing to kidney failure if not treated adequately, but they also have the most to gain from treatment when caught while the disease is still active. A high chronicity index, with extensive permanent scarring, points to a more guarded outlook, because that damage cannot be reversed, which is one reason early diagnosis and treatment matter so much. Reaching class VI, with widespread scarring, reflects advanced disease.

Beyond the biopsy, the factors most strongly linked with a poorer outcome are a large and persistent amount of protein in the urine, reduced kidney function at diagnosis, a failure to respond to initial treatment, and repeated flares of the disease. Because lupus nephritis can relapse, long-term monitoring is an essential part of care even after remission.

What happens after the diagnosis?

Lupus nephritis is managed jointly by a kidney specialist (a nephrologist) and often a rheumatologist, and treatment is guided closely by the class, the activity and chronicity scores, and the amount of protein in the urine. Treatment has expanded in recent years with newer medications added to the long-standing ones. This is a general overview; the specific plan is individual, and this article does not recommend any particular treatment.

Hydroxychloroquine for nearly everyone — This antimalarial drug is recommended for almost all people with lupus, including those with lupus nephritis, because it reduces flares and improves long-term outcomes.
Treating active proliferative disease — For classes III and IV, and often class V with heavy protein loss, treatment has two phases. An initial (induction) phase uses corticosteroids together with a stronger immunosuppressant, most often mycophenolate or cyclophosphamide, to bring the inflammation under control. This is followed by a longer maintenance phase, usually with mycophenolate or azathioprine, to keep the disease in remission and prevent relapse, generally continued for several years.
Newer add-on medications — Two drugs approved more recently are now added to standard treatment in many patients: belimumab, which reduces the immune cells that produce the harmful antibodies, and voclosporin, which helps control protein loss. Current guidelines support using one of these alongside the standard regimen, particularly in more severe disease or when extra protection is needed.
Supportive care — Blood pressure control and a drug that blocks the renin-angiotensin system (an ACE inhibitor or ARB) are used to lower protein in the urine and protect the kidney, alongside the immune-directed treatment.
Monitoring — Kidney function, protein in the urine, and lupus blood markers are followed over time. A repeat biopsy is sometimes done to reassess the class and the activity and chronicity scores, especially if the disease flares or does not respond, because the class can change over time.

Questions to ask your doctor

What class of lupus nephritis do I have, and what does that mean for my treatment?
Was more than one class present, such as class V combined with class III or IV?
What were my activity and chronicity scores, and what do they tell us?
How much of my kidney injury is active and treatable versus permanent scarring?
How much protein is in my urine, and what are we aiming for?
What is my current kidney function, and how will it be monitored?
Which treatment do you recommend for the induction phase, and for how long?
Would a newer medication such as belimumab or voclosporin be appropriate for me?
How will we know whether the treatment is working?
Am I at risk of a flare, and what would that look like?
Will I need a repeat biopsy at some point?
What signs should prompt me to contact my care team?