Reflex anal dilation (RAD) is the reflexive dilation of the human anus to a diameter greater than two centimeters in response to the parting of the buttocks or anal stimulation, such as brushing with a medical instrument. RAD was theorized to be a clinical marker associated with anal sexual assault in children, and has been associated with other signs of sexual assault but also appears in children with severe chronic constipation and those subject to invasive medical treatments of the anus. The finding of RAD alone is not considered indicative of sexual abuse, and a normative sample of children not suspected of having been sexually abused found that 49 per cent of children showed anal dilation either continuously or intermittently, though the dilation exceeded 20 mm in only 1.2 per cent of cases.
Used extensively in the Cleveland child abuse scandal, it was discredited during the trial as the sole indication of sexual abuse, determined to be considered a sign of sexual assault by a tiny minority of British physicians. RAD as a clinical marker for sexual abuse is now considered discredited.