UriFastBK: published analytical and clinical data

This page collects the published data on the urinary antigen test developed by SPyDiag’s founding team. It reports only results from published work or preprints, and does not constitute the instructions for use of a CE-marked device.

Principle and format

Characteristic	Published data
Format	Urinary lateral flow assay
Target	Major VP1 capsid protein of BK polyomavirus
Nature of the result	Antigen detection — assembled viral particles, not viral genome
Time to result	A few minutes
Antibody	Patented F6 anti-VP1 monoclonal antibody
Sample	Urine, non-invasive

Analytical characterisation

The characterisation described by Brochot et al. (Journal of Virological Methods, 2025) used three types of material diluted in urine matrix:

cell culture-produced virus;
pseudoviruses;
VP1 pentamers representing the four BKPyV genotypes.

Coverage of all four genotypes matters structurally: it determines whether the assay can detect replication regardless of the strain a given patient carries.

Data not yet public. The numerical limit of detection and cross-reactivity data against related polyomaviruses (JC virus, SV40) appear in the full publication and are not reproduced here. They will be added to this page once they can be communicated as a claim.

Clinical evaluation: haemorrhagic cystitis after allogeneic HSCT

A retrospective diagnostic study compared the rapid test with urine PCR in allogeneic haematopoietic stem cell transplant recipients with suspected haemorrhagic cystitis (Brochot et al., preprint 2025, DOI 10.2139/ssrn.5360002).

Parameter	Result
Sample size	49 samples from 49 different patients
Patients with haemorrhagic cystitis	20
Detected by the rapid antigen test	19 of 20, i.e. 95%
Specificity for haemorrhagic cystitis	86.2% (versus 62.1% for PCR)
Positive predictive value	82.6% (versus 64.5% for PCR)
Ethics approval	Local institutional review board, reference PI2022_843_0079

Of the 37 BKPyV PCR-positive samples, 31 were above 7 log₁₀ copies/mL, including all 20 patients with haemorrhagic cystitis. The authors conclude that this tool could be implemented as a point-of-care test to rapidly confirm or rule out suspected BKPyV haemorrhagic cystitis after HSCT.

The striking finding of this evaluation is that, in this indication, the antigen test outperforms PCR on specificity and positive predictive value. That is consistent with its nature: it detects effective replication, where PCR detects the presence of viral genomes, which is not always associated with true replication.

Evaluation in kidney transplantation

Clinical evaluation in the nephrology indication is ongoing. It follows a different logic from haemorrhagic cystitis: it is surveillance screening in asymptomatic patients, not confirmation of a clinical suspicion. The intended role in that setting is detailed on our viruria or viremia page.

Regulatory status and intellectual property

The patent covering the F6 antibody and its use in lateral flow assays has been granted in France and Europe since October 2024, and in the United States since April 2025. Details are on the Publications and intellectual property page.

UriFastBK is not currently a CE-marked in vitro diagnostic medical device. The data presented here are published research results.

Information intended for professionals. Etienne Brochot and Antoine Touzé, authors of the cited work, sit on the board of SPyDiag and are co-inventors of the patent; these relationships are disclosed in the publications.