Many prospective studies report Odds Ratio as measure of association between exposure and outcome, even though Risk Ratio / Rate Ratio can be directly measured in these studies. We reviewed the relationship between Odds Ratio and Risk Ratio in prospective studies and problems associated with the use of Odds Ratio as measure of association in them. Odds approximates probability and Odds Ratio approximates Risk Ratio only when probability of outcome is small (<10%). If association between exposure and outcome is positive, Odds Ratio will be higher than Risk Ratio and if that association is negative, Odds Ratio will be lower than Risk Ratio. Difference between Odds Ratio and Risk Ratio will increase with increasing outcome probability. Odds Ratio reported from a prospective study can be misinterpreted as relative risk, when in fact it can be quite different from risk ratio or rate ratio. We should avoid using Odds Ratio as measure of association in cohort studies and RCTs as it tends to exaggerate the magnitude of association between exposure and outcome. Even while reporting results of prospective studies from multi variable analysis, authors should calculate and report adjusted Risk ratio / Rate Ratio rather than Odds Ratio.
