Bladder, ureter & renal pelvis
BK virus

Topic Completed: 1 March 2015

Minor changes: 25 August 2020

Copyright: 2003-2019,, Inc.

PubMed Search: BK virus

Monika Roychowdhury, M.D.
Page views in 2019: 2,561
Page views in 2020 to date: 2,002
Cite this page: Roychowdhury M. BK virus. website. Accessed September 24th, 2020.
Definition / general
  • Cystitis due to infection with human polyoma BK virus
  • BK is a type of polyoma virus - others are JC virus and simian virus 40
  • Each encodes a highly homologous form of a large T antigen, a transcriptional and replicational regulatory protein
Clinical features
  • Real time PCR can detect BK virus DNA in urine, although high BK viruria is not necessarily associated with kidney or bladder damage (New Microbiol 2007;30:275)
  • Polyoma virus infection may be a risk factor for bladder carcinoma in immunocompetent patients (Diagn Cytopathol 2006;34:201)
Case reports
Cytology description
  • Decoy cells are epithelial cells containing intranuclear viral inclusions in urine (Adv Exp Med Biol 2006;577:201)
  • Decoy cells are easily identified in routine Papanicolaou stained urine cytology specimens, and can also be identified in unstained sediment by phase-contrast microscopy
  • In Papanicolaou stained cytology smears, decoy cells are comet-shaped cells with large basophilic, amorphous, homogenous, ground-glass like intranuclear inclusion bodies and a condensed rim of chromatin (Diagn Cytopathol 2011;39:933)

    The intranuclear inclusion can have different phenotypes:
    1. Classic: basophilic ground-glass inclusion surrounded by chromatin (most common)
    2. Cytomegalovirus like – intranuclear inclusion surrounded by a clear halo
    3. Multinucleated decoy cells
    4. Vesicular nuclei with clumped chromatin and nucleoli (Urine Cytology Findings of Polyomavirus Infections)

  • Urine cytology appears to be more sensitive than histology in detecting cells with polyoma virus (Am J Surg Pathol 2006;30:345)
  • The negative predictive value of urine cytology is 99%
  • Note: pancreatic enzymes can degrade decoy cells and granulocytes (Clin Transplant 2008;22:833)
  • When decoy cells are detected in high numbers, i.e. >5 DC cells/10 high power field or >10DC cells per cytospin preparation, they have a positive predictive value to indicate BKV associated nephropathy of 27%
Cytology images

Images hosted on other servers:

Various types of decoy cells

Decoy cells with
enlarged nuclei and
homogenized chromatin
due to viral inclusions

Renal tubular cells

Positive stains
  • SV40
Electron microscopy description
  • Densely packed viral particles with a diameter of approximately 40 nanometers
Electron microscopy images

Images hosted on other servers:

Densely packed viral particles

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