According to the National Institute of Diabetes and Digestive and Kidney Diseases, Crohn's Disease is an inflammation of the digestive system lining caused by the body's auto-immune system. It primarily occurs in the small intestine. The inflammation can get bad enough that it blocks up the intestines and causes diarrhea and other digestive symptoms. Crohn's disease can be difficult to diagnose because it displays symptoms common to other auto-immune conditions. The cause is still unknown, and there is no one test to diagnose Crohn's disease.

Blood Tests

When the body's auto-immune system starts to attack any part of the body, there is an increase in the number of white blood cells being produced. This is because the white blood cells normally fight disease for the body, but when the auto-immune system is damaged and starts attacking body parts, it uses white blood cells to do the job. Blood tests can help to determine if there is some sort of internal infection due to a high white blood cell count. Anemia may show up on a blood test, which could mean bleeding in the intestines. If there is bleeding in the intestines, the body is not getting enough red blood cells, and this causes anemia. A stool sample can be used to see if there is any intestinal bleeding as well.

Intestinal Endoscopy

The Crohn's and Colitis Foundation of America indicates that endoscopy exams are also used to diagnose Corhn's disease. One of the more common endoscopy tests is a colonoscopy: a small camera on a long tube is inserted into the colon through the anus, and the doctor looks for intestinal damage on a monitor. Another way doctors use imaging tests to diagnose Crohn's disease is by using a barium study and an X-ray. You would be asked to drink a barium solution, and if there is any infection in your intestines, the barium will highlight it when an X-ray is used. X-ray barium tests are normally used to see the overall scope of the condition and how much of the intestines is infected.

Scanning Tests

Imaging tests such as a CT scan or an MRI that do not require an invasive imaging technique such as the colonoscopy are also used to diagnose Crohn's disease. Unlike the X-ray barium test, which can give a general idea of the scope of the problem, an MRI or CT scan imaging procedure can give a more accurate idea of where the infection is occurring and how severe it may be.

Source:

National Institute of Diabetes and Digestive and Kidney Diseases - Crohn's Disease

Crohn's and Colitis Foundation of America - About Crohn's Disease

Medicinenet.com - Crohn's Disease