While two years ago it might have seemed that using a test for faecal blood was the best way to go, recent findings from a large trial involving a one-off flexible sigmoidoscopy (FS) procedure suggested that was no longer the case, he said.
Public health specialist and epidemiologist Prof Cox, who was involved with the development and monitoring of the country's breast and cervical screening programmes, said he hoped the "juggernaut of policy and planning is not so entrenched that it can't cope with new results as they emerge".
His comments follow the recent announcement by Health Minister Tony Ryall that there will be $24 million over four years provided in the Budget for a screening pilot involving an immunochemical faecal occult blood test (iFOBT).
Prof Cox said research published in Lancet of a randomised controlled trial in the United Kingdom covering 170,432 men and women using FS achieved a 31% reduction in bowel cancer mortality and a 23% reduction in the incidence of bowel cancer.
Trial participants were followed up over 11 years.
It showed 191 people would need to be screened to prevent one cancer diagnosis and 489 to prevent one death.
Prof Cox said given New Zealand's higher incidence of bowel cancer, he expected the detection rate would be higher in this country with such a programme.
He described Ministry of Health claims that up to 36% of deaths, or 270 a year, could be saved after 10 years with a nationwide programme using iFOBT as " extravagant".
If such claims had been used to help convince the Government to "go down this track", they were mischievous, he said.
In some ways it was lucky New Zealand did not rush in to have a screening programme earlier as it now had the opportunity to get it right and "we seriously need to make sure we get it right".
Mr Ryall's office advised that to achieve the saving of 270 lives annually after 10 years, about 55% of the eligible population would need to participate in the programme.
Asked why the predicted number of lives saved was more than double earlier estimates concerning a national programme, which suggested 10 to 15% of deaths could be avoided, Mr Ryall's office said it was because the type of test to be used was more accurate than other faecal occult blood tests.
Prof Cox said the results were based on modelling rather than actual observations.
He pointed out that the health minister in Ireland was only claiming a potential 15% reduction for the Irish programme.
In an email response to questions from the Otago Daily Times, Ministry of Health clinical director (bowel cancer) Dr Carol Atmore said the focus of the ministry had been on a faecal occult blood test-based screening programme.
This was in line with the recommendations of the 2006 Colorectal Cancer Screening Advisory Group, international evidence and best practice.
The 2009 Ministerial Bowel Cancer Task Force also approved this approach.
Emerging evidence suggesting other screening might be effective, such as flexible sigmoidoscopy, would be monitored.
In the meantime, the ministry would proceed with the pilot as planned, testing the suitability of the "current best option in the New Zealand setting".
The 2006 report to which Dr Atmore referred also estimated that a screening programme involving flexible sigmoidoscopy would be several million dollars cheaper than an iFOBT scheme.
While it would require more sigmoidoscopy capacity, it would reduce the demand for colonoscopies.
The total cost of the four-year iFOBT pilot, which would cover a population of 60,000 aged 50 to 74, is expected to be $31 million.
Mr Ryall said the pilot would give a clear idea of the sector's ability to do the required number of colonoscopies in the future.
In Otago last year, some doctors, concerned about the access their symptomatic patients had to the procedure, complained to the district health board, prompting an audit of some colonoscopy referrals.
As a result of the audit, the board is providing an extra 200 colonoscopies this year and looking at ways to improve the service.
Expressions of interest in running the pilot announced by Mr Ryall will be sought.
Whoever is chosen for the scheme will have to include at least 6000 Maori in the required age band.
No decision on a nationwide programme will be made until the feasibility study is completed in 2015 and all monitoring and evaluation data has been analysed.
The cost of a national iFOBT bowel cancer screening programme, for people 50 to 74, is estimated at $60 million a year once it was established.
