Screening usually spots bowel cancers at an earlier stage, and these often respond better to treatment than late-stage cancers.
But researchers from across the UK found that patients with cancers diagnosed through faecal occult blood testing (FOBT) survived longer than interval cancers detected in the two-year gap between regular screening.
The study, published in the British Journal of Cancer, found no difference in survival chances from the two means of diagnosis when an early stage cancer was detected. However, it did confirm that patients had better survival odds if their cancer was spotted at an earlier stage.
The researchers had compared 300 screen-detected bowel cancers with 200 same-stage interval cancers.
They suggested that some tumours may bleed less and may be missed by conventional screening.
Lead author Dr Michael Gill said: 'Our research suggests that the way we diagnose some patients seems to make a difference to how well those patients do. Even taking into account differences in gender, location of tumour, underlying health and backgrounds, patients with later-stage bowel cancer were more likely to survive the disease if they were diagnosed through screening.
'Now we want to find out why. But, even without all the answers, we know that bowel cancer screening is beneficial and saves lives. Cancer diagnosed through screening is likely to be at an earlier stage so easier to treat. But even if diagnosed at a later stage, our research shows diagnosis through screening seems to mean patients have a better chance of doing well.'
Low screening uptake
Jessica Kirby, Cancer Research UK’s senior health information manager, said only 58% of people offered screening completed their test kits and that this needed to improve. People with normal screening results should visit their GP if they notice bleeding or changes in bowel habits.
The UK's national bowel cancer screening programme offers FOBT to people aged 60-74, as well as 50-74 in Scotland, every two years.
More than 41,600 people are diagnosed with bowel cancer each year, and around 15,700 die from the disease. Early-stage bowel cancer has a 90% survival rate.