IBS is usually classified as a functional disorder. A functional disorder refers to a disorder where the abnormality in how the body works is the problem, but somehow or the other the causes of that disorder are difficult to identify.
Usually, through regular methods, a functional disorder cannot be diagnosed. The reason behind this is that it is not an infection. It is not an inflammation. And it is not a structural abnormality either. Because of that, commonly used testing methods such as x-rays or blood tests cannot detect it.
