A study was conducted to evaluate the risk for blindness in a patient with diabetes over a 5-year period. 4 people became blind in Group A which consist of 100 people. 3 people became blind in group B which consisted of 100 people. All participants in group B were given a trial of a new drug whereas group A did not receive any treatment. Given the data above, which statement is correct?
A. Absolute risk is 1
B. Relative risk is 3
C. Relative risk is 0.75
D. Absolute risk is 0.75
E. Relative risk is 1
Risk ratios are an important topic in PLAB as it is one of the main epidemiologic questions asked.
Absolute risk (AR) of a disease is your risk of developing the disease over a time period. The same absolute risk can be expressed in different ways. For example, say you have a 1 in 10 risk of developing a certain disease in your life. This can also be said to be a 10% risk, or a 0.1 risk - depending on whether you use percentages or decimals.
Relative risk (RR) is used to compare the risk in two different groups of people. For example, consider the risk for blindness in a patient with diabetes over a 5-year period. If the risk for blindness is 4 in 100 (4%) in a group of patients treated conventionally (control group) and 3 in 100 (3%) in patients treated with a new drug (experimental group), the relative risk is the ratio of the two risks. Given the data above, the relative risk is:
3% ÷ 4% = 75% (or 0.75)
Expressed as a relative difference, the new drug reduces the risk for blindness by a quarter.
An RR of < 1 means the event is less likely to occur in the experimental group than in the control group.
An RR of > 1 means the event is more likely to occur in the experimental group than in the control group.
A good formula to remember for PLAB part 1 is:
RR = risk of disease in exposed / risk of disease in unexposed
In this scenario, it would be:
RR = risk of blindness in diabetics taking new drug / risk of blindness in diabetics with no drug