High incidence of BK virus‐associated hemorrhagic cystitis in children after second or third allogeneic hematopoietic stem cell transplantation

: BKV-HC is a serious complication of allogeneic HSCT. To characterize the incidence, risk factors, and clinical outcomes of post-HSCT BKV-HC, we retrospectively analyzed 112 patients who underwent one or more allogeneic HSCTs at our hospital between 2001 and 2017. Twenty underwent second or third HSCT thereafter. Ten patients developed BKV-HC at a median of 30 days after HSCT. The 100-day cumulative incidences of grade 0-4 and grade 2-4 BKV-HC were 7.8% and 6.2%, respectively. HSCTs performed in 2011-2017 associated with significantly higher 100-day cumulative incidence of grade 2-4 BKV-HC (14.0%) than HSCTs performed in 2001-2010 (1.3%, P = 0.004). On multivariate analysis, second or third HSCT was the only independent significant risk factor for development of grade 2-4 BKV-HC (P = 0.015). Serial PCR monitoring of urine and blood BKV load did not predict BKV-HC. The recent increase in the incidence of BKV-HC may reflect recent innovations in transplant technologies that facilitate second or third HSCT, which are known to cause prolonged immune deficiency. If safe and effective treatment or prophylaxis becomes available, it could be used to target the high-risk patients for BKV-HC.