Report in English with a Dutch summary (KCE reports 45A)

KCE reports vol.45 Screening for Colorectal Cancer 17
Wilson & Jungner criteria, adding additional criteria on practical 4 and ethical 5
issues:
11. Treatment started at an early stage should be of more benefit
than treatment started later.
12. The time between test and result and between result and
treatment must be as short as possible.
13. The recruitment procedure should not limit people in their
freedom to participate or not in the screening program.
14. Potential participants should receive adequate information about
pro and cons of participation. Benefits and risks should also be
well known to health care providers 6.
15. Public education should promote a broad accessibility of the
program. It should however not include a moral pressure effect.
16. There should be quality assurance (QA) and quality control
(QC) procedures for the whole screening program.
17. Screening programs are concerted actions meeting
organisational and managerial requirements.
1.3 PITFALLS AND POSSIBLE HARMS
1.3.1 Biases
To many people, screening intuitively seems an appropriate thing to do, because
catching disease earlier seems better. However, no screening test is perfect.
There will always be problems with incorrect results and adverse effects.
Before a screening program is implemented, it should thoroughly be studied to
ensure that putting it in place would do more good than harm.
Various factors can cause the screening test to appear more successful than it
really is. A number of different biases can distort the results 7-13:
1. Length-time bias arises from the fact that intermittent screening
tests will tend to pick up slow-growing, indolent disease that is
likely to have a better prognosis than the rapidly advancing
disease, which is more likely to appear with symptoms between
screening intervals. As a consequence, screening in general may
tend to detect some cancers that would not have killed the
patient or even not have been detected prior to death from
other causes.
2. Lead-time bias arises from early diagnosis itself. By screening, we
intend to diagnose a disease earlier than it would be without
screening. Even if in both cases a person dies at the same time,
the survival time since diagnosis is longer with screening, simply
because in the latter case we diagnosed disease earlier. No
additional life has been gained but, looking at raw statistics,
screening will appear to increase survival time: this gain is called
lead time. If we do not pay attention to what survival time
actually means in this context, we might attribute success to a
screening test that does nothing but advance diagnosis.
3. Selection bias can arise from the fact that not everyone
necessarily will participate in a screening program. There can be
factors that differ between those willing to get tested and those