While a great number of studies examined risk factors for BKV infection, the only meta-analysis published which systematically studied risk factors identified 8 risk factors associated with increased risk for BK viremia (maintenance therapy regimen including tacrolimus, allograft from a deceased donor, recipient of male sex, history of previous transplant, age at transplantation, ureteral stent use, delayed graft function and acute rejection episodes) and two associated with increased risk for BKVAN (maintenance therapy regimen including tacrolimus and acute rejection episodes) [19]. Another meta-analysis specifically examined the risk of therapeutic regimens including mammalian target of rapamycin (mTOR) inhibitors vs. calcineurin inhibitors (CNI) did not find any association of drug regimen with risk of BKV infection [20]. When examining data from individual studies, risk factors for BKVAN can be divided into recipient-, donor- and allograft specific and include: male sex, older age of donor and recipient, cold ischemia time, delayed graft function, episodes of rejection, ureteral stent, use of anti-thymocyte globulin in induction, use of tacrolimus in maintenance therapy, ABO blood group system incompatibility, ischemia/reperfusion injury and recipient or donor seropositivity. Conversely, mTOR inhibitor use has been shown to be a protective factor [9,21,22,23,24].

The current BKVAN therapy protocol consists mainly of immunosuppressive therapy reduction [21]. Moreover, an important point is distinguishing BKVAN from allograft rejection, since the two may present similarly, but are treated in completely opposite ways, i.e., treatment of misdiagnosed rejection in the presence of BKVAN might lead to allograft loss [25,26]. Furthermore, histological surveillance of BKVAN is also problematic from a clinical point of view, which has been thoroughly studied. Menter et al. explored the histopathology of resolving BKVAN which found that this stage is morphologically indistinguishable from interstitial rejection [27]., For an accurate diagnosis it is imperative to obtain adequate and deep samples as BKV has a tropism for renal medulla [28]. Aside from reduction of immunosuppression, which is the only viable treatment strategy, other therapies have been tested and occasionally used. Intravenous immunoglobulins have been shown to be useful in patients who do not respond to the initial reduction of immunosuppression [29] and might lead to additional BKV clearance [30]. A recent study possibly partially elucidated the mechanism behind this, demonstrating that intravenous immunoglobulin administration increases the titer of neutralizing antibodies specific for BKV [30]. On this basis, a very recent proof-of-concept study has stated that intravenous immunoglobulins might be useful in clinical practice and potentially reduce the risk of allograft loss [30]. Some studies showed good results using leflunomide, a prodrug to an antimetabolite A77 1726, usually by replacing mycophenolate mofetil, however, other studies demonstrated conflicting results and their use is controversial [31,32,33]. Cidofovir is an antiviral agent, used also for BKV infection, [34], but its usage is limited due to low efficacy and potential nephrotoxicity [35]. While initial studies showed an effect of quinolone antibiotics [36,37], this was disproven in further, better-designed randomized controlled studies [38,39]. Immunosuppression with everolimus after switching from CNIs were shown to be promising in one retrospective study [40]. Given the limited options of therapy and the established viruria-viremia-nephropathy sequential course, screening aimed at early detection of BK viruria and viremia is of paramount importance. It has been shown that screening for BKV DNAemia enables identification of at least 90% of patients at risk before significant repercussions for the allograft. However, quantitative nucleic acid testing (NAT) is still underutilized [41,42]. It has been demonstrated that most cases of BKVAN occur in the first 6 months or 1-year post-transplant and recent research showed that only around 20% to 30% of BKV DNAemia events occur later than 6 months post-transplant, which strongly points to the need for strict early surveillance [43,44,45]. Several screening and intervention strategies have been developed, based on testing frequency and detection method. Kidney Disease Improving Global Outcomes (KDIGO) guidelines for kidney transplant recipients suggest screening all recipients of BKV using quantitative plasma NAT at least monthly for the first 3 to 6 months after transplantation, then every 3 months until the end of the first post-transplant year, and subsequently whenever there is an unexplained rise in serum creatinine and after treatment for acute rejection [46]. American Society of Transplantation (AST) guidelines recommend quantitative NAT as the main testing method to be performed monthly up to month 9 post-transplant followed by testing every 3 months up to 2 years post-transplant or at the time of surveillance and indication of an allograft biopsy. Stepwise reduction of immunosuppressive medications is recommended when BKV plasma NAT is persistently (for 3 weeks and longer) greater than 1000 copies per milliliter (mL) [47]. Complementary to quantitative polymerase chain reaction (qPCR) DNA testing, several institutions also use other methods, such as urine cytology (decoy cells) and virus RNA [46]. Nankivell et al. found that, although decoy cells detection have a high specificity and negative predictive value for BKVAN, quantitative viremia determination by qPCR was superior having high sensitivity, specificity and negative predictive value [48]. However, urine cytology, especially quantification of decoy cells, might be a useful additional tool in experienced centers especially in situations when kidney biopsy cannot be performed. Given a recent study on the need for deep sampling during kidney biopsy for obtaining an adequate tissue sample, urine cytology might be useful in cases where no or scarce medulla was obtained [28]. Looking for an integrated method of surveillance, a review by Comoli et al. summarized the current knowledge on BKV-specific cellular immunity, found that it is associated with viral clearance and that prospective monitoring for viremia coupled with specific immunity and B-cell alloimmune surveillance might lead to prevention and better outcomes in BKVAN [49].

One of the first important studies focused on outcomes in patients with BKVAN found that in a cohort of 1001 renal and renal/pancreas allograft recipients, 41 patients developed BKVAN with median of 318 days to diagnosis and that allograft survival at 6 months, 1, 3, and 5 years was 97%, 90%, 58%, and 47%, respectively, compared to a contemporaneous cohort of patients without BKVAN which had significantly better 6 months, 1, 3 and 5 year allograft survival (94%, 92%, 83%, and 76%, respectively). Allograft loss occurred in 46% of subjects and BKVAN diagnosis was preceded with a steep fall in estimated glomerular filtration rate (eGFR) [50]. A small Chinese prospective study found that 5.6% of patients developed BKVAN in the first year post-transplant and that a reduction in immunosuppression led to resolution in all patients [51]. In a study of 213 kidney transplant recipients, high BK viremia (defined as (≥10⁴ copies/mL) occurred in 17.4% and low viremia occurred in 49.3% of patients, while BKVAN occurred in 4.2% of patients [52]. In another retrospective long-term study (mean follow up of over 7.7 years), the incidence of BKVAN was 4.0% and 60.0% of patients with BKVAN and acute rejection progressed to allograft loss [53]. In patients sequentially monitored for BK viruria and viremia in which the CNI dose was reduced first when sustained BK viremia occurred (>1000 copies/mL) at median follow up of 5 years, 11% of patients developed rejection [43]. A report that included 609 patients of which 130 developed BK viremia during the first post-transplant year and who were then classified as transient low viremia, transient high viremia, persistent low viremia, and persistent high viremia (based on BK viral load cut-off of 10,000 copies/mL and infection duration cut-off of 3 months) demonstrated that low viremia (either transient or persistent) did not affect long-term outcomes. Contrary to this, persistent high viremia was associated with higher risk for BKVAN and subsequent allograft dysfunction and transient high viremia was associated with worse long-term allograft function [54]. An analysis of outcomes of Chinese renal allograft recipients treated for BKVAN showed that after a mean follow up of 14.4 months, BK viruria disappeared in 19.5% and BK viremia in 90.2%. One-year graft survival was excellent, while 5-year graft survival was 85.7% [55]. In a study of 1904 renal allograft recipients of which 17.3% had BK viremia and 3.6% had BKVAN, high serum creatinine, early BKVAN (defined as occurring within 6 months post-transplant) and microvascular inflammation were independently associated with higher risk of allograft loss [56].

Re-transplantation after allograft loss due to BKVAN is considered safe after complete resolution of viremia and is frequently performed, especially considering that most BKV infection is donor-derived. A very recent study showed that there were no differences in death-censored graft survival, acute rejection episodes and patient survival between patients who underwent re-transplantation after first allograft loss due to BKVAN versus other causes [57].