jejunal biopsy: jejunal biopsy removal of a piece of the lining (mucosa) of the upper smail intestine. This can be done by a surgical operation but is usually performed by a special metal capsule, swallowed by the patient. When the capsule is in the *jejunum a small knife within it is triggered by suction on an attached tube, cutting off a small piece of mucosa. The specimen may be examined microscopically to assist in the diagnosis of *coeliac disease or intestineal infection, or its enzyme content may be measured chemically to detect.