Useful tests of usefulness of new risk factors: tools for assessing reclassification and discrimination.

New risk factors for various diseases are suggested at an increasing pace, with promise of clinical usefulness for risk prediction, but almost unvaryingly without formal testing of that property. We propose that a risk factor clinically relevant for risk prediction can be defined as one that correctly alters predicted risk to a clinically relevant extent in persons with a relevant absolute risk, such that it affects clinical decision making. We recommend that investigators who suggest a new risk factor for clinical use investigate if the new risk factor adds capacity to discriminate between persons who will subsequently experience the outcome from those who will not. For that purpose, we provide tools for calculating the net reclassification improvement, NRI, and the integrated discrimination improvement, IDI, using major statistical packages.