Some people with Crohn’s disease will experience severe symptoms. Factors that may influence progression to severe disease include age at diagnosis, smoking, elevated CRP biomarkers, location of disease, and perianal disease.

Crohn’s disease is a chronic form of inflammatory bowel disease (IBD).

The severity of the condition can vary widely. Some people may experience no or only mild symptoms, while others may experience severe disease.

Numerous factors may play a role in the condition progressing to the severe stage.

Learn more about assessing disease severity in Crohn’s, why it matters, and what factors may contribute to severe Crohn’s disease.

Crohn’s disease can be highly variable among people living with the condition. Some individuals will have a mild form of the condition, while others may experience severe disease and complications.

A physician will work with a person to evaluate their severity level.

A doctor may take into account factors such as:

  • overall effect on a person’s health
  • specific symptoms
  • overall quality of life
  • degree of disability due to the condition
  • extent of inflammation
  • location of Crohn’s in the body
  • the severity of the effects on the bowel
  • any structural damage

A doctor may use various methods to evaluate the severity of the condition. These may include:

  • a full medical history
  • imaging
  • endoscopy
  • biopsy
  • Crohn’s disease activity index (CDAI)

Experts consider the CDAI the gold standard in measuring the extent of disease activity in Crohn’s. It is a common tool in clinical trials for the condition.

Developed in 1976, the CDAI allows doctors to assess the status of Crohn’s disease based on questions about the frequency and severity of symptoms over the past 7 days.

Factors for assessment in the CDAI include:

  • abdominal pain rating
  • number of liquid or loose stools in a week
  • general well-being
  • use of antidiarrheal medications
  • anemia
  • weight changes
  • abdominal mass
  • complications of Crohn’s
  • extraintestinal manifestations
  • fever

A doctor then calculates these factors to give a person with Crohn’s an overall CDAI score. From this, they may be able to determine the severity of the person’s condition:

  • Less than 150: This may mean a person is in symptomatic remission. They may have no symptoms at all.
  • Between 150 and 220: This may indicate mild to moderate Crohn’s disease.
  • Between 220 and 450: This may mean an individual has moderate to severe Crohn’s. They may have not responded to treatments for mild to moderate disease, or they may have more significant symptoms such as fever, abdominal pain, weight loss, and nausea or vomiting.
  • Over 450: This may indicate severe Crohn’s disease. A person may have persistent symptoms even with treatment with biologics or corticosteroids and may have significant symptoms that affect their quality of life.

A person with severe Crohn’s may experience various symptoms including:

  • persistent diarrhea
  • rectal bleeding
  • urgency with bowel movements
  • high fever
  • persistent vomiting
  • intestinal obstruction
  • loss of muscle mass
  • abscess

Those with severe Crohn’s may also be at risk for serious complications, such as:

  • fissures — a tear in the lining of the anus
  • fistula — an irregular channel that can appear between areas of the intestine or between the intestine and the skin, bladder, or vagina
  • strictures — a narrowing of a part of the intestine that forms due to scar tissue on its wall

Crohn’s disease can progress from mild to severe stricturing or penetrating disease — the formation of strictures or fistulas. More than 50% of people with Crohn’s develop stricturing or penetrating disease.

Scientists believe several factors contribute to progressing to the severe form of the condition. It is important for a person living with Crohn’s to have an understanding of these.

Such an understanding can also inform doctors on how to make the best treatment decisions. An optimum treatment decision may help improve symptoms or stop the condition from progressing to severe stages.

Factors that may contribute to the development of severe Crohn’s disease include:

  • smoking
  • elevated CRP levels
  • location of Crohn’s disease at the time of diagnosis
  • being below 40 years at diagnosis
  • presence of perianal disease
  • lack of response to treatment


Cigarette smoking is one of the most widely studied risk factors associated with Crohn’s disease. It is not only linked with the onset of Crohn’s disease but also with severe disease and less favorable outcomes.

Cigarette smoking changes the composition of the gut microbiome and also modifies the immune system response.

Researchers state that exposure to chronic smoke can change the integrity of the lining of the gut. This can make people susceptible to inflammation in the intestine.

People with Crohn’s disease who are current smokers have a worse outlook for their condition than those who do not smoke. They have an increased need for surgeries related to their condition, as well as an increased need for immunosuppressants and corticosteroids.

Research has found that people with Crohn’s disease who smoke had frequent relapses even if they were on immunosuppressant medication.

C-reactive protein (CRP)

CRP is a marker of inflammation. Research from 2019 suggests that this biomarker may be one of the strongest predictors of moderate to severe disease activity in Crohn’s.

There is a significant association between CRP and CDAI scores.

An increased level of CRP has a link with a relapse of Crohn’s.

Research also suggests a correlation between CRP and endoscopic and histological Crohn’s activity. This refers to Crohn’s disease being found via endoscopy and microscopically in the study of tissues.

Researchers have also found CRP levels correlate with severe inflammation on a biopsy.

CRP levels are higher in those with severe Crohn’s than in people with mild Crohn’s disease.

Location of Crohn’s disease

Crohn’s disease can appear anywhere in the gastrointestinal tract between the mouth and the anus.

The location of Crohn’s disease at diagnosis may also contribute to the severity of the condition.

A 2021 review suggests that Crohn’s disease located in the ileum — the last part of the small intestine — is associated with severe disease.

Ileocolonic Crohn’s disease, which affects the end of the small intestine and the large intestine — the colon — is also associated with severe disease.

People with ileal or ileocolonic Crohn’s disease may be 5–7 times more likely to experience complications like strictures or fistulas compared with people with isolated colonic Crohn’s.

Involvement of the upper GI tract may also be associated with more severe Crohn’s disease.

Age at diagnosis

Most people receive a diagnosis of IBD after the age of 15 years. However, experts believe that when doctors diagnose IBD during childhood, the condition may take a more severe course and be more extensive than Crohn’s disease diagnosed in adulthood.

About 5% of all IBD cases in the United States are in children. Crohn’s disease occurs twice as much as ulcerative colitis in this group.

Research from 2022 suggests that being below 40 years at the time of diagnosis is associated with moderate to severe disease progression.

Perianal disease

Numerous studies have indicated that perianal disease is a factor in progression to severe Crohn’s disease.

Perianal disease is inflammation at or near the anus and may show up as skin tags, fissures, fistulae, abscesses, or stenosis. These may occur even before a Crohn’s disease diagnosis. The symptoms of perianal disease include pain, itching, bleeding, pus, and bowel incontinence.

The presence of perianal disease in Crohn’s disease diagnosis is associated with less favorable outcomes.

Perianal disease has significant associations with hospitalizations due to Crohn’s, surgery, colon cancer, and steroid use and has less favorable outcomes.

Several factors may contribute to the progression of severe Crohn’s disease.

Smoking, elevated CRP biomarkers, age at diagnosis, location of Crohn’s disease in the GI tract, and perianal disease are some factors that may influence disease severity.

It is important for people to understand these influencing factors and speak with a doctor to discuss treatment options. This can help improve symptoms or prevent the condition from worsening.