Technical report: results of immunological tests on faecal extracts are likely to be extremely misleading.

Clinical investigation of gut immunity is difficult because of the need to study intestinal tissues or secretions directly. Others have reported that immunoglobulins, antibodies and cytokines can be detected in saline extracts of faeces. We have assessed the validity of this approach by measuring immunoglobulins, albumin, alpha 1-antitrypsin and isotype-specific antibodies in matched samples of faeces and whole gut lavage fluid. Results were compared as estimated output per day, and by using haemoglobin as a common reference substance. Samples were obtained from 10 patients with active inflammatory bowel disease and 10 with other benign GI diseases. For immunoglobulins, albumin and antibodies, the amount detected in faeces varied from < 0.01% to 35.5% (based on estimated daily output) and < 0.01% to 18.5% (based on haemoglobin) of the amount known to be produced in the gut from results of assays on whole gut lavage fluid (WGLF); there were significantly higher rates of recovery in faecal specimens from patients with active gut inflammation than from other patients. Detection rates and titres of specific antibody in faeces were even lower than those for immunoreactive IgA. These data indicate that immunological tests on saline extracts of faeces do not represent the true status of the gut humoral immune system, and such studies should be strongly discouraged.