A method of diagnosing small intestinal bacterial overgrowth or a condition caused by small intestinal bacterial overgrowth, said condition being selected from the group consisting of irritable bowel syndrome, fibromyalgia, chronic fatigue syndrome, depression, attention deficit/hyperactivity disorder, systemic lupus erythematosus, and multiple sclerosis, comprising:
- obtaining a serum sample from a human subject having a suspected diagnosis of small intestinal bacterial overgrowth or the condition caused by small intestinal bacterial overgrowth;
- and
- analyzing said serum sample for the presence of antibody specific for or cross reactive with a surface epitope of Saccharomyces cerivisiae, the presence of said antibody in said serum sample corroborating the suspected diagnosis of small intestinal bacterial overgrowth or the condition caused by small intestinal bacterial overgrowth.