Low proportion of false-negative smears in the Finnish program for cervical cancer screening

BACKGROUND: We assessed the performance and validity of cytology in the Finnish screening program by considering high-grade neoplasia and cervical cancer (CIN3+) rates as detected in the program and by reevaluating cases observed after a negative screening test.

METHODS: This retrospective study included 915 screen-detected CIN3+ cases and 421 cases observed after a negative screen. Randomized and blinded reevaluation of potential false-negative screening tests covered 345 archival case smears from women without a referral to colposcopy, as well as 689 control smears for estimating performance and validity measures.

RESULTS: The false-negative rate at the cutoff of low-grade squamous intraepithelial lesion or worse was 35% (95% confidence interval, 30-40%). In the subpopulation with original screening result of Pap I, the false-negative rate was 23% (18-28%). Sensitivity of screening laboratory rereading for detecting low-grade lesions or worse as atypical was 75% (67-82%) and specificity 93% (91-94%). Reproducibility of specific cytologic diagnoses was only fair. False negatives constituted 11% of all CIN3+ diagnoses in the screened population; those false negatives with an original Pap I screening result constituted 5%.

CONCLUSIONS: Although screen failures in the form of diagnostic false negatives occur within the Finnish screening program, their effect on cancer incidence is fairly small and cannot be readily decreased without sacrificing the high specificity of screening or without high incremental costs. Feedback for the screening laboratories is needed, however, to improve the reproducibility of cytologic diagnoses to optimize the burden of intensified follow-up and treatment of precancerous lesions.