Plasma HHV-6 viral load-guided preemptive therapy against HHV-6 encephalopathy after allogeneic stem cell transplantation: a prospective evaluation. 
Human herpesvirus 6 (HHV-6) causes life-threatening encephalopathy in recipients of allogeneic SCT, but no consensus has been reached regarding appropriate preventive methods.  This study evaluated a plasma HHV-6 viral load-guided preemptive approach against HHV-6-associated encephalopathy.  Plasma real-time PCR assay was performed once a week.  Among 29 patients, 19 developed positive plasma HHV-6 DNA.  Median maximum plasma HHV-6 DNA was 4593.5 copies/ml plasma (range, 150.0-127 891.0 copies/ml plasma).  In one of eight events with low-level HHV-6 DNA (defined as <1000 copies/ml plasma) and four of seven events with mid-level HHV-6 DNA (1000-9999.5 copies/ml plasma), HHV-6 loads in plasma subsequently continued increasing.  Ganciclovir was administered against six of nine patients with high-level HHV-6 DNA (> or =10,000 copies/ml plasma).  High-level HHV-6 DNA resolved similarly in both groups with or without ganciclovir therapy.  Among the nine patients with high-level HHV-6 DNA two developed encephalopathy.  As encephalopathy developed before the detection of high-level HHV-6 DNA in plasma, these two patients had not received preemptive ganciclovir therapy.  In conclusion, our preemptive approach against HHV-6-associated encephalopathy cannot prevent all cases of HHV-6 encephalopathy in SCT recipients due to the dynamic kinetics of plasma HHV-6 viral load.