A prospective study of 500 patients reports a hazard ratio (HR) of 1.85 (95% CI: 1.10–3.12, p=0.02) for developing cardiovascular disease in patients with metabolic syndrome vs those without. Which interpretation of this hazard ratio is MOST accurate in a Cox proportional hazards model?

• A 85% of CVD cases are caused by metabolic syndrome
• B Patients with metabolic syndrome live 85% shorter than those without
• C The probability of CVD is 1.85 times higher in the metabolic syndrome group at the end of follow-up
• D Patients with metabolic syndrome have an 85% higher instantaneous risk (hazard) of developing CVD at any given point in time, compared to those without metabolic syndrome ✓

Explanation

The hazard ratio from Cox proportional hazards regression represents the ratio of the instantaneous rate (hazard) of the event in the exposed group relative to the unexposed group, at any given point in time (under the proportional hazards assumption). An HR of 1.85 means the hazard of developing CVD at any point during follow-up is 85% higher in patients with metabolic syndrome compared to those without. It is NOT a relative risk at end of study, NOT a proportional mortality, and NOT a survival time ratio. The HR approximates the relative risk when events are rare (incidence <10%) but is interpretable as an instantaneous risk ratio in event-time analysis.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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