Types of Risk: Absolute Risk and Relative Risk
It is important to understand what "risk" means when considering your risk for developing diseases. Risk is a simple word that is used to explain how likely it is that something will happen. There are two kinds of risks that are used when talking about a person's risk for developing diseases: absolute risk and relative risk.
Absolute risk is an individual's risk of developing a given disease over a period of time. For example, if you have a 1 in 10 chance of developing diabetes in your lifetime, you are said to have a 10% absolute risk.
Relative risk is used to compare the risk between two groups of people; one group has a certain risk factor, and the other group does not. For example, you could compare the risk for heart disease of a group of people who smoke to a group of people that do not smoke. The risk given to each group would be called their relative risk, meaning their risk compared to another group's risk.
A relative risk of 1 means there is no difference in risk between the two groups. If a person's relative risk is greater than 1, it means their risk for developing that disease is higher than the other group. If a person's relative risk is lower than 1, they have a lower disease risk than the other group.
Additionally, an individual may have a high relative risk in comparison to a certain group, but a low risk of developing that disease overall. For example, if you compare heart disease risk of a person who occasionally exercises to a group of athletes, the relative risk of the occasional exerciser person could be high. Overall, this person is not high at risk for heart disease because they do exercise, but their relative risk is high based on the comparison group.
The CPMC presents risk in terms of relative risk. When possible, information about how common the disease is in the population is provided to help you interpret your risk.
Genetic Variant Relative Risk
Suppose you have one copy of a risk variant (rs1333049) for Coronary Artery Disease (CAD). Your relative risk for developing CAD is 1.3. This means you are 30% more likely to develop coronary artery disease than someone who does not have any copies of the risk variant.
Having a protective genetic variant can actually decrease the likelihood that you will develop a disease. The protective variant rs9272346 is associated with a lower risk of developing type 1 diabetes. If you compare the risk of a person that has two copies of this protective variant to a person that has no copies of the protective variant, the individual with two copies of the protective variant is 92% less likely (relative risk = 0.08) to develop type 1 diabetes.
Family History Relative Risk
For example, if you have one parent with rheumatoid arthritis, your relative risk of rheumatoid arthritis based on your family history is 3.0, or your risk is 3 times that of someone who does not have any parents or siblings with rheumatoid arthritis. If you do not have any parents or siblings that have rheumatoid arthritis, your family history relative risk is 1.0.
Lifestyle Relative Risk
For example, if you are a current smoker, your risk for developing age-related macular degeneration is 2 times that of someone who does not currently smoke. A non-smoker would have a relative risk of 1.0.
Relative Risk Estimates Cannot be Combined
It is not currently possible to accurately combine your genetic relative risk, family history relative risk, and environmental/lifestyle relative risks into one relative risk number. This is because it is unknown exactly how each risk factor influences your overall disease risk or how much these risk factors interact with each other. Therefore, the CPMC only provides you with a single relative risk number for each risk factor.