Mo1191 An Investigation of High gFOBT Positivity During the First Five Days Following Sample Collection

Abstract

Introduction The NHS Bowel Cancer Screening Programme (BCSP) in England has used the guaiac-based faecal occult blood test (gFOBt) since 2006. Test positivity is higher during the first 5 days after sample collection. Methods The gFOBt kits used by the BCSP have six windows (three pairs) lined with guaiac-impregnated philtre paper. The screening participant is asked to apply two faecal samples from three separate stools. The samples usually arrive dry at the laboratory, which is likely to reduce the degree of haemoglobin (Hb) degradation and provide the best opportunity to detect bleeding. BCSP policy is that every kit is logged on the day of receipt and read as soon as possible thereafter. BCSP Southern Hub data from the Bowel Cancer Screening System (BCSS) for 01/2008–07/2012 were analysed for subjects aged 60–69 years completing the first prevalent round of screening. The interval between each sample date and the date the sample was analysed (elapsed time) was calculated. Spot positivity was assessed by elapsed time, stratified by sex. Clinical outcomes were extracted from BCSS for subjects who had a positive gFOBt result. The relationship between positivity, elapsed time and clinical outcomes was examined. Results During the period of observation, nearly 1.2 million kits were returned to the Hub and 92% were read within five days of the last sample collection. For samples analysed one day after the last sample collection, spot positivity was 2.7% for women and 3.8% for men. Positivity declined thereafter until it reached a steady level at day 5 (women 1.3%; men 2.0%). Positivity for the most recently collected sample (usually spots 5 and 6) was slightly higher than for the spots collected earlier. There were 20,408 subjects (8,343 women; 12,065 men) who accepted further investigation. The outcome of those investigations was not associated with elapsed time between faecal sampling and test kit reading; the proportion of cancers, high-, intermediate- and low-risk adenomas was fairly constant as elapsed time increased. Conclusion The higher positivity associated with a shorter elapsed time did not appear to be associated with more false-positive tests. A number of factors may contribute to the pattern of positivity observed: (a) vegetable peroxidases present in the diet cause short term increases in gFOBt positivity; (b) faecal Hb denatures gradually before analysis, and (c) screening invitees worried about their health may return test kits more rapidly than others and a proportion of those individuals will be found to have colorectal cancer and related disease. None of those factors alone can account for the marked elevation in the ‘early positivity rate’ and further studies are required to elucidate the reasons for this effect. Disclosure of Interest None Declared.