Bowel Cancer New Zealand says failures with the country's bowel screening programme are putting patients at risk of "unnecessary deaths".
The organisation has serious concerns that many participants in the screening programmes are not being told whether there is blood in their stool.
Authors of a letter published today in the New Zealand Medical Journal say this breaches the Code of Health and Disability Consumers' Rights (COR), which may need to be revised.
Tests results are interpreted by the National Bowel Screening Programme (NBSP) before a participant is told if their test meets the threshold of a pass or fail.
The threshold for a "positive" or "negative" test is based on a cut-off value of 200ng of haemoglobin (Hb) per ml of buffer.
The letter's authors, Otago University research associate professor Brian Cox, surgeon Phil Bagshaw, journalist Ainslie Talbot and Otago University senior research fellow Mary Jane Sneyd, said they had no issue with the cut-off, rather the reporting of results.
"For example, a participant in the current NBSP with a test result of 185ng/ml will receive a report from the programme that their test was negative," the letter reads.
"We suggest that, to comply with the COR, the numerical result should be provided with explanation of why colonoscopy is, or is not, recommended by the NBSP.
"The participant can then discuss the result with their general practitioner, as occurs for other cancer screening tests."
The authors said participants might assume their "negative" result meant there was no evidence of bleeding, which could be inaccurate.
Bowel Cancer NZ spokeswoman Mary Bradley said failure to alert patients to their actual test results could lead to "unnecessary deaths".
She added the process for participants to get their results was lengthy and hinged on people knowing they had the right to request the information.
"Not sending the full report with the concentration of blood found is keeping participants in the dark and providing false reassurance that they don't have bowel cancer," Bradley said.
"When in fact all a negative result signals is that blood sufficient to meet the screening programme level has not been detected.
"We would like to see GPs getting this reading as a matter of course, which they currently do not, as it will allow them to better monitor their patient should they present with further symptoms or have a family history."
National Bowel Screening Programme clinical director Dr Susan Parry did not agree the Code of Rights had been breached but would consider the published research.
Participants were told the results would be reported as positive or negative. This was in line with advice from the test's manufacturer and with practices in other countries that used the screening test. It had also been agreed by a range of experts.
Participants could request their numeric test results but this was not widely publicised.
"There may be other medical reasons why there is blood in faeces, which is why New Zealand's National Bowel Screening Programme (NBSP) will test participants every two years from the age of 60 to 74 years. Participants are encouraged and informed that if their bowel habits change, or they can see blood in their poo (bowel motion), they should seek advice from their general practitioner (GP)."