Background
One of the most common types of cancer diagnosed is large bowel or colorectal cancer (CRC). Early detection, before symptoms appear, makes it easier to treat bowel cancer and increases the chance of survival. Taking part in a bowel cancer screening program can lead to early detection and removal of large or advanced polyps (advanced adenomas), which are considered to be a precursor to bowel cancer. Simple faecal tests are used to detect the presence of blood in stool, which could be an early sign of bowel cancer or polyps. Two types of faecal blood tests used in population screening are: guaiac-based faecal occult blood tests (gFOBTs) and faecal immunochemical tests (FITs). Large, older studies have shown that screening with gFOBTs can reduce mortality. In a systematic review of the literature, we compared the accuracy of these two tests in order to assess which test gives the best results in population screening for bowel cancer, and, secondarily, for advanced neoplasia (which comprises bowel cancer and advanced polyps together).
Study characteristics
We carried out a detailed search of online databases for studies that evaluated or compared (one of) these two tests in CRC screening. The review included only studies in average-risk individuals over 40 years of age without symptoms. The reference standard to compare the test results with was a total endoscopic examination of the large bowel with a camera on a flexible tube passed through the anus (colonoscopy). We reviewed two types of studies: those in which all participants underwent both the stool test and colonoscopy; and those in which only participants with an unfavourable result on the stool test underwent colonoscopy (in these studies, participants who did not have a colonoscopy after the stool test were followed for at least one year to see if they would be diagnosed with colorectal cancer). The evidence is current until 25 June 2019. We ran a top-up search on 14 September 2021, which yielded only one potentially eligible study, currently awaiting classification.
Test characteristics
The gFOBT 'screenees' – i.e. those who participate in screening – are instructed to collect two faecal samples from three consecutive bowel movements and to smear this on six stool panels. If there is blood in the stool, the panel changes colour. The number of coloured panels for referral to colonoscopy varies between screening programs. In most programs, a single coloured panel is sufficient for referral; however, in others, the number of panels is set at five out of six.
The FIT screenees are instructed to collect one faecal sample from one bowel movement, and to collect this with a brush or spatula into a tube. This tube is then send to a laboratory where the concentration of blood in the stool can be measured. Depending on the height of this concentration, above or below the so-called cut-off or threshold, the screenee is referred for colonoscopy. This cut-off differs per screening program.
Key results
We analysed 63 studies including almost 4 million individuals. The results of this review indicate that if, in theory, 10,000 people take part in screening with a faecal blood test and 100 people in this group have CRC:
- out of the 100 people with CRC, 24 will be missed in those being screened with FITs.
- out of the 100 people with CRC, 61 will be missed in those being screened with gFOBTs.
We also looked at participants with large polyps, CRC, or both. If, in theory, 10,000 people take part in screening with a faecal blood test and 1000 people in this group have large polyps, CRC, or both:
- out of the 1000 people with large polyps, CRC, or both, 850 will be missed in those being screened with gFOBTs.
- out of the 1000 people with large polyps, CRC, or both, 670 will be missed in those being screened with FITs.
In this theoretical group of 10,000 screenees:
- 594 people being screened with FITs will be offered an 'unnecessary' colonoscopy – unnecessary because they do not have CRC; and
- 594 people being screened with gFOBTs will be offered an 'unnecessary' colonoscopy.
From the results described above, we can see that FITs miss less CRC than gFOBTs, while an equal number of screenees from each type of blood test undergo an unnecessary colonoscopy.
How reliable are the results of the studies in this review?
The results of the studies are reliable, as the included studies mostly met the quality criteria we specified before commencing the review.
Future research
More research is needed to investigate whether, in the long term, FIT screening can reduce the number of bowel cancer cases and deaths, and to compare these findings with those from gFOBT screening.