The proportion of people who returned a National Bowel Cancer Screening Program (NBCSP) screening test in a defined 12-month period who were diagnosed with colorectal cancer (not involving a positive NBCSP screen and positive assessment) in the following 24-month period, or before their next screen, whichever comes first.

An interval cancer is a colorectal cancer that is diagnosed after a screen that detected no cancer and before the next screen or in the following 24 months, whichever is earlier. Interval cancers are inevitable in a population based screening program; a low interval cancer rate is desirable.

A high interval cancer rate reduces the potential for the program to achieve reductions in morbidity and mortality from colorectal cancer. Monitoring interval cancer rates is also important to assess the diagnostic assessment component of the screening pathway.

Monitoring the interval cancer rate by various stratifications may also reveal emerging positive or negative trends that need to be investigated and rectified if necessary.