Rapid ganciclovir susceptibility assay using flow cytometry for human cytomegalovirus clinical isolates

Rapid, quantitative, and objective determination of the susceptibilities of human cytomegalovirus (HCMV) clinical isolates to ganciclovir has been assessed by an assay that uses a fluorochrome-labeled monoclonal antibody to an HCMV immediate-early antigen and flow cytometry. Analysis of the ganciclovir susceptibilities of 25 phenotypically characterized clinical isolates by flow cytometry demonstrated that the 50% inhibitory concentrations (IC₅₀s) of ganciclovir for 19 of the isolates were between 1.14 and 6.66 μM, with a mean of 4.32 μM (±1.93) (sensitive; IC₅₀ less than 7 μM), the IC₅₀s for 2 isolates were 8.48 and 9.79 μM (partially resistant), and the IC₅₀s for 4 isolates were greater than 96 μM (resistant). Comparative analysis of the drug susceptibilities of these clinical isolates by the plaque reduction assay gave IC₅₀s of less than 6 μM, with a mean of 2.88 μM (±1.40) for the 19 drug-sensitive isolates, IC₅₀s of 6 to 8 μM for the partially resistant isolates, and IC₅₀s of greater than 12 μM for the four resistant clinical isolates. Comparison of the IC₅₀s for the drug-susceptible and partially resistant clinical isolates obtained by the flow cytometry assay with the IC₅₀s obtained by the plaque reduction assay showed an acceptable correlation (r² = 0.473; P = 0.001), suggesting that the flow cytometry assay could substitute for the more labor-intensive, subjective, and time-consuming plaque reduction assay.