Addressing bias in polyp size measurement

The purpose of most colonoscopies is to prevent colorectal cancer, and to achieve this goal at the lowest reasonable cost and without harm. Because colonoscopy is subject to marked and quite variable levels of operator error [1] [2], these goals are sometimes, or evenly frequently, not all achieved. Enormous errors in management are introduced by failed detection, which occurs to such an extent that some endoscopists miss > 90 % of adenomas [3] and serrated class lesions [4], and many endoscopists miss more than half [5]. There is up to a threefold variation between endoscopists in effectiveness of polyp resection [6]. Given the same findings at colonoscopy, many endoscopists advise surveillance at intervals shorter than those recommended in guidelines [7]. Substantial fractions of the average-risk US Medicare population are subjected to screening colonoscopy at 5-year intervals, even though no guideline has ever endorsed screening this group at intervals of less than 10 years [8].

Adding to these huge sources of error in effective and cost-effective management is the problem of inaccurate assignment of polyps into size categories. From a cost perspective, the assignment of polyps into categories of ≤ 9 mm vs. ≥ 10 mm is of greatest importance, because lesions ≥ 10 mm are considered advanced in both the conventional adenoma and serrated classes, and advanced histology frequently leads to shorter follow-up in most surveillance guidelines [9] [10]. There are several a priori sources of error in assigning size to these two categories. One source is financial incentive, in which doctors receiving fee-for-service payment move small polyps into the ≥ 10 mm size category so that patients will come back for more follow-up examinations. Another source is fear of missing lesions (and the potential consequences), in which endoscopists up-size polyps to the ≥ 10 mm category in order to justify shorter surveillance because they are afraid that they might have missed a lesion, and hope that the shorter interval may protect the patient from an interval cancer. The origins of this behavior must be either lack of faith in one’s own inspection performance or lack of faith in surveillance guidelines [11]. The best documented source of error in assigning the size category is simple inaccurate measurement of size, which is apparently both rampant and probably highly variable between endoscopists [12] [13].

In this issue of Endoscopy, Plumb et al. advance our understanding of size category misclassification by documenting another source of error in assigning size category, that of terminal digit preference [14]. This bias represents an extremely simple methodology to assess the precision of any clinical measurements. It has been known since the 19th century that when people report numerical results, certain preferred end-digits are reported substantially more often than the expected pattern of distribution suggests, and these digits are typically multiples of 5 and 10. Digit preference bias is a proxy for the precision of the measurement, and systematic peaks at 5 – and 0-end digits indicate a tendency for individuals to inaccurately report numerical data. Terminal digit preferences have been reported in situations where clinicians are responsible for recording measurements, such as blood pressure and birth weight [15] [16]. In a retrospective analysis of 92 124 polyps, Plumb et al. showed unexpected peaks of clustering for all measurements ending in 5 or 0, such as for 10 mm or 15 mm polyps. This was mirrored by an under-representation of measurements ending in 1 and 9, such as 9 mm or 11 mm polyps. When limiting the analysis to 10 mm cut-off, it was estimated that the absolute rate of over-diagnoses of patients with ≥ 10 mm polyps ranged between 2.4 % and 5 %, according to the clinical setting.

Recognition of terminal digit preference gives us additional important understanding of how polyps are incorrectly sized, and is clearly a tendency we should teach colonoscopists to avoid. Arguably, however, terminal digit preference is the least important of the sources of misclassification of polyp size in clinical practice, particularly when it occurs for polyps with a true size of near 10 mm. Thus, downsizing polyps in the 11 – 15 mm range to 10 mm has no effect on the surveillance interval and therefore is of minimal importance. Upsizing polyps in the 8 – 9 mm range to 10 mm by terminal digit preference is more of a problem, but how much of a clinical problem is likely to be relative to some of the other sources of error discussed above. First, the biologic factors that drive cancer potential in polyps undoubtedly increase along a continuum of size, and setting the cutoff for an “advanced” lesion at 10 mm in size is, at least partly, arbitrary. In fact, originally setting the cutoff at 10 mm rather than 12 mm or some other size may have in part reflected terminal digit preference. How much different in biology is a 9 mm from a 10 mm polyp? Does it matter much if a patient with a 9 mm polyp is sometimes managed like a patient with a 10 mm polyp? Second, Plumb et al. found that 2.4 % – 5 % of patients had inflation of their polyp size category to ≥ 10 mm [14]. This quantitative level of error is dwarfed by the errors introduced into surveillance intervals by failed detection [3] [4] [5] and lack of knowledge of guidelines or lack of willingness to apply the intervals recommended in guidelines [7] [8] [9] [11].

Even though terminal digit preference is probably a relatively small source of error in colonoscopy, we certainly endorse measures to improve the accuracy of lesion size measurement, and the quality and consistency of size documentation. While the development of electronic real-time calipers is anticipated and eagerly awaited, we suggest several steps that can be taken now to improve assignment of polyps into correct categories ([Table 1]).

Table 1 Suggested measures to improve measurement and categorization of polyp size in colonoscopy.

1	Teach precise polyp size measurement to the mm level and avoidance of terminal digit preference
2	Photograph all lesions prior to resection
3	For lesions in the diminutive size range, consider photography with a closed snare tip or biopsy forceps of known size adjacent to the lesion
4	For lesions in the 6 – 15 mm range, photograph with open snare of known diameter adjacent to the polyp (graduated snares and forceps designed specifically for this purpose have been described recently)

First, photography goes a long way to enhance not only endoscopic documentation but also performance. Although photography is unlikely to be as effective as video recording in enhancing performance [17], it could be expected to positively impact performance in polyp size measurement. If a physician systematically overestimates polyp size, quality review of endoscopy reports with photographs should make it readily apparent. Systematic photography should make it more difficult to overestimate polyp size because of financial bias or fear of missed lesions. Furthermore, systematic photography with instruments of known size adjacent to the lesion is easily feasible and could enhance size measurement [18] [19] [20] [21]. Although measurement of polyps using uncalibrated devices of known size is not perfect [13] [18], it is likely to enhance accuracy, and perhaps particularly for classifying lesion as < or ≥ 10 mm. Calibrated devices further improve accuracy [18] [19] [20] [21]. Photography with a device of known size in view may be informative and educational to doctors making systematic errors in size measurement. Diminutive lesions can be measured against a closed forceps tip or snare sheath tip of known diameter. Lesions in the 6 – 15 mm size range can be photographed with an open snare of known diameter. Stiff diminutive snares maintain their specified size well when open and are particularly useful for this purpose, especially if calibrated devices are not available.

In general, better documentation goes hand in hand with quality improvement. In the long term, new and accurate automated software programs will overcome the problems of incentives, bias based on fear, inaccurate measurement, and terminal digit preference to correctly place polyps into size categories. In the interim, we suggest that the measures presented in [Table 1] are reasonable steps to improve polyp size category assignment. These and other related measures warrant use in clinical practice and testing in clinical trials in order to optimize and validate a set of rules that overcome the deficiencies of current practice in polyp size measurement.

• References

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