 Various clinical and radiological outcome measures have been investigated to quantify pCD activity. Clinical assessment is crude and participant to inter-operator-variability, with response loosely defined as a reduction of 50 in the number of draining fistulas and remission the absence of draining fistulas on two consecutive visits. This assessment has never been validated, and takes no account of the inflammatory state of the disease deep within the perineum. A Perineal Disease Activity Index PDAI was developed to assess perianal disease severity and response to therapy. It is based on the assessment of quality of life and disease severity. The PDAI score is 87 accurate and wrongly classifies patients disease activity in 10 of cases, but is insensitive to change after medical therapy and its remission cut-off never been properly determined. 