From the Guidelines
A relative risk of 1.50 means that the risk of an outcome in one group is 50% higher than the risk in another group. This is a measure used in epidemiology and medical research to compare the likelihood of an event occurring between two groups, as explained in the 2018 expert consensus document on optimal use of ionizing radiation in cardiovascular imaging 1. For instance, if the lifetime rate of developing leukemia in an unexposed population is 500 cases per 100,000 (0.5%) and the rate in an exposed population is 1,250 cases per 100,000 (1.25%), the excess relative risk for the exposed population is (1,250/500) - 1 = 1.5, indicating a 50% increased risk. Some key points to consider when interpreting relative risk include:
- The baseline risk: a 50% increase of a very small risk might still represent a small absolute risk increase
- The absolute risk impact: if a disease has a large background incidence rate and a small excess relative risk for exposure, the absolute risk impact of exposure may be large in terms of the number of cases attributable to the exposure
- The excess absolute risk: which is the rate of disease in an exposed population minus the rate of disease in an unexposed population, and can be expressed as the number of exposures needed to harm one individual. Relative risk is calculated by dividing the incidence rate in the exposed group by the incidence rate in the unexposed group, providing a more intuitive understanding of increased risk, making it valuable for clinical decision-making and patient communication about potential benefits or harms of interventions or exposures, as noted in the context of cardiovascular imaging 1.
From the Research
Definition of Relative Risk
- A relative risk of 1.50 means that the risk of an event occurring in the exposed group is 1.5 times higher than the risk of the event occurring in the non-exposed group 2.
- In other words, if the risk of an event is 10% in the non-exposed group, a relative risk of 1.50 would mean that the risk of the event is 15% in the exposed group.
Interpretation of Relative Risk
- Relative risk is a measure of the strength of association between a risk factor and an outcome 2.
- A relative risk of 1.50 indicates a moderate increase in risk, but the absolute risk may still be low if the baseline risk is low 3.
- It is essential to consider both relative and absolute risk measures when interpreting the results of a study, as they provide different information about the strength and importance of the association 2, 3.
Limitations of Relative Risk
- Relative risk can be misleading if not interpreted in the context of the baseline risk 4, 3.
- A high relative risk may not necessarily translate to a large absolute risk reduction if the baseline risk is low 3.
- Therefore, it is recommended to report both relative and absolute risk measures, along with their confidence intervals, to provide a more complete understanding of the results 2, 3.