A case-control study of the protective benefit of cervical screening against invasive cervical cancer in NSW women

Objective To examine the effects of different Pap screening patterns in preventing invasive cervical cancer among women in New South Wales, Australia. Methods A total of 877 women aged 20-69 years diagnosed with invasive cervical cancer during 2000-2003 were matched with 2,614 controls by month and year of birth. Screening behavior patterns in 4 years preceding the time of cancer diagnosis in the cases were classified into none (no Pap test in the 4 years), 'irregular' (1 of the 4 years with Pap test(s)), and 'regular' (2 or more of the 4 years with a Pap test), and compared with those in the matched non-cases over the same period. Conditional logistic regression modeling was used to estimate the relative risk, approximated by the odds ratio, of invasive cervical cancer for regular and irregular cervical screening compared with no screening in the previous 4 years, before and after controlling for potential confounders including the first recorded Pap test result in the preceding 6-year reference period. Results Compared with no screening, irregular Pap screening in the 4 years preceding the cancer diagnosis is estimated to reduce the risk of invasive cervical cancer by about 85% (RR = 0.15, 95% CI: 0.120-0.19); regular Pap screening reduces the risk by about 96% (RR = 0.04, 95% CI: 0.03-0.05). After adjusting for the index Pap test result, the relative risks for invasive cervical cancer were 0.19 (95% CI: 0.13-0.27) for irregular screening and 0.07 (95% CI: 0.04-0.10) for regular Pap screening. Conclusions Regular and irregular Pap tests among women aged 20-69 years were highly effective in preventing invasive cancer. At-risk women with no Pap test history should be encouraged to undergo a Pap test every 2 years, but any Pap screening over a 4-year period remains highly protective against future invasive cervical cancer.