[Informative value of Hemoccult test according to the number of positive slides in mass screening of colorectal cancer].

Our aim was to study the relationship between the level of positivity of the Hemoccult colorectal cancer screening test and the positive predictive value on one hand, and the characteristics of the screened neoplasms on the other. This study focuses on four successive colorectal cancer screening campaigns in a population of 45,642 subjects born between 1914 and 1943. There were 1 or 2 positive slides in 50.1% of cases, 3 or 4 in 30.7% of cases and 5 or 6 in 19.2% of cases. The positive predictive value was 11.1% for cancer, 17.4% for adenoma > or = at 1 cm and 10.1% for adenoma < 1 cm. For a cancer or adenoma > or = at 1 cm, the positive predictive value varied between 18.6% when there were 1 or 2 positive slides, and 52.5% when there were 5 or 6 positive slides. Dukes A cancers are less likely to have 5 or 6 positive slides than more advanced cancers. On the contrary, neither cancer localisation nor characteristics of adenomas > or = at 1 cm (localisation, size, degree of dysplasia) influenced the number of positive slides. Owing to intermittent colorectal cancer bleeding, it seems necessary to take several successive samples. Two samples per stool over three successive stools seem like a good compromise. The informative value of the test increases with the number of positive slides.