Early figures on cancers missed in the Waitemata bowel screening pilot have prompted fresh concern about the limitations of the national programme and the need for comprehensive information for screening participants to avoid false reassurance. Elspeth MacLean reports.
Provisional data shows 17 of these people later died of colorectal cancer.
The Ministry of Health has released provisional figures on these cancers (known as interval cancers because they are found in the interval between screenings) under the Official Information Act, after repeatedly refusing to release any information on them last year.
Outspoken critic of the adequacy of the national programme, Associate Prof Brian Cox, who was closely involved with the introduction of New Zealand's breast and cervical screening programmes, estimates the number of interval cancers in the pilot programme would have been at least 92 using the national programme's blood/faeces threshold of 200ng/ml.
"These cancers would have been generally more advanced than the interval cancers of the pilot study, with poorer survival."
The ministry data showed 18 of the 56 interval cancers were still confined to the bowel when diagnosed, but in nine cases, the cancers had spread to distant organs at the time they were found.
In 10 cases, the stage at which the cancer was diagnosed was unknown.
The 56 people had all completed negative faecal immunochemical tests (Fit) in 2012, 2013 and 2014, and had cancers found before their next routine two-yearly test.
Bowel Cancer New Zealand spokeswoman Prof Sarah Derrett said the "very real concern" was that a substantially greater proportion of cancers would be missed in the national roll-out compared with the pilot.
"It is vital that people are not falsely reassured by a "negative" Fit test, due to the higher threshold of detectable blood. Accurate information about a range of bowel cancer symptoms should be provided to people when they are informed about the Fit test being "negative" to help people recognise symptoms should these occur."
The letter which goes out to those who return a negative test spells out two possible symptoms, a change in bowel habit or blood in faeces, compared with six listed in Bowel Cancer NZ information.
Information provided to participants and prospective participants does not contain an estimate of the proportion of cancers likely to be missed by the national programme.
People already participating in the Waitemata pilot who are being moved to the national programme with its higher blood/faeces threshold are not given any information about the cut-off change.
The ministry has previously stated only 62% of the people found with cancers in the pilot programme in Waitemata would have had them detected if the pilot programme had used the age range and blood/faeces test level of the national programme.
To the end of March last year, 375 pilot participants had been diagnosed with cancer, but if the higher age range and less sensitive test threshold for the national pilot had been used, about 142 of those people would not have had cancer detected.
Prof Cox said with the limited information given on the interval cancers, it appeared the provisional screening sensitivity for the pilot was acceptable.
(Sensitivity is the ability of a test to correctly identify those with disease. A highly sensitive test rarely gives a false negative result.)
He estimated the provisional sensitivity at 79.1%. A similar programme in the Netherlands had recorded 77% sensitivity.
It was difficult to be sure without all of the information and without knowing the thoroughness of the data collection.
He based his calculation on 212 cancers detected in the first 36 months and 56 interval cancers found.
The ministry emphasised that the interval cancer figures given were provisional as full analysis of the information was not complete.
A comprehensive report is expected late this year.
Ministry director of service commissioning Jill Lane said it was "very likely numbers will change as the full analysis is completed and reviewed".
"There are a number of factors why an individual may develop an interval cancer: the cancer was "missed" on the first screen, the cancer wasn't visible at the first screen, and cancers becoming detectable and symptomatic after the screening."
The information released by the ministry was missing provisional data on any cancers found in people in the five-year interval after a "normal" screening colonoscopy.
Under the programme, participants are offered colonoscopies after a positive Fit result and, after a clear colonoscopy, participants are not recalled for five years.
Ms Lane refused to release any numbers on these, saying insufficient time had passed to compile a complete year's data set on such cancers.
Ms Lane said the pilot produced a pronounced shift in the proportion of patients diagnosed with early stage cancer, with 39% of cancers found at stage 1 in the pilot, compared with 13% found at this stage in the unscreened population.
Asked whether these figures were likely to be replicated in the national programme with its increased threshold and the change in the age of those screened (50-74 in the pilot compared with 60-74 in the national programme), Ms Lane said this was unknown at this time.
Earlier, it had been reported the Sapere Research Group, which evaluated the pilot, used international data and some information from the pilot to estimate the sensitivity of the screening test at different Fit cut-offs.
At the national programme cut-off of 200ng Hb/ml buffer, it estimated the sensitivity of a single Fit at 50.4% for stage 1 or 2 cancer (59.7% in the pilot) and 70.6% for Stage 3 or 4 cancer (compared with the pilot 85.9%)
Asked whether the comprehensive report later this year was expected to give some indication of what might be expected to happen to the interval cancer rate in the national programme, with its increased threshold, Ms Lane said she could not answer this because the shape of the final analysis had not been decided.
Following a complaint to the Ombudsman last year about the ministry's refusal to provide the interval cancer numbers, the ministry acknowledged it had not given adequate reasons for refusal but continued to resist releasing the information, on the grounds of privacy and because the analysis on the data had not been completed.
After the ministry described the process involved in gathering and analysing the cancer data, the Ombudsman suggested a fresh request be made.
This resulted in the provisional Fit negative data release.
