Question:

Item 2 of 2

Based on the abstract, which of the following is true about the magnitude of the risk of gout among patients adopting a Western diet in the highest fifth as compared to patients adopting a Western diet in the lowest fifth?

A Western diet in the highest fifth does not seem to increase the risk for gout when compared to a Western diet in the lowest fifth.

A Western diet in the highest fifth is associated with a 17% reduction in the risk for gout when compared to a Western diet in the lowest fifth.

A Western diet in the highest fifth is associated with a 20% increase in the risk for gout when compared to a Western diet in the lowest fifth.

A Western diet in the lowest fifth is associated with an 80% increase in the risk for gout when compared to a Western diet in the highest fifth.

A Western diet in the lowest fifth is associated with an 83% reduction in the risk for gout when compared to a Western diet in the highest fifth.

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Explanation:

Relative risk (RR) can be calculated in observational or experimental follow-up studies as follows:

RR = Risk of disease in exposed group / risk of disease in unexposed group

The RR usually indicates how high or low the risk for disease is among the exposed compared to the unexposed group. However, the RR can also be extended to compare risk for disease at different levels of an intervention or risk factor. The comparison is interpreted depending on which group is used as the denominator (ie, reference group):

RR <1.0 indicates decreased risk in the group in the numerator
RR = 1.0 indicates no difference in risk between the groups
RR >1.0 indicates increased risk in the group in the numerator

In this case, the numerator is the group of patients adopting a Western diet in the highest fifth, whereas the denominator (ie, reference group) is the group of patients adopting a Western diet in the lowest fifth. Therefore:

RR = (348 / 192,778) / (287 / 190,572) = 0.0018 / 0.0015 = ~1.2

RR = 1.2 (= 120%) indicates that adopting a Western diet in the highest fifth increases the risk of gout by 20% (= 1.2 – 1 = 120% – 100%) when compared to adopting a Western diet in the lowest fifth (Choices A and B).

(Choices D and E) Assuming the reference group (ie, denominator) is patients adopting a Western diet in the highest fifth, the RR can be calculated as follows:

RR = (287 / 190,572) / (348 / 192,778) = 0.0015 / 0.0018 = ~0.83

RR = 0.83 indicates that the risk of gout among patients adopting a Western diet in the lowest fifth is 0.83 times (ie, 83%) the risk of gout among patients adopting Western diet in the highest fifth. An alternate interpretation is that adopting a Western diet in the lowest fifth is associated with a 17% (= 1 − 0.83 = 0.17) decrease in risk for gout when compared to adopting a Western diet in the highest fifth.

Educational objective:
Relative risk is a measure of the strength of association between exposure and disease in follow-up studies. Typically, it is the risk of the exposed group divided by the risk of the unexposed group. However, the exposure status may be redefined to designate different levels of an intervention or risk factor.