This study aims to determine: a) whether those patients with 'low level' viral load results (between 200 and 3,000 copies/ml) could be monitored as opposed to starting preemptive therapy with valganciclovir, ganciclovir and/or foscarnet; b) whether those patients with 'high level' viral load results (above 3,000 copies/ml) could stop preemptive therapy earlier, thus maximising the benefits of therapy and minimising its risks.