Risk of invasive cervical cancer after three consecutive negative Pap smears.

OBJECTIVES To determine the factors that influence risk of cervical cancer after three consecutive negative Pap smears. METHODS A cohort study was conducted using data from the British Columbia Cervical Cancer Screening Program and British Columbia Cancer Registry. Analysis was based on a one percent sample of women aged 20-69 years with Pap smears enriched with all invasive cervical cancer cases diagnosed between 1994-99. Screening intervals, after three negative screens, were created with the following variables: age at beginning of interval, interval length, previous cytologic abnormality and previous cervical procedure. The risk of cervical cancer by histologic type was calculated using survival analysis methods. RESULTS The sample consisted of 10,509 women, who contributed 28,309 intervals, and 371 cervical cancer cases. The incidence rate of invasive squamous cervical cancer increased with time since last screen up to six years. Women with a history of dysplasia remained at elevated risk for squamous cancer, hazard ratio=2.6 (95% confidence interval [CI]=1.9, 3.4) but age or previous procedure were not related to risk. No relationship between time since last screen and non-squamous cancer risk was found although history of a previous procedure was significant. The marginal effectiveness of Pap smears declined with increasing frequency of use. CONCLUSIONS This study confirmed the preventive effect of Pap smear screening and its dependency on frequency of use. Women with a history of dysplasia, prior to three consecutive negatives, were at increased risk of developing invasive squamous cervical cancer compared with women with no such history.