BK virus screening after kidney transplantation: viruria or viremia?
International guidelines now favour viremia monitoring. That position is well founded. It does not make urinary monitoring useless — it redefines its role. This page reviews the available evidence, including the evidence that argues against the urinary approach.
What the guidelines say
The 2019 guidelines from the American Society of Transplantation Infectious Diseases Community of Practice (AST-IDCOP) recommend screening by PCR on blood or plasma, monthly until month nine post-transplant and then every three months up to two years. The earlier 2009 KDIGO guidelines allowed either viruria or viremia.
The rationale for this shift towards plasma is positive predictive value. Viremia above 104 copies/mL shows 100 % sensitivity, 88–96 % specificity and a positive predictive value of 50–82 % for BK virus nephropathy. Viruria above 107 copies/mL tops out at a positive predictive value of 31–67 %.
The objection, stated plainly
The criticism levelled at urinary screening is explicit in the recent literature: urine BK PCR is not recommended as a first-line screening test because of specificity and cost concerns; a positive result always requires plasma confirmation; and nearly half of patients with viruria will never develop viremia. The French reference laboratory Eurofins Biomnis takes the same position.
This objection is valid. A positive urine result alone does not establish a diagnosis of BK virus nephropathy, nor does it justify a change in therapy.
What the same literature says on the other side
The negative predictive value of viruria is 100 %. This figure is reported by the very sources that criticise its positive predictive value.
It changes the nature of the question entirely. A patient with no detectable urinary replication does not have BK virus nephropathy, and will not develop it in the near term: viremia and nephropathy do not occur without prior high-level viruria. Urinary replication precedes viremia, which precedes tubulointerstitial injury.
One study cited in the screening literature reports that BK virus is found in serum in 74 % of cases when urinary load reaches 107 copies/mL, against 9 % below that threshold. Viruria is therefore not a poor predictor of disease: it is an excellent predictor of the need for a plasma PCR.
Viral genome versus actual replication
One point deserves more attention than it usually gets: quantitative molecular tests detect viral genomes. As Brochot et al. note in the Journal of Virological Methods (2025), the detection of viral genomes by a positive PCR is not always associated with true viral replication. This is one of the recognised structural limits of molecular methods in BKPyV monitoring, and one of the reasons new biomarkers are being sought.
An antigen test answers a different question. By targeting the VP1 capsid protein, it detects assembled viral particles — the product of effective replication. That is not the same information as a viral load, and it is not inferior information: it is complementary information.
Two-stage screening
This is the logic applied by several transplant teams: universal urinary screening followed by confirmatory plasma PCR in positive patients only. Published institutional protocols describe this scheme, and the approach is also described as the strategy of choice in resource-limited settings, for its cost-effectiveness.
What this strategy requires is a first stage that is fast, inexpensive and repeatable. That is precisely what urine PCR is not: it uses the same instruments, the same turnaround times and the same costs as plasma PCR, without its decision-making value. Two-stage screening loses its economic rationale if the first stage costs as much as the second.
Where UriFastBK fits
UriFastBK is being developed as a lateral flow urinary antigen test directed against the VP1 capsid protein of BK polyomavirus. Its purpose is not to replace quantitative plasma PCR, nor to provide a viral load. It is to serve as the first stage: identifying, without laboratory infrastructure, the patients for whom plasma monitoring is warranted.
This positioning has a direct consequence for monitoring frequency. A test that can be run outside a molecular biology laboratory allows screening cadence to increase without a proportional increase in cost or logistics — where the current schedule is constrained by virology laboratory capacity.
Published work on this approach is detailed on our Publications page.
Frequently asked questions
Is viruria a poor marker of BK virus?
It is a poor marker of established nephropathy, with a positive predictive value of 31–67 %. It is, however, an excellent exclusion marker, with a negative predictive value of 100 %, and an early marker of viral reactivation.
Why do guidelines favour plasma?
Because the question they answer is one of diagnosis and treatment decisions, for which positive predictive value is the determining criterion. Triage calls for a different criterion: negative predictive value.
Does a urine test remove the need for plasma PCR?
No. A positive urine result calls for confirmation by quantitative plasma PCR. The aim of a triage test is to reduce the number of unnecessary plasma PCRs, not to eliminate them.
How common is BK virus after kidney transplantation?
Recent cohorts report asymptomatic viruria in around 40 % of kidney transplant recipients, viremia in around 20 %, and BK virus nephropathy in up to 10 %.
Does a positive PCR always mean active replication?
No. The literature acknowledges that detection of viral genomes by PCR is not always associated with true viral replication, which is what motivates the search for complementary biomarkers.
References
- Hirsch HH, Randhawa PS, AST Infectious Diseases Community of Practice. BK Polyomavirus in solid organ transplantation — Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice. Clin Transplant. 2019;33(9):e13528.
- KDIGO Clinical Practice Guideline for the Care of Kidney Transplant Recipients. 2009.
- Brochot E, Fourdinier O, Demey B, Aubry A, Collet L, Descamps V, Morel V, Gaboriaud P, Castelain S, Helle F, Touzé A. Development of a urinary antigen test for BK Polyomavirus to help clinicians in patients’ follow-up. J Virol Methods. 2025.
- Advances in BK Virus Complications in Organ Transplantation and Beyond. Kidney Med. 2020.
- BK Virus Nephropathy in Kidney Transplantation: A State-of-the-Art Review. 2022. PMC9330039.
- Early Detection Strategy of BK Polyoma Virus Infection in Kidney Transplant Recipients. Indian J Nephrol. 2024.
Information intended for professional and educational purposes. It does not replace medical advice or the protocols in force at your transplant centre.