In classification of perianal CD, a distinction should be made between the detailed anatomic description of perianal fistulas and an assessment of fistula activity [43]. There are two disease activity measures currently used in clinical trials to follow perianal CD activity: the Perianal Disease Activity Index (PDAI) and the Fistula Drainage Assessment (Table 46.6). The PDAI was developed and validated by Irvine and colleagues based on the assessment of quality of life factors [44]. It contains 5 items, each scored 0–4, with higher scores representing more severe disease. In the validation cohort, it had moderate correlation with both physician and patient assessment of perianal disease activity (r = 0.72 and 0.66, respectively). As expected, it correlated poorly with the CDAI and HBI (r = 0.23 and 0.21, respectively) implying that perianal disease can present in the absence of other CD symptoms. The PDAI scores were reproducible in patients with stable disease over 4–8 weeks, implying test-retest reliability. Although the PDAI changed in patients who improved or deteriorated on repeat visits, robust and accepted responsiveness statistics were not presented. The second instrument, the Fistula Drainage Assessment (Table 46.7), was introduced as part of a clinical trial of infliximab therapy for perianal CD [45]. A fistula was considered closed when it no longer drained despite gentle finger compression. A response was defined as a reduction of 50% or more in the number of draining fistulas, and remission was defined as absence of any draining fistulas on two consecutive visits. In this study, the Fistula Drainage Assessment was significantly lower in the treatment versus the control group, similar to the PDAI calculated simultaneously. Although these results have been replicated in other clinical trials [46–48], the PDAI has several drawbacks. “Gentle finger compression” is an investigator-dependent measure, and fistula remission is dependent on external appearance, while MRI studies demonstrate that the internal fistula tract lags behind clinical remission by a median of 12 months [49, 50].

Incorporating MRI-based scoring features in an attempt to combine anatomical fistula description with features reflecting active inflammation, van Assche et al. generated a score which, not surprisingly, correlated poorly with PDAI but in which absence of MR enhancement was found to correlate well with clinical remission [51, 52]. Of MRI features of perianal disease, recent pediatric data suggests perianal fistula length assessed by MRI was found to be the best predictor of treatment response [53].

There is no unique perianal disease activity index for children. In order for the PDAI to be considered in children, the sexual dysfunction component should be modified to reflect more age-appropriate issues. At present, the use of the Fistula Drainage Assessment definitions appears to be the preferred perianal outcome in pediatric clinical trials, as an anchor for physician assessment of disease activity; however, improved instruments to score perianal CD fistula activity are required [43].

The earliest classification of UC disease activity was a qualitative scale published by Truelove and Witts in 1955 (Table 46.8) [54]. Because of simple gradation, significant ambiguity exists in defining change in disease activity with this index. Arbitrary quantitative indices have since been introduced, including the Powell-Tuck Index [55], the Mayo Clinic score [56], Rachmilewitz Index [57], and Lichtiger Score [58] with the Mayo score currently in widespread use. The first three scores include an endoscopic evaluation of the rectosigmoid as part of the global assessment. Their validation has been largely a side product of clinical trials in which they have been used and developed. Seo and colleagues developed and evaluated a UC disease activity index [59, 60], weighted against the Truelove and Witts classification. The initial development process was biased toward severe disease since the investigators used a retrospective cohort of hospitalized patients. Walmsley and colleagues developed a Simple Clinical Colitis Activity Index that removed all laboratory parameters [61]. It correlated highly with both the Powell-Tuck Index and Seo index. The Endoscopic-Clinical Correlation Index (ECCI) was developed prospectively in 137 adults with items chosen based on their ability to predict endoscopic outcome [62]. The ECCI is highly correlated with the endoscopy colitis score (r = 0.81), higher than the Seo, Truelove and Witts, Powell-Tuck, and Walmsley’s simple colitis index; however, separate validation is not available to assess reliability and responsiveness. In a prospective head-to-head study in adults of all noninvasive UC disease activity indices, the Walmsley index and PUCAI (see below) were best in assessing disease activity when compared to a number of parameters including the Mayo score [63] (Table 46.9).