Tissue & Blood Helminths — Topic Test

A renal transplant recipient on tacrolimus develops fever, dyspnoea and a rising serum creatinine at week eight. A peripheral blood smear and tissue biopsy would show greatly enlarged cells bearing a single large basophilic intranuclear inclusion separated from the nuclear membrane by a clear halo, giving an owl's-eye appearance. Which is the investigation of choice to guide pre-emptive therapy in this setting?