Interlaboratory Comparison of Cytomegalovirus Viral Load Assays

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To assess interlaboratory variability in qualitative and quantitative cytomegalovirus (CMV) viral load (VL) testing, we distributed a panel of samples to 33 laboratories in the USA, Canada and Europe who performed testing using commercial reagents (n = 17) or laboratory-developed assays (n = 18). The panel included two negatives, seven samples constructed from purified CMV nucleocapsids in plasma (2.0–6.0 log10 copies/mL) and three clinical plasma samples. Interlaboratory variation was observed in both actual (range, 2.0–4.0 log10 copies/mL) and self-reported lower limits of detection (range, 1.0–4.0 log10 copies/mL). Variation observed in reported results for individual samples ranged from 2.0 log10 (minimum) to 4.3 log10 (maximum). Variation was greatest at low VLs. Assuming ± 0.5 log10 relative to the expected result represents an acceptable result, 57.6% of results fell within this range. Use of commercially available reagents and procedures was associated with less variability compared with laboratory-developed assays. Interlaboratory variability on replicate samples was significantly greater than intralaboratory variability (p < 0.0001). The significant interlaboratory variability in CMV VL observed may be impacting patient care and limiting interinstitutional comparisons. The creation of an international reference standard for CMV VL assay calibration would be an important step in quality improvement of this laboratory tool.

Key words

CMV
interlaboratory variation
quantitative NAT (QNAT)
standardization
viral load

Abbreviations

CMV
cytomegalovirus
VL
viral load
QNAT
quantitative nucleic acid testing;
PCR
polymerase chain reaction
ASR
analyte-specific reagents
EM
electron microscopy
GM
geometric mean
SD
standard deviation
QRS
quantitative reference standard;
CV
coefficient of variation
LOD
limit of detection

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