Based on family history, people can be placed into one of three categories (described below) of relative risk of developing colorectal cancer. Age is an independent risk factor for colorectal cancer. These categories were developed by the Cancer Council Australia Colorectal Cancer Guidelines Working Party (2019).

General practitioners and primary care nurses should educate patients and encourage them to participate in the screening appropriate to the patient’s level of risk.

Category 1: Near average risk

Risk factors for category 1 patients include:

• no first- or second-degree relative with colorectal cancer
• one first-degree relative with colorectal cancer diagnosed before the age of 55
• one first-degree and one second-degree relative with colorectal cancer diagnosed at 55 years or older.

People who have one relative with colorectal cancer diagnosed at age 55 or older should be advised that their own risk of developing colorectal cancer could be up to twice the average risk but is still not high enough to justify colorectal cancer screening by colonoscopy.

Category 2: Moderately increased risk

People in this category should be advised that their risk of developing colorectal cancer is at least three times higher than average but could be up to six times higher than average, if they have any of the following:

• one first-degree relative with colorectal cancer diagnosed before the age of 55
• two first-degree relatives with colorectal cancer at any age
• one first-degree relative and at least two second-degree relatives diagnosed with colorectal cancer at any age.

Category 3: High risk

People in this category should be advised that their risk of colorectal cancer is at least seven times higher than average but could be up to 10 times higher than average, if any of the following apply:

• at least three first-degree relatives diagnosed with colorectal cancer at any age
• at least three first-degree or second-degree relatives with colorectal cancer, with at least one diagnosed before age 55 years
• they are a member of a family in which a gene mutation that confers a high risk of bowel cancer has been identified.