gastroenterologist doctors wellington

What is a colorectal (bowel) cancer screening?

Screening is the process of doing a test is someone with no symptoms in order to try and identify a disease at an early stage. It is a fraught area because most tests used are not perfect – they may fail to pick up signs of disease when it is present (false negative), leading to false reassurance, or may show positive when disease is not present (false positive). False positive tests can lead to potentially harmful investigations in healthy individuals and create unnecessary alarm.

In terms of screening tests for colorectal cancer, a number of approaches are used around the world currently. In the US, for instance, most private health care plans allow for a colonoscopy at 50 years of age, and 10 yearly thereafter. Colonoscopy is certainly an effective screening test as it will rarely be falsely negative (i.e. miss a cancer), and can remove large polyps at the time of screening and thereby prevent cancer. For an individual, colonoscopy is currently the best screening test for colorectal cancer, but for a public health system, such as in New Zealand, it is too expensive and resource intensive, and in the NZ public health system is only offered to individuals who have a predicted risk of colorectal cancer at least twice that of the average (see 'who is at increased risk?').

In much of the UK, Australia, and the Waitemata (NZ) trial, the bowel cancer screening test used for average risk people between the ages of 50 and 74 is one that tests for tiny amounts of blood in the stool, ‘faecal occult blood’. If blood is detected then a colonoscopy is scheduled. This sort of screening test is subject to both false positive and false negative results but performs acceptably.  

The first roll-out of the NZ bowel screening programme began in Wairarapa and the Hutt Valley in July 2017.  Currently screening is only being offered to those from ages 60 to 74, and involves a 'faecal occult blood' test performed every 2 years.  Other hospital board regions are expected to commence screening over the next 2 years.  Wellington City/Porirua/Kapiti will probably start in the first half of 2019.

A radiological exam on the colon called CT colonography, where the bowel is filled with gas and then a CT scan is performed, is commonly used as an alternative to colonoscopy to investigate bowel symptoms. For the purposes of screening, however, it is less ideal as it may not identify significant flat polyps, especially serrated polyps (described in the About colonoscopy section).

For those able to afford it, however, colonoscopy is the best test it has the unique ability amongst all the screening methods of preventing the development of cancer by removing polyps before they can grow into cancer.  A screening colonoscopy at the age of 50 is one of the best investments the average person can make in his or her future health, along with stopping smoking, moderating drinking, and getting regular exercise.