The adenoma detection rate (ADR) is the primary quality indicator for colonoscopies. The polyp detection rate (PDR) is available from administrative data and does not depend on histology verification. The correlation between PDR and ADR and the ADR/PDR conversion factor in preventive colonoscopies were evaluated. In the prospective study, asymptomatic individuals aged 45-75 years with preventive colonoscopy in 2012-2016 were included. Spearman's correlation coefficient was used to assess PDR/ADR for each endoscopist. Conversion factor predicting ADR from PDR was obtained by linear regression and subsequently compared with adenoma to polyp detection rate quotient. One thousand six hundred fourteen preventive colonoscopies performed by 16 endoscopists in 8 screening colonoscopy centres in the Czech Republic were analysed. Correlation between PDR and ADR in all preventive colonoscopies was high and statistically significant (Rs 0.82; P < 0.001). There was a strong correlation between PDR and ADR in men (Rs 0.74; P = 0.002) and in screening colonoscopies (Rs 0.85; P < 0.001). The conversion factor to convert ADR from PDR was 0.72 in all preventive colonoscopies, 0.76 in FOBT+ colonoscopies and 0.67 in screening colonoscopies. ADR may be replaced by PDR in the assessment of colonoscopy quality. The value of the conversion factor varies according to colonoscopy indication and gender of examined individuals; in this Czech study, it was 0.72 in all preventive colonoscopies. The minimum requested ADR of 25 % corresponds to a PDR of 35 %, when converted with the appropriate conversion factor.