A recent paper by Fogazzi et al. [1] shows how decoy cells can be identified not only by Papanicolau stain on fixed urine or by immunocytochemistry, but also by phase-contrast microscopy without any stain.
BK Polyomavirus is a double-stranded DNA virus belonging to the Papovavirus family, and infects up to 90% of the general population. After primary infection, generally occurring in childhood without evident symptoms, the virus can remain latent in the urinary tract. Reactivation can be enhanced by immunosuppressive conditions, leading to overt clinical disease [2]. The most important clinical manifestations are haemorragic cystitis in bone marrow transplantation, ureteral stenosis and interstitial nephropathy in kidney transplant recipients [3].
BK virus nephropathy (BKN) has been identified as a frequent complication affecting renal transplantation recipients, possibly associated with the degree of immunosuppression, and leading to allograft dysfunction in 50% of patients [4,5].
Histopathology is the gold standard test for diagnosis. Surrogate markers, such as detection of Polyomavirus-inclusion bearing cells (decoy cells) in the urine [6] and quantification of BK virus DNA in the plasma by polymerase chain reaction (PCR), have been used for diagnosis and management of polyomavirus BKN [7]. The presence of decoy cells in the urine is a 100% sensitive sign of elevated BK virus replication in the urogenital tract, but the positive predictive value for BKN may be <20%. Decoy cells are not exclusive to the BK virus. If PCR for BK virus is negative in urine specimens with presumed decoy cells, this does not exclude the presence of other viruses such as JC virus [8], and certain adenoviruses (e.g. type 11). Detection of BK virus DNA in plasma suggests significant allograft involvement (sensitivity 100%, specificity 88%) [5].
We confirm the results of the authors, reporting the case of a 61-year-old man, who received a kidney transplant in July 2002 from a cadaveric donor. In October 2002, his serum creatinine was 2.0 mg/dl, which then progressively increased up to 6.0 mg/dl in June 2003, while the patient was treated with tacrolimus and mycophenolate mofetil.
The urinalysis by phase-contrast microscopy showed 2 decoy cells/high-power field, at x400 (tubular and uroepithelial cells with enlarged nuclei and a gelatinous appearance) (Figure 1).
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In conclusion, it is possible to detect the presence of decoy cells on spot-morning urine sediment analysis by phase-contrast microscopy. This is important as it means that the nephrologist can screen patients for BKN with this simple and feasible test.
The key point remains the clinical suspicion that must be signalled, as in this case, from the clinicians to the laboratory.
Conflict of interest statement. None declared.
University of Torino A.S.O. San Giovanni Battista Torino Italy Email: massimogai{at}katamail.com
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