FIG. 1.

Recovery of IC DNA from plasma at the single-molecule level. (A) Reference plasma was supplemented with IC DNA to a concentration of 4 molecules per 200 μl of plasma. DNA was extracted from 20 200-μl aliquots, and one-quarter of the extracted DNA was used in the PCR (bars 1 to 20). DNA was also extracted from reference plasma only, which served as a negative control (bars c). The amount of amplimers was determined and expressed in LU. The cutoff level is indicated with a broken line. (B) In the same experiment IC DNA was used directly in 10 separate PCRs at an input expected to contain a mean of 1 molecule of IC DNA per PCR (bars 1 to 10). The amount of amplimers was determined by ECL and expressed in LU. The cutoff level is indicated with a broken line.