Large performance variation does not affect outcome in the Finnish cervical cancer screening programme

OBJECTIVE: Cytology screening for prevention of cervical cancer can reduce incidence and mortality by more than 80% in settings with good organization and rigorous quality control. Audit studies are essential for reaching and maintaining a high quality of screening. The aim of this study was to evaluate variation in performance indicators by screening laboratory and assess the impact on the effectiveness of screening as indicated by cervical intraepithelial neoplasia grade 3 and above (CIN3+) rates after a negative screen.

METHODS: Seven cytology screening laboratories operating during 1990-1999 with a total of 953 610 screening tests performed were included in the study. By linking screening and cancer register files, all cases of CIN3+ diagnosed in the screened population were identified. For 395 CIN3+ cases with a preceding negative screen and 787 controls, a re-evaluation of smears was undertaken to uncover false negative screening tests. Performance parameters and rates of CIN3+ after a negative screen were analysed for interlaboratory heterogeneity.

RESULTS: The rates of follow-up recommendations and referrals varied by up to 3.6- (2.8-10.2%) and 4.0-fold (0.03-0.12%), respectively. CIN1, CIN2 and CIN3+ screen detection rates differed by up to 8.5- (0.02-0.17%), 5.4- (0.05-0.25%) and 3.3-fold (0.05-0.18%). False negative rates determined by re-evaluation showed up to 2.1-fold differences (29-62%). Rates of CIN3+ after a negative screen (0.023-0.048%) and as a proportion of total CIN3+ (15-31%) in the screened population were low and did not vary significantly.

CONCLUSIONS: There were large variations in the sensitivity-specificity trade-off between laboratories, reflected in all performance indicators as well as in the test validity estimates of the re-evaluation phase, but not in screening effectiveness. Even though performance variations do not always have an impact on the effectiveness of screening, they lead to variations in cost, treatment and psychological burden, and should be addressed.