About half of those who return a positive test in the upcoming bowel cancer screening pilot will turn out to either have cancer or pre-cancerous polyps.
As the test to be used in New Zealand can pick up any human blood in faeces, the Ministry of Health has had to decide the level of blood which will register as a positive result requiring further investigation.
If the test is too sensitive, more people than actually have cancer would be asked to undergo further testing such as the examination of the bowel by a colonoscopy.
Ministry national clinical director of cancer programmes Dr John Childs said there were a variety of reasons people might have blood in their faeces, including after having a bleeding nose.
After considerable study of the international data, it had been decided that for the pilot about half of the 5% or 6% of people expected to return a positive test would later be found to have cancer or pre-cancerous polyps.
The percentage of false negative results expected - where people who may have cancer do not show up as having blood in their faeces - was somewhere between 5% and 10% , Dr Childs said.
In any screening programme the test was not foolproof and people are always advised not to ignore symptoms even if they have had a negative test.
