Validation of low-volume plasma dilution protocols for human cytomegalovirus DNA quantification using the cobas CMV assay.

Quantitative nucleic acid amplification tests (QNATs) for human cytomegalovirus (CMV) DNA are widely used for diagnosing and monitoring CMV infection. However, automated CMV-QNATs require relatively large plasma volumes, which may limit testing when specimen volume is limited, particularly in pediatric patients. Here, we evaluated the analytical and clinical performance of two plasma dilution protocols (1:2.5 and 1:5) for the cobas CMV assay to enable CMV DNA quantification using reduced plasma volumes. Analytical sensitivity and linearity were assessed using the World Health Organization International Standard for CMV, and precision was evaluated using pooled plasma. Clinical performance was evaluated using 135 plasma specimens by comparing results from diluted samples with those from the standard protocol. The estimated limits of detection for the 1:2.5 and 1:5 dilution protocols were 28.4 and 195.9 IU/mL, respectively, and the corresponding lower limits of quantitation were 200 and 1,000 IU/mL. Both protocols demonstrated acceptable linearity and excellent precision. Qualitative agreement with the standard protocol was high, with positive percent agreements of 97.9% and 94.7% and negative percent agreements of 95.0% and 100% for the 1:2.5 and 1:5 dilution protocols, respectively. Bland-Altman analysis showed minimal mean differences between diluted and undiluted measurements (-0.03 and -0.08 log10 IU/mL). Although both dilution protocols showed agreement with the standard assay, the 1:2.5 dilution protocol demonstrated superior analytical sensitivity and a broader linear range. This protocol enables reliable CMV DNA quantification using as little as 200 μL of plasma and may facilitate CMV testing when plasma volume is limited.IMPORTANCEAutomated quantitative nucleic acid amplification tests for human cytomegalovirus (CMV) DNA require relatively large plasma volumes, which can limit testing in pediatric patients and other settings with insufficient specimen volume. We evaluated 1:2.5 and 1:5 plasma dilution protocols for the cobas CMV assay using WHO International Standard materials and clinical plasma specimens. Both protocols showed strong agreement with the standard undiluted assay, while the 1:2.5 protocol provided better analytical sensitivity and enabled CMV DNA quantification using approximately 200 μL of plasma. Because no manufacturer-recommended plasma dilution protocol is available for this assay, these findings provide practical performance data that may support laboratories implementing low-volume CMV testing on widely used cobas 6800/8800 systems.