Assays of samples with discrepant results were repeated by line blot. Results by line blot were consistent after repeat analysis, with the exception of weak-positive signals, which were inconsistently amplified. To ascertain the possibility of irreproducibility due to sampling error from low concentrations of viral DNA, we added HPV 16 plasmid DNA to a PCR premix for a final concentration of 1.27 x 1024 fg/ml, the equivalent of a single target per 100-ml reaction mixture. This mixture was aliquotted to 80 PCR tubes and amplified under sensitive amplification profiles. Analysis of the products by
strip analysis indicated that only 42 of 80, or 52.5%, were positive for HPV DNA. Human DNA was included at a concentration of 2.5 ng per PCR and was amplified in all 80 reactions.