This is a medical question of type : exams and tests 
 How to diagnose What I need to know about Crohn's Disease ? 
 A doctor will perform a physical exam and tests to diagnose Crohns disease. During your visit, the doctor will ask about your symptoms and medical history.
                
The doctor may order blood tests, which involve drawing blood at a health care providers office or commercial facility and sending the sample to a lab for analysis. Blood tests can show anemia caused by bleeding. Anemia is a condition in which red blood cells are fewer or smaller than normal, which means less oxygen is carried to the bodys cells. Blood tests can also show a high white blood cell count, a sign of chronic inflammation.
                

                
You may also be asked for a stool sample. A stool test is commonly used to rule out other causes of GI diseases, such as infections. The doctor will give you a container for catching and storing the stool. The sample is returned to the doctor or a commercial facility and sent to a lab for analysis. A stool sample can also be used to check if you have bleeding or inflammation.
                
Other tests may be needed to diagnose Crohn's disease. The following tests are all performed at a hospital or outpatient center.
                
- Colonoscopy. Colonoscopy is the most commonly used test to specifically diagnose Crohns disease. This test is used to look inside your rectum, entire colon, and ileum. The health care provider will give you written bowel prep instructions to follow at home before the test. You may need to follow a clear liquid diet for 1 to 3 days before the test. You will need to take laxatives and enemas the evening before the test, and you will likely have one or more enemas about 2 hours before the test. A laxative is medicine that loosens stool and increases bowel movements. An enema involves flushing water, laxative, or sometimes a mild soap solution into the anus using a special squirt bottle.  For the test, you will lie on a table while the doctor inserts a flexible tube into your anus. A small camera on the tube sends a video image of the intestinal lining to a computer screen. The doctor can see inflammation, ulcers, or bleeding. The doctor may also perform a biopsy. The doctor will look at the tissue with a microscope to confirm the diagnosis of Crohns disease. In most cases, youll be given a light sedative, and possibly pain medicine, to help you relax. You will not feel the biopsy.  Cramping or bloating may occur during the first hour after the test. Driving is not permitted for 24 hours after the test to allow the sedative time to wear off. Before the appointment, you should make plans for a ride home. By the next day, you should fully recover and go back to your normal diet.    - Flexible sigmoidoscopy. This test is used to look inside the rectum and lower colon. The health care provider will give you written bowel prep instructions to follow at home before the test. You may need to follow a clear liquid diet for 1 to 3 days before the test. You may also need a laxative or enema the night before the test. And youll have one or more enemas about 2 hours before the procedure.  For the test, you will lie on a table while the doctor inserts a flexible tube into your anus. You will not need a sedative for the test. A small camera on the tube sends a video image of the intestinal lining to a computer screen. The doctor can see inflammation, ulcers, or bleeding. The doctor may also perform a biopsy by snipping a bit of tissue from the intestinal lining. The doctor will look at the tissue with a microscope to confirm the diagnosis of Crohns disease. You will not feel the biopsy.  You can usually go back to your normal diet after the test, though you may have cramping or bloating during the first hour after the test.    - Computerized tomography (CT) scan. A CT scan uses x rays and computers to create images of the inside of the body. For the test, you will lie on a table that slides into a tunnel-shaped device where the x rays are taken. The technician may give you a solution to drink and an injection of a special dye through a needle inserted into an arm vein. You will not need a sedative for the test. CT scans can be used to help diagnose Crohn's disease.    - Upper GI series (x rays). An upper GI series may be done to look at the small intestine. No eating or drinking is allowed for 8 hours before the procedure. You will not need a sedative for the test. During the procedure, you will stand or sit in front of an x-ray machine and drink barium, a chalky liquid. The barium coats the small intestine, making signs of the disease show up more clearly on x rays.  After the test, you may go back to your normal diet, though you may have nausea or bloating for a short time. Traces of barium in the GI tract cause stools to be white or light colored for a few days after the test.    - Lower GI series (x rays). A lower GI series may be done to look at the large intestine. The health care provider will give you written bowel prep instructions to follow at home before the test. You will be asked to follow a clear liquid diet for 1 to 3 days before the test. A laxative or enema is usually used the evening before a lower GI series. Enemas are sometimes repeated the morning of the test.  For the test, you will lie on a table while the doctor inserts a flexible tube into your anus. You will not need a sedative for the test. The large intestine is filled with barium, making signs of the disease show up more clearly on x rays. After the test, you may go back to your normal diet, though you may have bloating. You also may have some soreness of the anus. Traces of barium in the GI tract cause stools to be white or light colored for a few days after the test.