Age-specific patterns of unsatisfactory results for conventional Pap smears and liquid-based cytology: data from two randomised clinical trials.

Objective: To investigate the rate of unsatisfactory cervical cell samples in liquid-based cytology (LBC) versus conventional cytology (CC) by age.
Design: Randomised clinical trials.
Setting: Population-based cervical cancer screening in the Netherlands and Italy.
Population: Asymptomatic women invited for screening enrolled in two randomised trials: Netherlands ThinPrep versus conventional cytology (NETHCON; 39 010 CC, 46 064 LBC) and New Technologies in Cervical Cancer Screening (NTCC; 22 771 CC, 22 403 LBC).
Methods: Comparison of categorical variables using Pearson's chi-square test, logistic regression and trend tests.
Main Outcome Measures: Proportion of unsatisfactory samples, ratio of LBC versus CC, and variation by 5-year group.
Results: In NETHCON, a lower percentage of LBC samples were judged to be unsatisfactory compared with CC samples (0.33 versus 1.11%). There was no significant trend in unsatisfactory results by age group for conventional cytology (P(trend) = 0.54), but there was a trend towards an increasing percentage of unsatisfactory results with increasing age for LBC (P(trend) < 0.001). In NTCC, a lower percentage of LBC samples were judged to be unsatisfactory compared with conventional cytology (2.59 versus 4.10%). There was a decrease in the unsatisfactory results by age group with conventional cytology (P(trend) < 0.001) and with LBC (P(trend) = 0.01), although the latter trend arose from the 55-60-years age group (P(trend) = 0.62 when excluding this group).
Conclusions: The clinical trial in which the results were collected and the cytologic method used were the most important determinants of unsatisfactory cytology. In all situations, the proportion of unsatisfactory samples was lower in LBC compared with CC. The effects of age depended on the criteria used to define unsatisfactory results.