A Simple, Unified Approach to Bayesian Risk Calculations

A simple, unified approach for calculating Bayesian risks is presented and illustrated with examples. Although new genetic tools have reduced the need for these risk calculations, situations still exist in which consultands need to know these kinds of risks (for example, when no direct test is available for a particular deleterious mutation, or when a consultand wants to know his/her risk before deciding whether to undergo a direct test). The “Unified Approach” presented here is straightforward and ensures calculating the correct risks. It can be applied to a wide variety of genetic counseling situations, including but not limited to: calculating recurrence or carrier risks for dominant and recessive diseases; incorporating false-positive and false-negative rates on genetic tests into risk calculations; and determining the probability that an isolated case in a family represents a new mutation. The method is based on basic principles of probability and likelihood theory but can be used without sophisticated knowledge of this theory. The method relies on two rules, the “Rule of All Configurations” and the “Rule of Fundamental Probabilities,” which are explained and illustrated. The Unified Approach does not represent new or original mathematics but should make it easier for users to calculate risks accurately. A warning is included that complex calculations should always be independently verified by another individual.