Although not all patients with Crohn’s disease will develop irreversible bowel damage leading to complications such as strictures and fistulas, there is a large number that will. Identifying those people at higher risk of developing these complications is not always easy, but there are certain risk factors that have been associated with disease that is more likely to progress over time.

In general, people who develop strictures or intestinal fistulas tend to have small bowel or upper GI tract inflammation as opposed to inflammation in the colon. The location of the disease is therefore an important factor but alone is not enough to predict complications. If however the Crohn’s is in the colon, having deep colonic ulcers is a predictor for severe disease, and these patients are more likely to require a colectomy.

Perianal fistulas are also considered a marker of severe disease, not only because they are themselves difficult to treat and often require surgical intervention, but because they may be predictive of the progression of small bowel disease.

The age at which a person is diagnosed is also an important factor. Crohn’s disease in children and adolescents tends to be more severe and is more likely to result in complications. A patient requiring steroids for treatment of a flare-up within the first 3 months of diagnosis is also a risk factor for severe disease.

Lastly, smoking is a well-known environmental risk factor for Crohn’s disease. Patients with Crohn’s disease who smoke are more likely to develop strictures and fistulas. Furthermore, these complications tend to develop faster in smokers compared to nonsmokers.

The more the above characteristics are present, the higher the likelihood it is to develop strictures and fistulas long term. Once these complications occur, it means that there has been irreversible damage to the bowel at which point surgery is often the only option. Because of this, it is important to identify people at higher risk early as they may benefit from aggressive medical treatment to prevent progression of the disease to strictures, fistulas, and eventually surgery.

Crohn’s disease is a chronic relapsing inflammatory condition that is progressive and characterized by the development of complications over time. Of patients presenting with uncomplicated Crohn’s disease, it has been estimated that between 40 and 60 % will develop either stricturing or penetrating complications over a 10-year period [1, 2]. These complications have a detrimental effect on quality of life and in many cases require surgical intervention [3, 4].

The ability to predict which patients are more likely to develop complications would potentially allow us to aggressively treat those patients at the highest risk with effective medicines early on in their disease to prevent disease progression.

There have been a number of studies aimed at predicting the patient characteristics that increase the risk of developing penetrating and stricturing disease over time.

Disease location, specifically ileal, ileocolonic, or upper GI involvement, has been shown in several studies to be significantly associated with disease progression [5–7]. It has been proposed that the reasons behind the differences in the rates of disease progression between small bowel and colonic disease are potentially related to differences in the diameter of the lumen and the intensity of inflammation leading to permanent bowel damage [8].

Other characteristics that have been shown to independently be associated with disabling disease include young age at diagnosis (<40 years) and the need to use steroids for the first flare [9, 10]. The definition of disabling disease in these studies however is much broader and includes not only the development of stricturing and penetrating complications but also multiple steroid courses, hospitalizations, and disabling symptoms. This likely explains the finding that these two characteristics, although likely associated with more severe disease, have not been consistently shown to increase the risk of disease progression when the definition of progression is limited to stricturing and penetrating disease behavior [11].

Perianal disease has been associated with a disabling disease course in multiple studies [9, 12], but whether it is an independent risk factor for intestinal fistulization is controversial. A population-based cohort showed a strong association between intestinal and perianal fistulization [13], and in another more recent study, perianal disease was of borderline significance in predicting progression of disease [5]. Other studies however have shown that perianal disease is not a risk factor for intestinal penetrating disease [6, 14].

Smoking has also been found in several studies to predict a change in disease behavior from inflammatory disease to stricturing or penetrating disease [8, 15], and it has also been shown to accelerate the rate of progression to complicated disease [16]. In addition, Eglington et al. [15] showed that smoking is a risk factor for disease progression independent of medical therapy and disease location.

When considering colonic disease specifically, a colectomy would be considered a severe complication. In this case, the presence of deep ulcers covering at least 10 % of a colonic segment has been associated with up to 60 % risk of colectomy over a 3-year period [17].

Serological markers have also been found to be predictive of complicated disease. Reactivity to ASCA (anti-Saccharomyces cerevisiae antibody), OmpC (E. coli outer membrane porin C), antiI2 (anti-CD-related bacterial sequence I2), and CBIr1 flagellin has been associated with early onset Crohn’s disease, fibrostenotic disease, penetrating disease, and the need for early small bowel surgery [18–20]. Furthermore, it appears that the more antigens that are present and the higher their titers, the higher the frequency of disease complications [21]. These serological markers, however, may increase with disease duration in parallel with the development of complications. This may limit the prognostic value of these antibodies.