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Notes: Objective To determine whether the cytological detection of persistent cervical intraepithelial neoplasia (CIN) after local ablative treatment is improved by the use of sampling devices other than the Ayre's spatula.Design A randomized controlled study.Setting Lothian Area Colposcopy Clinic.Subjects 856 patients who had received local therapy (CO2 laser or cold coagulation) for CIN II or III between 9 and 30 months earlier.Intervention Each patient had three consecutive cervical smears taken, one with the Ayre's spatula, one with either the Aylesbury, the Rocket or the Multispatula device, and finally one with the Cytobrush. The allocation of which spatula and the order of the first two was randomized. Each patient had a colposcopic examination immediately after the smears were taken.Main outcome measures A comparison of the detection of histologically proven persistent CIN by the Ayre's spatula with the detection of persistent disease by alternative sampling devices.Results Of the 856 patients 130 had histologically proven persistent CIN. Another 98 had suspicious findings on colposcopy but punch biopsies reported as histologically normal. Of the remaining patients with normal colposcopy 130 were randomly selected to form a control group. The cervical smears from these 358 women were reported. Significantly fewer Ayre’ s samples contained endocervical cells than Aylesbury samples (47%vs 59%, difference 12%; 95% CI 3%-21%; P〈0.1), Rocket samples (47%vs 67%; difference 20%, 95% CI; 12%-32%; P〈0.001) or Multispatula samples (47%vs 76%; difference 29%, 95% CI 19–38%; P〈0.001). When punch biopsies contained CIN, dyskaryotic cells were seen in 10% of Ayre's samples, 4.3% of Aylesbury samples, 8.3% of Rocket samples, and in no smear taken with the Multispatula. Obtaining a third smear with the Cytobrush did not substantially improve the detection rate of dyskaryosis. Neither the order of use of the spatulas, the form of initial treatment nor the size of the transformation zone had any apparent effect on the cytological detection of persistent CIN.Conclusions We recommend that surveillance of patients who have received local ablative therapy for CIN should be by both cytology and colposcopy, and that cytological samples should be obtained using the Ayre's spatula.