What Do S and T Lesions Mean for IgA Nephropathy Prognosis?

An S (segmental glomerulosclerosis) or T (tubular atrophy/interstitial fibrosis) finding on your IgA nephropathy biopsy indicates that portions of your sampled kidney tissue have experienced structural damage or scarring. These findings are recognized risk markers associated with an increased chance of kidney function decline over time ^{[1] [2]}. However, discovering S or T lesions does not mean your kidney function will inevitably or rapidly fail. While established scars cannot typically be reversed, your doctor will evaluate these scores alongside your daily clinical measurements—like your blood pressure, protein levels, and current kidney function—and other biopsy features to determine your actual risk and the best treatment plan ^{[3] [4]}.

What Are S and T Lesions?

When a pathologist examines an IgA nephropathy biopsy, they use the Oxford MEST-C scoring system. The S and T portions evaluate specific types of chronic kidney damage, while other letters (like M, E, and C) evaluate different kinds of cellular activity or inflammation.

S Lesions (Segmental Glomerulosclerosis)

The “S” score looks at your glomeruli, which are the tiny filtering units of the kidney. Segmental glomerulosclerosis means that a segment (part) of some filtering units has scarred or adhered to surrounding tissue.

• It is scored as S0 (absent) or S1 (present).
• Having an S1 score is associated with a somewhat faster decline in eGFR (estimated glomerular filtration rate, a measure of how well your kidneys filter blood) ^{[5] [2]}.

An S1 score is generally viewed as a marker of established scarring ^[2]. While researchers have found that the S1 category can be microscopically diverse, having an S1 score by itself does not prove you have active inflammation, nor does it automatically mean you need strong immunosuppressive medications like corticosteroids ^{[6] [7]}. Your doctor will look at the entire biopsy report—including the other MEST-C scores—to see if there are signs of active, treatable inflammation.

T Lesions (Tubular Atrophy and Interstitial Fibrosis)

The “T” score evaluates the tubules (the tiny tubes that process the fluid filtered by the glomeruli) and the interstitium (the tissue surrounding them). Tubular atrophy refers to the shrinking or loss of these tubes, and interstitial fibrosis refers to the buildup of scar tissue between them.

The T score is one of the most consistently associated histologic findings with long-term kidney outcomes in IgA nephropathy ^{[1] [2]}. It is scored based on the percentage of the sampled cortical area in the biopsy that shows this damage:

• T0: 25% or less of the sampled area shows scarring ^{[8] [9]}.
• T1: 26% to 50% of the sampled area shows scarring ^{[8] [9]}.
• T2: More than 50% of the sampled area shows scarring ^{[8] [9]}.

In large studies, scores of T1 and T2 are independently associated with a higher relative risk of reaching specific endpoints, such as a sustained 50% decline in eGFR from baseline or kidney failure ^[2]. It is important to note that this is a statistical risk used for estimation, not a certainty that you will personally experience this outcome.

Are These Scars Permanent?

Tubular atrophy and interstitial fibrosis (T lesions) represent established, chronic structural damage that is permanent ^{[1] [2]}. Similarly, many S lesions represent established scars. Because a biopsy is only a tiny sample of tissue taken at a single moment in time, it does not perfectly predict exactly how your remaining healthy kidney tissue will behave ^{[1] [2]}.

While you cannot erase permanent scar tissue, your test numbers can still improve. With appropriate treatment, the amount of protein in your urine can fall, and your eGFR can stabilize or sometimes even improve if there was a reversible acute component to your illness. Clinical care focuses on protecting the healthy tissue that is left to prevent further scarring.

The Big Picture: Biopsy Plus Clinical Health

Kidney specialists do not look at S and T lesions in isolation. A biopsy shows what has already happened to the kidney, but your ongoing clinical markers show what is happening right now.

Studies show that combining your biopsy scores with clinical measurements—such as your eGFR, blood pressure, and urine protein trends—improves risk stratification over using the biopsy alone ^{[3] [4]}. For example, a higher T score is most concerning when it is combined with heavy proteinuria (high levels of protein in the urine), which indicates ongoing stress on the kidneys ^[10].

Working with your treating clinician to achieve good control of your blood pressure and reduce urine protein can help slow the rate of kidney decline ^[4]. The goal of treatment is to use individualized strategies to ease the burden on your kidneys, minimizing future damage and preserving your current function.