Rationale for combining different screening strategies.

It is generally accepted that screening programs should be quite safe, and that the benefits should substantially outweigh the harms. As Cochrane and Holland stated: We believe that there is an ethical difference between everyday medical practice and screening. If a patient asks a medical practitioner for help, the doctor does the best he can. He is not responsible for defects in medical knowledge. If, however, the practitioner initiates screening procedures he is in a very different situation. He should, in our view, have conclusive evidence that screening alters the natural history of disease in a significant proportion of those screened. If this is so, one should recommend the combination over either test alone only if there is sufficient evidence that the combination is more effective and no more dangerous. There is a difference of opinion over whether the evidence, which is certainly not strong, is nevertheless sufficient. This poses a dilemma. Many expert groups prefer that screening for colorectal cancer be done with both FOBT and sigmoidoscopy rather than either alone. Yet, the strength of the evidence for additional effectiveness, and information on the magnitude of that effect if it is present, is substantially less than for the individual tests. This being the case, the author believes that it is premature to advocate the combination over either test alone, especially when the most pressing national priority in colorectal cancer screening is to get a large proportion of the adult population to be screened at all.