Concentration (copies/ml) | Log_{10} concentration | % CV (total) | SD | 95% CI (log viral load) | Log change 95% CI | Fold change 95% CI |
---|---|---|---|---|---|---|

250,000 | 5.39 | 1.93 | 0.104 | 5.11-5.67 | 0.56 | 3.6 |

5,000 | 3.69 | 1.75 | 0.130 | 3.43-3.95 | 0.53 | 3.3 |

300 | 2.48 | 6.91 | 0.164 | 2.15-2.81 | 0.66 | 4.6 |

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*a*Three concentrations of analyte spanning the reportable range were used to evaluate the precision of a quantitative assay for human cytomegalovirus. Samples at each concentration were tested in duplicate in one run per day over 20 days using two operators. Between-day, operator-to-operator, and total imprecision were evaluated using analysis of variance of log_{10}-transformed data. The coefficient of variation (CV) and standard deviation (SD) were calculated for each concentration. Total imprecision (% CV) values for the three concentrations ranged from 1.75% to 6.91%, indicating that the assay is less precise at lower concentrations. Relatively little variance was attributable to between-day or operator-to-operator components except for the low-concentration sample, for which the day-to-day imprecision was much greater than with the other concentrations (data not shown). Calculation of 95% confidence intervals (CI) revealed that a 3- to 4-fold change at mid- to high viral loads and about a 5-fold change at viral loads near the limit of detection may represent imprecision of the assay rather than a true biological difference.