Survivor bias and risk assessment

In the current issue of the European Respiratory Journal , Lee et al. [1] have done excellent work developing and validating a risk equation for incident pulmonary arterial hypertension (PAH) patients in the UK. In addition, they have very helpfully summarised the differences and similarities between the newly developed Scottish composite score (SCS) and other contemporary PAH survival equations. One of the most important methodological choices was the decision to include only incident cases in the analysis, and this is an issue which deserves further discussion because of the implications for the design of registries and other observational studies in PAH. Prevalent cohorts are prone to inherent bias. Prevalent cohorts in PAH, and in fact for any disease, are subject to a survivor bias because patients who die soon after disease onset are less likely to be included in a prevalent cohort. For instance, consider two different subgroups, one with a mean survival of 4 yrs and another with a mean survival time of 8 yrs. If the incidence rate is constant over time, patients from the latter cohort, with the better prognosis, will be twice as likely as the former group to appear in a prevalent cohort. The latter group will therefore be overrepresented if one is interested in the distribution of an incident cohort. Survivor bias of this nature impacts the development of survival estimates in two distinct ways. First, an analysis cohort consisting of prevalent patients will have a disproportionately higher number of lower risk patients. In PAH prevalent cohorts, we see more females, fewer functional class (FC) IV patients and more patients with longer 6-minute walk distances (6MWDs). Because there are more of these patients, they receive more weight in the development of equations. Second, the future prognosis of this lower risk cohort …