Effect of HPV assay choice on perceived prevalence in a population-based sample.

Human papillomavirus (HPV) immunization programs clearly have considerable potential to reduce HPV-associated disease; they are also resource-intense; so, it is essential that their effectiveness is determined accurately and in a timely way. Measuring circulating HPV types in a population can provide an early measure of vaccine impact. We assessed the impact of HPV assay on the observed population prevalence of HPV in women who provided samples as part of a National HPV Immunisation Surveillance Exercise. A total of 1145 liquid-based cytology samples, 326 self-taken swabs, and 371 urine samples were tested with a line-blot assay (the Digene reverse hybridization HPV genotyping assay) and a luminex-based assay (the Mulitmetrix HPV genotyping assay). Assay agreement was determined for the different sample types. Positivity (according to assay) was compared at different levels ranging from positive for HPV 16 and/or 18 to positive for any one of the 18 HPV types common to both assays. The luminex assay consistently detected a higher prevalence of HPV--up to 10% for HPV types common to both assays. In addition, disagreement for HPV 16 and/or 18 was observed in around 9% of the overall sample, with an associated κ score of 0.74. These data indicate that assay choice has a significant impact on observed prevalence of HPV, including vaccine types. The impact of any change of assay during longitudinal surveillance programs should thus be taken into account to avoid confounding the assessment of any vaccine-induced changes.