Significance of leukocyte concentration in the performance of the quantitative cytomegalovirus (CMV) antigenemia assay

Abstract

Background: The quantitative cytomegalovirus-antigenemia (CMV-Ag) assay is an important technology in the regimen of tests utilized in the care and management of acquired immunodeficiency syndrome (AIDS) and other immunocompromised patient groups. Performance of this assay is contingent upon the appropriate processing of the polymorphonuclear leukocyte (PMNL) compartment of the peripheral blood. However, a cell input standard in the performance of the CMV-Ag assay, has not been established. Interpretive differences between laboratories utilizing the CMV-Ag assay may reflect this lack of test uniformity. Objectives: To determine the effect of different PMNL concentrations on the quantitation of CMV in peripheral blood. The leukocyte concentration resulting in optimal rates of viral detection, will be compared with the shell vial assay-indirect immunofluorescent assay (SVA-IFA), and conventional tube culture (TC-CPE). Study design: A total of 74 freshly collected blood specimens were tested by the CMV-Ag assay, using cytospin preparations consisting of 2×10 5, 4×10 5 and in preliminary experiments, 8×10 5 PMNLs/slide. Data obtained from these studies were compared to SVA-IFA and TC-CPE. Viral load was monitored among 11 symptomatic patients through sequential testing of these patients at the start of ganciclovir (GCV), foscarnet (PFA), or combination drug therapy. Results: Among 74 blood specimens tested by the CMV-Ag assay, cytospin preparations consisting of 4×10 5 compared with 2×10 5 PMNLs/slide, affected a mean positive cell increase of 215% ( P=0.03). PMNL slide preparations consisting 8×10 5 cells produced background levels which prevented accurate reading of slides. The CMV-Ag assay was more sensitive than the SVA-IFA, but equivalent to TC-CPE. Among 11 patients started on drug therapy, viral load was markedly reduced in 8 within 2–3 weeks; three patients (2 deceased within 3 weeks after receiving therapy), showed no decrease in viral load. One patient was identified as harboring a PFA resistant strain. Conclusions: A PMNL concentration of 4×10 5 cells facilitated the reading of CMV-Ag assay slide preparations. The modified CMV-Ag assay furthermore, is applicable in the monitoring of viral load for the tracking of susceptible or resistant CMV strains.