Proponents of bowel cancer awareness say the GPs are being left in the dark because the results of the screening program are very simplistic.
Researchers at the University of Otago are calling for a review of the program, which only tells patients whether they tested positive or negative for blood in their stool samples, based on a cutoff point.
New Zealand's bowel cancer spokeswoman Mary Bradley said a negative result could provide a false assurance that everything was fine – even if there was a high blood level.
"General practitioners must, of course, receive the test results because with their patient they can see if there is a family history, they can see if that patient is showing more symptoms and only gives a clearer picture, gives them more information, "said Bradley.
Bowel cancer was the second largest cancer killer in New Zealand, so it was important for doctors to have the best data possible, she said.
Otago University Preventive and Social Medicine researcher Brian Cox said it may be illegal to retain the full results of the screening of patients' bowel cancer.
According to the Bill of Rights, people should receive this information and more details than just passing or failing, Dr. Cox said.
"The results that have been given so far appear to have failed to meet the legal requirements of the rights code," he said.
"People who did tests should have the results sent to them now – and that's thousands of people.
"The level of blood in the stool that would be reported as negative by the intestinal screening program in many other countries – Australia and Denmark, for example – would be referred for colonoscopy."
Dr. Cox said that the way the tests were reported needed to be reviewed immediately to comply with the law.
New Zealand has the highest level of bowel cancer in the world, he said.
The Health Ministry has upheld the decision not to give patients the full test results of bowel cancer, saying that this could only confuse them.
National Bowel Screening clinical director Susan Parry said the program is in line with international guidelines.
Patients could request their numerical results, but the test results had to be reported in a way that was easily understood, she said.
Dr. Parry said there may be other medical reasons why there was blood in the stool and patients were encouraged to see their doctor if they saw blood or if bowel habits changed.