Polyomavirus (BK Virus) Nephropathy: Understanding Your Pathology Report

Jason Wasserman MD PhD FRCPC
20 de julho de 2026

Nefropatia por poliomavírus, também chamada BK virus nephropathy, is an infection of a transplanted kidney caused by the BK virus. Most people carry this virus harmlessly for life, kept in check by the immune system. After a kidney transplant, the medications that prevent rejection also lower the immune system’s defenses, and the virus can reactivate, multiply in the kidney, and cause inflamação and injury. It affects a small but important number of kidney transplant recipients and, if not managed, can damage the transplanted kidney.

The single most important idea in this diagnosis is a paradox worth understanding from the outset. Under the microscope, polyomavirus nephropathy can look almost identical to acute T-cell mediated rejection, yet the two need opposite treatments: rejection is treated by aumentando immunosuppression, while this infection is treated by redução it. Telling them apart is the central task of the biopsy. This article will help you understand the findings in your pathology report for polyomavirus nephropathy, what each term means, and why it matters for your care.

O que causa a nefropatia por poliomavírus?

The BK virus is extremely common; most people are infected in childhood, usually without ever knowing it, and the virus then lies dormant in the cells lining the urinary tract and kidney for life. In a healthy person, the immune system keeps it silent. After a kidney transplant, the medications taken to prevent the body from rejecting the new kidney deliberately weaken the immune system, and this can allow the BK virus to wake up and begin multiplying. When it multiplies within the transplanted kidney, it damages the cells lining the small tubes (tubules) that process urine, causing the infection called polyomavirus nephropathy. The risk is highest in the first year after transplantation and is greater when immunosuppression is more intense, which is why transplant teams monitor for the virus closely during that period.

Quais são os sintomas da nefropatia por poliomavírus?

Polyomavirus nephropathy usually causes no symptoms, especially early on, which is exactly why transplant teams screen for it rather than wait for it to declare itself. When signs do appear, they reflect kidney injury rather than anything specific to the virus, and may include:

A rise in the blood creatinine level, a waste product the kidney normally clears; this is usually the first clue, found on routine bloodwork.
Sangue na urina (hematúria).
A gradual decline in kidney function.
Occasionally, discomfort over the transplanted kidney.

Screening: how the infection is usually caught first

Because the infection is silent, most transplant programs test blood and urine for BK virus at regular intervals after transplantation, using a PCR test that measures the amount of viral genetic material. The infection tends to progress in a recognized order: first the virus appears in the urine (viruria), then in the blood (viremia), and only then does it invade the kidney tissue to cause nephropathy. Catching the virus in the blood before it reaches the kidney allows the transplant team to act early, often by adjusting medications, which is the most effective way to prevent nephropathy from developing. A rising blood viral level is frequently what prompts the biopsy that leads to this diagnosis.

Como é feito o diagnóstico?

Polyomavirus nephropathy is diagnosed by biopsia of the transplanted kidney, in which a thin needle is used to obtain one or more small tissue cores que um patologista examines under the microscope. Under an ordinary microscope, the pathologist looks for the effects of the virus on the cells lining the tubules: enlarged cells with altered nuclei, called viral inclusões intranucleares, along with inflammation and injury in the surrounding tissue.

Because these changes can be patchy and can resemble other conditions, the diagnosis is confirmed with a special stain called SV40, applied using a technique called imuno-histoquímica. The SV40 stain attaches to a protein made by the BK virus, so it lights up the infected cells directly. A positive SV40 stain confirms the diagnosis, and it is the single most important test for this condition. Because the virus can affect only part of the kidney, it is sometimes missed on a small sample, so the result is always interpreted alongside the blood and urine viral levels. The pathologist also carefully assesses whether rejection is present simultaneously, as discussed below.

How polyomavirus nephropathy is classified

When the diagnosis is confirmed, pathologists grade its severity using a classification system, because the severity predicts how likely the kidney is to recover. Two systems exist, and your report may use either. Both are built from how much virus is present and how much scarring has developed, because scarring reflects permanent, long-standing damage.

The Banff classification (three classes)

The most widely used system, developed by the international Banff group, combines two scores into three classes:

Polyomavirus load (the “pvl” score) — How much of the kidney is infected, measured as the percentage of tubules showing the virus. pvl 1 is 1% or less, pvl 2 is more than 1% up to 10%, and pvl 3 é superior a 10%.
Interstitial fibrosis (the “ci” score) — How much scarring is present in the supporting tissue of the kidney, graded from ci 0 (minimal) to ci 3 (severe).

These combine into three classes of rising severity:

Class 1 — Little virus and little scarring (pvl 1 with minimal fibrosis). This class has the best outlook, with kidney function usually preserved and a high chance the virus will clear with treatment.
Class 2 — A moderate amount of virus or scarring. This is the most common class, with an intermediate outlook.
Class 3 — Extensive virus together with severe scarring (pvl 3 with advanced fibrosis). This class carries the highest risk of lasting kidney damage, and recovery is less certain even with treatment.

The AST-IDCOP classification

An older system from the American Society of Transplantation sorts the disease into stages, from Class A (early, minimal infection with the best outlook), through Class B (established disease, subdivided into B1, B2, and B3 by increasing extent of infection and inflammation), to Class C (advanced scarring with the poorest outlook). Both systems capture the same underlying idea: more virus and more scarring mean a higher risk of losing kidney function.

O que o patologista vê ao microscópio

Your report may describe several features that together establish the diagnosis and its severity:

Viral inclusions — Enlarged tubular cells whose nuclei have a smudged, glassy appearance caused by the virus multiplying inside them. These are the classic sign of infection.
Positive SV40 staining — The confirmatory stain described above, which marks the infected cells.
Lesão tubular — Direct damage to the cells lining the tubules, which may appear swollen, flattened, or detached.
Interstitial inflammation — A buildup of immune cells in the tissue around the tubules, often including células plasmáticas. This is the feature that most closely resembles rejection.
Intersticial fibrose - Scarring of the supporting tissue, which reflects longer-standing damage and contributes to the classification score.

Why this infection can be mistaken for rejection

This is the most important section of the report to understand. Polyomavirus nephropathy and acute T-cell mediated rejection can look strikingly similar under the microscope, because both produce inflammation in the tissue around the tubules and inflammation within the tubule walls. Yet they sit at opposite ends of immune balance: rejection means the immune system is too active against the kidney, while this infection means the immune system has been suppressed too much and the virus has taken advantage. Their treatments are therefore opposite. Increasing immunosuppression to treat what appears to be rejection can fuel the virus, and reducing it to treat the virus can allow true rejection to emerge.

This is why the SV40 stain matters so much: a positive result indicates the virus, whereas a negative result is SV40-negative. The pathologist weighs the SV40 result together with the pattern and location of inflammation and the blood viral levels to reach the right answer. Complicating matters, the two conditions can occur simultaneously, and reducing immunosuppression to control the virus can itself trigger rejection. When both are present, the transplant team faces a genuine balancing act, carefully adjusting medications while closely monitoring the kidney. If your report mentions both the virus and the rejection, this is the situation it describes.

Qual é a perspectiva?

The outlook for polyomavirus nephropathy depends heavily on how early it is caught and how much scarring has already occurred. When the infection is found early, with little virus and little scarring (Banff class 1 or AST-IDCOP class A or B1), the virus clears in the vast majority of people, and kidney function is usually preserved. When it is found late, with extensive infection and severe scarring (class 3, or AST-IDCOP class C), the chance of clearing the virus is lower and some loss of kidney function is more likely, because scarring cannot be reversed. This strong link between early detection and good outcomes is why transplant programs screen for the virus so diligently, aiming to catch and treat it before nephropathy develops or scarring sets in.

O que acontece depois do diagnóstico?

Polyomavirus nephropathy is managed by the kidney transplant team, with the guiding principle of helping the immune system regain control of the virus while protecting the kidney from rejection. The classification and the blood viral levels guide how this is done. This is a general overview; the specific plan is individual, and this article does not recommend any particular treatment.

Reducing immunosuppression — The main treatment is to carefully lower the medications that suppress the immune system, allowing the body to fight the virus. This is usually done in stages, often by reducing or stopping one medication first, while the blood viral load and kidney function are closely monitored. The reduction is deliberate and gradual, because lowering these medications too far raises the risk of rejection.
Monitoring the response — The blood viral load is monitored over time to confirm the virus is clearing, and repeat biopsies are sometimes performed to confirm that the infection is resolving and that rejection has not appeared.
Antiviral and other medications — No drug reliably cures BK virus, and antiviral options have limited and unproven benefit, so they are used selectively rather than routinely. Reducing immunosuppression remains the mainstay.
Managing complications — Rarely, the virus can also affect the tube that drains urine (the ureter) and cause a blockage, which may need a procedure to relieve it.

Because the balance between infection and rejection is delicate, close follow-up with the transplant team is central to a good outcome, and early detection through screening remains the most powerful tool.

Perguntas para fazer ao seu médico

What class or stage was my polyomavirus nephropathy, and what does that mean for my kidney?
How much scarring did the biopsy show, and how much healthy kidney remains?
Did the biopsy show any signs of rejection along with the virus?
How will my immunosuppressant medications be adjusted, and over what time frame?
How will we know whether the virus is clearing?
Will I need a repeat biopsy or ongoing blood tests to monitor this?
Does reducing my medication put me at risk of rejection, and how will that be watched for?
Are antiviral medications appropriate in my case?
What is the long-term outlook for my transplant after this infection?
Quais sintomas devem me levar a entrar em contato com a equipe de transplante?

Nefropatia por Poliomavírus (Vírus BK): Entendendo seu Laudo Anatomopatológico