Risk variables affecting high-grade Pap smears at second visit: Effects of screening interval, year, age and low-grade smears

Our aim was to study the effect, during the period 1979-1996, of the potential risk factors (i) year, age at second visit and first screening interval on the frequency of detection of Low- and high-grade smears at the second visit after a normal smear at the first visit; (ii) year, age at second visit and low-grade smears at first visit on the detection of high-grade smears at second visit; (iii) detection of low- and high-grade smears by calendar year at second visit after a normal first visit (period 1981-1996); (iv) proportion of high-grade smears at second visit attributable to low-grade smears at first visit (exposed group); and (v) effect of increasing the screening interval from 2 to 5 years. The results were as follows: (i) low-grade smears increased significantly with years, high-grade smears increased significantly with screening interval and both grades decreased significantly with age; (ii) high-grade smears increased significantly with low-grade smears at first visit and with year but decreased significantly with age; (iii) a significant increase in low-grade smears at second visit with years; (iv) 97% of high-grade smears at second visit were attributable to low-grade smears at first visit; and (v) the risk of high-grade smears was 60% higher when the screening interval was 5 years rather than 2 years after a normal visit at age 20. The strongest risk factors for high-grade cell changes were low-grade smears at first visit [odds ratio 10.2 (ii) and 29.0 (iv)] and first screening interval [odds ratio 1.6 (v)].