Results of delayed triage by HPV testing and cytology in the Norwegian Cervical Cancer Screening Programme

Background. High-risk human papilloma virus (hrHPV) testing was added to the cytology triage of women with equivocal screening smears in the Norwegian programme for cervical cancer screening in 2005. In this population-based observational before and after study we assessed the effect of changing the screening algorithm. Material and methods. In periods before and after the change 75 852 and 66 616 women, respectively, were eligible for triage, i.e. they had smear results of unsatisfactory, atypical squamous cells of undetermined significance (ASC-US), or low-grade squamous intraepithelial lesion (LSIL) at routine screening. The triage was delayed as supplementary testing started six months after the initial screening. The groups were compared with respect to results of triage and later three-year cumulative incidence of cervical intraepithelial neoplasia grade 2 or worse (CIN2+). Results. Before and after the change in the screening algorithm 5.2% (3964/75 852) and 8.1% (5417/66 616) of women, respectively, were referred to colposcopy. Among women referred to colposcopy cumulative incidence of CIN2+ (positive predictive value of referral) increased from 42.0% [95% confidence interval (CI): 40.3 - 43.7%] in the period with cytology only to 48.0% (95% CI 46.6 - 49.4%) after the start of HPV testing. For women recalled to ordinary screening the three-year cumulative incidence decreased from 2.7% (95% CI 2.5 - 2.9%) to 1.0% (95% CI 0.9 - 1.2%) during the same period. Among women with LSIL at routine screening and HPV testing in triage, 52.5% (1976/3766) were HPV positive. Conclusion. The new algorithm with HPV testing implemented in 2005 resulted in an increased rate of referral to colposcopy, but in a better risk stratification with respect to precancerous disease.