Ioannidis1 nicely addressed key challenges of “personal” genetic prediction for common diseases. Expectations are huge in this domain. I argue that some of these expectations may be favored by the term personal and that it would be better to use the term stratified.2- 4
Patient's characteristics such as age, sex, lifestyle, socioeconomic status, biomarkers, past environmental exposure, or genetic variants can help identify groups or strata of patients who are more (or less) likely to develop a disease or respond to a treatment.3 Such characteristics can improve our ability to estimate the probability of getting a common disease. Nevertheless, probability is a group property and should not be confounded with individual determinism. At the individual level, either you get or do not get the disease; there is no probability. Suppose there are 2 patients with exactly the same characteristics, including genetic makeup, and these characteristics are predictive of getting a disease. These 2 patients are in the same risk stratum, which is associated with a given—and sometimes quantifiable—likelihood of getting the disease. Still, 1 of these 2 patients could get the disease and not the other, and it is not possible to know a priori which one will be afflicted eventually.