Healthy Volunteer Effect and Cardiovascular Risk

Abstract

To the Editor: Most cardiovascular risk prediction functions are developed using data from cohort studies. Invariably, a proportion of invitees will not participate in such studies because relatively good health status is required for a person to agree to undergo the examinations. This implies that persons enrolled in a study requiring active participation are healthier than those who declined to participate. It is thus unclear whether the cardiovascular risk distributions among study participants adequately reflect the risk distribution of the source population. We aimed to quantify the consequences of this “healthy volunteer effect.” Within the Rotterdam Study,1 a prospective population-based cohort, we investigated the association between participation in the third examination (1997–1999), all-cause mortality,2 and coronary risk (Framingham point score; assessed at enrollment 1990–1993)3 (see eAppendix, https://links.lww.com/EDE/A779 for details). Of 5423 eligible invitees (mean age 73.5 years; 39% men), 87% participated, of whom 76% visited the research center (eTable 1, https://links.lww.com/EDE/A779). Nonparticipants had lost interest (50%), had physical complaints (34%), or considered themselves too old to participate (12%; mean age 86.9 years). Persons who were elderly, women, less educated, and with higher levels of specific cardiovascular risk factors were less likely to participate (eTable 1). Nonparticipation was strongly associated with mortality (hazard ratio [HR] = 1.71 [95% confidence interval (CI) = 1.56–1.88]). This was most pronounced shortly after invitation (0–3 months, HR = 4.85 [2.43–9.71]), with a diminishing healthy volunteer effect during follow-up (test for trend, P< 0.001) (Table). Every percentage-point increase in coronary risk yielded an approximately 3% lower probability of participating (eTable 2, https://links.lww.com/EDE/A779). Those categorized as “high risk” were least likely to participate (odds ratio = 0.56 [95% CI = 0.45–0.71]; eTable 2). There was a slightly lower proportion of high-risk persons among the examined participants compared with all invitees (23% vs. 24%) (eTable 3, https://links.lww.com/EDE/A779).TABLE: Risk of Mortality Associated with Nonparticipation in the Third Examination of the Rotterdam Study, by Time Since Invitation (1997–1v999)More than 5 decades ago, investigators from the Framingham Study observed higher mortality rates in those who refused to participate.4 They speculated that mortality in participants and nonparticipants might converge later during follow-up. We indeed noticed declining differences in mortality rates at long-term follow-up. The large difference shortly after invitation is presumably attributable to clinical or subclinical disease that makes invitees less likely to volunteer.5 Residual differences at long-term follow-up could reflect a health-care-aversive attitude or a lower awareness of health in general.6 However, long-term benefits of participation in population-based research cannot be ruled out because most studies (including the Rotterdam Study) disclose results from measurements (eg, blood pressure and cholesterol levels) and incidental findings (eg, aneurysms and indolent cancers) to the participants.7 Our focus on distributions of absolute cardiovascular risk is new information. This is important in light of the recent shift in cardiovascular epidemiologic research toward evaluating new risk markers on the basis of their clinical utility in risk prediction (ie, whether the predicted risks with addition of a new marker change sufficiently to alter recommended therapy) rather than the marker’s association with cardiovascular disease expressed as an odds ratio or HR. In contrast to the study of associations, the risk distribution in the study population affects the magnitude of measures of risk reclassification, such as net reclassification improvement.8 Given that most people have a low predicted cardiovascular risk (eTable 3), underrepresentation of persons at higher cardiovascular risk will usually mean that smaller proportions of persons are reclassified, resulting in lower estimates of net reclassification improvement8; therefore, the contribution of emerging risk markers could be underestimated. We observed a small underrepresentation of persons at high cardiovascular risk (1.2% less at high risk) (eTable 3); thus, results from analyses on absolute cardiovascular risk are not likely to be severely affected. However, future simulation studies could quantify the degree of bias on measures of risk reclassification. Maarten J. G. Leening Jan Heeringa Department of Epidemiology Erasmus MC–University Medical Center Rotterdam Rotterdam, The Netherlands Jaap W. Deckers Department of Cardiology Erasmus MC–University Medical Center Rotterdam Rotterdam, The Netherlands Oscar H. Franco Albert Hofman Jacqueline C. M. Witteman Bruno H. Ch. Stricker Department of Epidemiology Erasmus MC–University Medical Center Rotterdam Rot