Quantitative ILPCR dose–response curve for CEA added to CEA-negative human serum. A 100-μl sample volume (antigen diluted in CEA-negative human serum) was used for all concentrations. The black circles are the average of three replicate Ct measurements over a concentration range of 10-8 to 10-18 M CEA; the standard deviation of the Ct values are shown as blue vertical bars. The orange line is the linear regression fit of the data from 10-10 to 10-16 M CEA. The green dotted lines depict the trend in the data outside the major linear region of the dose–response curve. The solid grey line is the average Ct value of the blank with the standard deviation shown in blue at each end of this line. The blank Ct value was plotted as a line rather than a single point for ease of visualization. The dashed magenta line is the detection threshold of the assay, which is defined as the average Ct value of the blank minus 3 times the standard deviation of the blank.