Reporting test results
The National Bowel Screening Programme (NBSP) uses a sensitive Faecal Immunochemical Test (FIT) which is designed to detect small amounts of blood in a stool sample (poo).
The NBSP reports a person’s test result as positive or negative. It does not give the quantitative figure of how much blood was found.
Before participating in the NBSP potential participants are informed that their FIT result will be reported as either positive or negative. The practice of reporting test results in this way is recommended by the manufacturer of the FIT kit and is consistent with what is done in other countries using the FIT.
How a test result becomes positive or negative
Within New Zealand’s bowel screening programme, a positive result is triggered if the blood (haemoglobin (Hb)) found in a sample is over 200 nanograms of Hb in a millilitre of buffer, the threshold set for the NBSP. The bowel screening pilot found that readings at this level and higher accounted for 80 percent of bowel cancers found and therefore indicated a higher likelihood of bowel cancer.
There is not enough evidence to precisely calculate the likelihood of bowel disease or cancer for all possible FIT results. For every 100 people with a FIT result above the NBSP threshold, we know that approximately eight will have a cancer found in the first round of screening. For every 100 people with FIT results below the threshold there will be very few cancers found. Consequently, it is not possible to provide clinical advice to someone without symptoms based on a single quantitative FIT result.
In the screening context, the international screening community is gathering evidence to determine if the actual Hb quantitative level will impact the subsequent screening for an individual.
In the symptomatic setting, we are aware that the National Health Service (NHS) England is using FIT in patients with bowel symptoms to determine who would most benefit from proceeding to colonoscopy. Information from this study will be reviewed by the National Bowel Cancer Working Group as part of ongoing updates to the referral criteria for direct access to outpatient colonoscopy or computed tomography colonography.
At this stage, the Ministry of Health’s position is that reporting the quantitative result could be confusing and potentially increase anxiety for NBSP participants. Therefore, until there is a clearer understanding, the NBSP will continue to report results as positive or negative.
Participants with a negative test result, who remain in the eligible age range, are offered a further test every two years, providing further opportunities to detect disease. They are informed that if they notice blood in their bowel motions (poo) or a change in their usual pattern of going to the toilet, they should seek advice from their doctor.
Communicating results to participants and potential participants
The Ministry is working with its international screening counterparts, as part of the continuous improvement process for the new NBSP, on how screening results are provided. The information made available to participants and potential participants about their screening results will be included as part of this ongoing review to ensure they continue to meet international best practice.
More information for the technically minded
International studies have examined the performance of FIT tests and these are summarised in the July 2016 report prepared for the Ministry by the Sapere Research Group (the Group) titled “The cost effectiveness of bowel cancer screening in New Zealand: a cost-utility analysis based in pilot results”. The Group used international data and results from the New Zealand pilot to estimate the sensitivity of the screening test at different FIT cut-offs.
More detailed information can be found in Appendix 4 of The Final Evaluation Report of the Bowel Screening Pilot”(pages 26 -31 and 72-78) available on the Ministry website.
As the NBSP becomes established and more data becomes available, analysis is being done to find out the number of interval cancers that are occurring in people who have been screened. These are cancers diagnosed after a negative screen and before the person was due for their next screen. The analysis is being done to test that the FIT sensitivity threshold set for New Zealand’s NBSP is correct or may need adjusting.
Information about the NHS England pilots can be found on the NICE website.