Screening for colorectal cancer--the patient and the population.

All colorectal cancer screening strategies lead to colonoscopy, the definitive procedure for identifying and removing premalignant adenomas as well as some early cancers. Other screening strategies owe much (in the case of flexible sigmoidoscopy) or all (in the cases of fecal occult blood testing and barium enema) of their benefit to the direct effects of colonoscopic procedures. No other screening test or combination of tests is quite as sensitive as colonoscopy (1), particularly for identifying adenomatous polyps. For these reasons, there is increasing consideration of bypassing preliminary tests and going straight to colonoscopy, once or twice in a lifetime, as screening for average-risk patients (2,3). Whether this makes sense should be determined from two perspectives: that of the individual patient and that of the population. Cost-effectiveness modeling is helpful for both. For patients who intend to comply with screening and with all recommended follow-up for positive findings, a model with compliance set at 100% may provide the best available estimates of the benefits and value of screening. Five recently published, carefully conducted cost-effectiveness comparisons of colorectal cancer screening strategies have done just this (4 – 8), including the report by Vijan et al. (8) in this issue of the Journal. Despite distinct approaches to modeling and some variation in assumptions, all five suggest that when compliance is 100%, screening by a variety of approaches, including screening colonoscopy, is both highly effective and cost effective. Estimated differences in mortality reduction between strategies are small, inconsistent between studies, and dependent on assumptions about costs, and safety. So, for individual patients of at least average risk, colonoscopy is one reasonable approach to initial screening. Its major advantages are less frequent screening and fewer procedures when screening is positive; its disadvantages include a modest absolute increase in risk of complications, greater initial cost, and a more uncomfortable bowel preparation. Vijan et al. extend their model to give a population perspective. They recognize that noncompliance can diminish both the overall population benefit and, in some cases, the cost effectiveness of screening programs. Not surprisingly, they find that noncompliance presents greater problems for screening strategies involving sequential steps, where noncompliance or dropout can occur at more than one point. Colonoscopy, which combines screening, follow-up, and, in most cases, definitive treatment in a single examination, preserves more of its benefit as population-wide compliance decreases. The strategy of fecal occult blood testing and flexible sigmoidoscopy has as many as three decision points and is the most susceptible to decreasing compliance. Two limitations of this analysis, noted by the authors, bear repeating. The authors compared strategies at fixed levels of compliance. However, we have no idea of the extent to which populations would accept screening colonoscopy, with its rigorous bowel preparation and intravenous sedation. If acceptance of colonoscopy should turn out to be substantially lower than for less invasive approaches, population benefits would also be much lower. Ironically, cost-effectiveness calculations could still favor colonoscopy, especially if screening costs are calculated per examination performed, ignoring start-up costs related to equipment purchase and training, which are relatively fixed regardless of volume. Cost-effectiveness analyses, including the present one, tend to undervalue start-up costs, particularly in low-volume settings. At the population level, we would lose lives and perhaps money by shifting resources prematurely to a less acceptable screening strategy. In their base case, the authors also set compliance for recommended follow-up tests at the same level as compliance with initial screening. However, it seems possible that patients willing to undergo initial screening would be more likely to comply with follow-up testing, especially when presented with knowledge that their screening test was positive. In a mass sigmoidoscopy screening program at Kaiser Permanente, more than 83% of 2379 patients with a positive result of flexible sigmoidoscopy (ie, multiple or advanced adenomas) underwent follow-up colonoscopy within 6 months (9). The authors pointed out that if compliance with follow-up testing is greater than compliance with initial screening, the relative advantage of colonoscopy decreases. Despite these concerns, the point is well taken that a potential advantage of Am J Med. 2001;111:662– 664. From the Division of Research, Kaiser Permanente Northern California, Oakland, California Requests for reprints should be addressed to Joe V. Selby, MD, MPH, Division of Research, Kaiser Permanente Northern California, 3505 Broadway, 13th Floor, Oakland, California 94611-5714.