Prognostic and treatment-predictive factors-is there a difference?

The terms risk factor, prognostic factor and predictive factor are often imprecisely used. We defined a prognostic factor as a patient characteristic that identifies subgroups of untreated patients having different outcomes, and a factor predictive of treatment effect as a patient characteristic that identifies subgroups of treated patients having different (as a consequence of treatment) outcomes. To illustrate this, theoretical graphical examples were constructed and data from the literature on prostate cancer were used to substantiate the theoretical examples. In most situations, but not necessarily always, a prognostic factor is also pre- dictive of the effect of specific treatments. Whether a prognostic factor is also predictive of treatment effect or not can only be assessed in a valid comparative setting such as in a randomized trial. A factor that is predictive for treatment effect may not be predictive for another treatment. Prostate Cancer and Prostatic Diseases (2000) 3, 265-268