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Crohn's disease can be difficult to diagnose because the symptoms are nonspecific, meaning that they occur with many different gastrointestinal disorders. Upon hearing your symptoms, a number of possibilities will suggest themselves to your health care provider. He or she will conduct a detailed medical interview to try to pinpoint the diagnosis. You will be asked questions about your symptoms, medical problems you have had in the past, surgeries you have had, what medications you take, your family history, and your diet, habits, and lifestyle. You will be examined carefully to look for physical signs that might reveal the diagnosis.

Blood tests may be done to test for Crohn's disease. The purpose of these is to detect inflammation or nutritional deficiencies.

• Blood cell counts: Abnormalities may indicate anemia or inflammation.
  
• Electrolytes: Low levels may be an indication of problems in absorbing nutrients from foods in the intestine.
  
• Protein (albumin): Again, a low level may indicate absorption problems in the digestive tract.
  
• C-reactive protein and orosomucoid: These are markers of inflammation, and their levels correlate with how active the disease is.
  
• Erythrocyte sedimentation rate: This is another marker of inflammation and disease activity.
  
• Perinuclear antineutrophil cytoplasmic antibody (p-ANCA) antigen and anti-S cerevisiae antibodies [ASCA]): These tests are useful in distinguishing Crohn's disease from ulcerative colitis. A test result positive for p-ANCA antigen and negative for ASCA suggests the diagnosis of ulcerative colitis; a test result positive for ASCA and negative for p-ANCA antigen suggests Crohn's disease.
  

A stool sample may be collected to check for blood and signs of inflammation or infection, including parasites that could cause similar symptoms.

You may undergo imaging studies (x-ray films) to detect the extent of Crohn's disease and any complications that may have developed.

• Barium contrast studies: This is a series of x-ray films taken after you drink a contrast material containing a chalky substance called barium. The barium allows the intestine to show up better than on a plain x-ray film. Barium studies are very useful in defining the nature, distribution, and severity of the disease. The barium studies may include an upper GI series (x-ray films of the upper part of the digestive system) and a small bowel follow-through (x-ray films of the small intestine).
  
• Barium enema: This works on the same principal as the barium contrast studies of the upper digestive system, but the barium is introduced into the lower digestive tract through the rectum. This test is done to see whether your colon and rectum are involved, and to what extent.
  
• CT scan or, in some cases, ultrasound is helpful in assessing complications outside of the intestine, such as fistulas, an abscess, or abnormalities of the liver, bile duct, or kidneys. MRI may be used instead.
  

Endoscopy is often necessary if imaging studies do not give a definite diagnosis.

• Endoscopy involves inserting a thin tube with a light and a tiny camera on the end into a body cavity or organ. The camera transmits pictures of the inside of the organ so that the doctor can see inflammation or bleeding or other signs of problems.
  
• Both the upper and the lower parts of the digestive tract can be examined endoscopically. Endoscopy of the lower part of the digestive tract is called colonoscopy. Endoscopy of the upper digestive tract is usually called simply upper endoscopy.
  
• In both cases, the doctor can use the endoscope to take a biopsy. A biopsy is a tiny sample of tissue taken from the mucosal lining of the digestive tract. These tissues are examined under a microscope by a pathologist (a doctor who specializes in diagnosing diseases by examining tissues and cells in this way).
  
• Endoscopic retrograde cholangiopancreatography (ERCP) is another endoscopic procedure that is helpful for both diagnosis and treatment in people who have Crohn's disease in their pancreas or bile ducts.

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