Diagnosing Crohn’s disease

In order to diagnose Crohn's disease, the following biochemical tests may be ordered by your doctor: 

• Haematological blood tests: If you have Crohn’s, your white blood cell counts will be high and there’ll be other signs of inflammation. Anaemia may also be present (a reduced number of red blood cells).
  
• Autoantibody tests: These help to tell the difference between inflammation caused by Crohn's disease and inflammation caused by other inflammatory bowel diseases.
  
• Stool tests: These can detect blood in the stool from irritated intestines.

The following imaging tests may be done:

• Upper gastrointestinal series: An X-ray is taken after the consumption of a barium solution. This test can reveal places in the small intestine that are narrowed (strictures), and can also highlight ulcers and fistulas.
  
• Flexible sigmoidoscopy or colonoscopy: This provides a picture of what the inside of the large intestine looks like. 
  
• MR enterography: This test uses an MR scan instead of X-rays to take pictures of the entire intestine.
  
• Wireless capsule endoscopy: This involves swallowing a pill-sized object that contains a tiny video camera. The camera then takes pictures of the small intestine and wirelessly sends them to a nearby computer.
  
• A biopsy may also be indicated. This involves removing a small tissue sample from the lining of the intestine and examining it in a laboratory for signs of inflammation. This can help confirm Crohn's disease and rule out other conditions.

Differential diagnosis

Crohn's disease shares symptoms and signs with other conditions, which will have to be ruled out.

1. IBS vs. IBD 
Irritable bowel syndrome (IBS) is a very different condition to inflammatory bowel disease (IBD), although some of the symptoms are similar. Both conditions are associated with abdominal pain, bloating and bouts of diarrhoea or constipation. 

However, in people with IBS, pathological findings such as the inflammation typical of Crohn’s disease won’t be present. Some people with Crohn’s disease may, however, develop IBS-like symptoms (e.g. they get diarrhoea even when their IBD is inactive). IBS is more common in people with IBD than in the general population.

For this reason, extra-intestinal features of Crohn's disease are valuable pointers towards making the correct diagnosis. Peri-anal disease affects up to 15% of people with Crohn’s disease. The presence of peri-anal disease implies the disease is more severe.

2. Crohn’s disease vs. ulcerative colitis 
The site of inflammation differs between Crohn's disease and ulcerative colitis. Crohn's disease can affect any part of the gastrointestinal tract, including the mouth, oesophagus, stomach, small and large intestines, rectum and anus. Ulcerative colitis, on the other hand, is located in the colon, usually starting from the rectum.

The histology of the gut is also different – in other words, the cells look different. With Crohn’s disease, there’s a cobblestone appearance; with ulcerative colitis, there are more ulcerations. 

Reviewed by Kim Hofmann, registered dietitian, BSc Medical (Honours) Nutrition and Dietetics, BSc (Honours) Psychology. December 2017.