Dr Henry WeiGastroenterology & Endoscopy
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Inflammatory bowel disease: understanding Crohn's and colitis

Inflammatory bowel disease (IBD) means ongoing inflammation in the digestive tract. Crohn's disease and ulcerative colitis are the two main types. Your tests help show where the inflammation is and how active it is. I use those results to discuss treatment and what we need to monitor.

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Know who is following your results and treatment

In New Zealand, IBD care often involves your gastroenterology service and GP, with an IBD nurse and dietitian where available. Make sure you know which team to contact about a flare, prescriptions or overdue results. An overseas treatment recommendation does not by itself establish funded access here.

Crohn's and Colitis New Zealand offers local information and support. Its 'I Can't Wait' toilet card can help explain urgent toilet needs when you are out, although it does not guarantee access.

Guidance behind this advice
Explore the explanation

IBD affects different parts and layers of the bowel

Choose a view
Explanatory diagram
The large bowel and the end of the small bowelFixed front-view colon schematic. The patient's right is image left. The caecum and ascending colon are on that side. The transverse colon crosses to the descending colon on the patient's left. The sigmoid joins the rectum and anal canal. The terminal ileum joins the medial caecum through the ileocaecal valve. The appendix is a separate branch. Only a short terminal ileum is shown; the rest of the small bowel, upper digestive tract, other organs and mesentery are omitted. No inflammation is highlighted in this orientation map. Colours are teaching highlights, not endoscopy colours, a severity scale, a time sequence or a personal result.Your rightYour leftFront viewAscendingcolonTransverse colonDescendingcolonCaecumSigmoid colonRectumAnusTerminal ileumAppendixColon and a short end section of small bowel. Other organs are omitted.Positions and bowel bends vary. This is a simplified front view, not to scale.

Start with the anatomy

The colon and rectum make up most of the large bowel. The terminal ileum is the last part of the small bowel. This map shows their connection; most of the small bowel is outside the picture.

Simplified illustrations, not to scale. These show an example, not your own anatomy or test result.

Read every diagram explanation

Start with the anatomy

The colon and rectum make up most of the large bowel. The terminal ileum is the last part of the small bowel. This map shows their connection; most of the small bowel is outside the picture.

Crohn's disease

Crohn's can affect any part of the digestive tract from the mouth to the anus. The terminal ileum, the end of the small bowel, and the colon are common locations, but the stomach and other parts of the small bowel can also be involved.

Ulcerative colitis

This diagram demonstrates inflammation of the left colon. People can have different extents of inflammation, from very limited inflammation of the rectum only (proctitis) to inflammation of the entire colon (pancolitis).

Putting the tests together

I use the symptom pattern, examination, blood and stool results, endoscopy and biopsies. A scan can add information where needed. These help distinguish IBD from infection and other causes, including symptoms that resemble IBS.

IBD and IBS are different conditions

IBD involves an abnormal immune response and inflammation that can damage bowel tissue. It is not an infection you can pass to someone else, and it is not caused simply by stress or eating the wrong meal.

Irritable bowel syndrome (IBS) involves altered gut sensitivity and movement, without the bowel inflammation of IBD. Some symptoms overlap. IBS-type symptoms can also occur in someone whose IBD inflammation is controlled, but I check for inflammation and other causes before attributing new symptoms to IBS.

Crohn's disease and ulcerative colitis affect the bowel differently

Crohn's disease and ulcerative colitis are different conditions, but their symptoms can overlap. This comparison shows the usual pattern; some people do not fit neatly into one category at first.

Crohn's disease and ulcerative colitis
FeatureCrohn's diseaseUlcerative colitis
Where it can occurAnywhere from the mouth to the anus. The terminal ileum and colon are common locations.The colon and rectum. It usually starts in the rectum and extends continuously.
PatternInflamed areas can be separated by areas that look less affected.Inflammation is usually continuous, without skip areas.
DepthInflammation can extend through the bowel wall.Inflammation mainly affects the inner lining, although severe disease can involve deeper tissues.

How tests confirm IBD

I ask about diarrhoea, bleeding, urgency, night-time symptoms, pain, weight, family history, travel and medicines. I also ask about painful joints, mouth ulcers, skin changes and sore eyes. IBD can affect more than the bowel, and bowel frequency alone does not show how unwell you are.

Blood tests check inflammation, anaemia and nutrition. Stool tests look for infection and may include calprotectin, a marker of bowel inflammation. Calprotectin does not diagnose IBD on its own. Colonoscopy with biopsies usually provides key information. MRI or specialist bowel ultrasound can assess areas beyond the camera and the bowel wall.

What else can cause similar symptoms?

Infection, coeliac disease and other conditions can resemble IBD. Coeliac testing is most reliable while you are eating gluten. A normal blood inflammatory marker does not exclude IBD, and a low calprotectin is less reassuring for some small-bowel disease.

FIT measures blood. It does not diagnose IBD or replace endoscopy recommended for persistent symptoms. I interpret the symptoms, examination, tissue results and scans together. A biopsy report saying 'inflammation' does not by itself mean Crohn's disease or ulcerative colitis.

What should I do during a flare?

A flare is a return or increase of disease activity. Remission means the disease is controlled, but feeling better does not always mean the bowel has healed. Conversely, new diarrhoea can be caused by infection or another problem rather than active IBD.

If symptoms worsen, contact your GP or IBD team early. Describe bleeding, stool frequency, urgency, pain, fever and your ability to eat and drink. Follow your agreed flare instructions. Do not start leftover steroids or increase treatment yourself unless your team has specifically agreed this plan.

How treatment helps

The aim is to improve daily life, heal inflammation and reduce future complications while avoiding repeated or prolonged steroids. Treatment differs between Crohn's and ulcerative colitis and is matched to disease location, severity, previous response, safety and your preferences.

Options include medicines, selected nutritional treatments and sometimes surgery. Steroids can settle a flare but are not a long-term maintenance treatment. Other medicines help keep inflammation controlled. Do not stop treatment just because you feel well. Food support should protect nutrition, rather than steadily remove more foods without a clear purpose.

Crohn's disease and ulcerative colitis are distinctive conditions, but some treatments overlap. The choice depends on where the inflammation is, how active it is, previous treatment, safety and your preferences.

What do we check at follow-up?

Follow-up may combine symptoms, blood tests, calprotectin, scans and endoscopy. I also review fatigue, iron or vitamin deficiencies, weight, bone health and medicine monitoring. Vaccination and infection checks are particularly useful before some immune treatments. Tell us about pregnancy plans before changing medicines.

Longstanding inflammation in the colon can increase bowel cancer risk, but the risk varies between people with IBD. I consider the extent and duration of disease, how active the inflammation has been, family history and associated bile-duct disease.

Your endoscopy and/or histology report will record the recommended follow-up once tissue results are available. Routine screening FIT does not replace that surveillance.

When to get help

Seek urgent assessment for severe or worsening abdominal pain, a swollen abdomen with repeated vomiting, heavy bleeding, fever with marked illness, dehydration or inability to keep fluids down. A painful swelling around the anus also needs prompt assessment. A painful red eye, light sensitivity or a change in vision needs urgent eye assessment.

Call 111 if you collapse, have heavy bleeding with faintness or are too unwell to travel safely. Do not wait for a routine appointment or an email reply.

Three things to remember

  • IBD is bowel inflammation. It is different from IBS, although symptoms can overlap.
  • Blood tests, poo tests, endoscopy and scans provide different information. No single test can confirm IBD on its own.
  • Keep taking prescribed treatment and follow your agreed flare plan. Contact your GP or IBD team if symptoms worsen.

Related information

Sources and further reading

English clinical master checked 16 October 2026.

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