Question: What is (are) Mastocytic enterocolitis ?
Answer: Mastocytic enterocolitis is a term describing the condition of chronic, intractable diarrhea in people with normal colon or duodenum biopsy results, but with an increased number of mast cells in the colonic mucosa (the innermost layer of the colon). The increase in mast cells is not associated with systemic or cutaneous mastocytosis. It is unclear whether the accumulation of mast cells is a response to, or cause of, the mucosal inflammation that causes the symptoms of the condition. Most individuals with this condition respond well to drugs affecting mast cell function.
Source: GARD

Question: How to diagnose Mastocytic enterocolitis ?
Answer: How is mastocytic enterocolits diagnosed? Mastocytic enterocolitis is diagnosed after an endoscopic procedure in which the doctor takes samples of tissues (biopsies) from the lining of the intestines. The tissue is then sent to a pathologist who looks at it under the microscope. Mast cells may be hard to see on biopsies without a special stain for tryptase, an enzyme present in mast cells. Mastocytic enterocolitis is diagnosed when excess mast cells are present in the small bowel or the colon.
Source: GARD