Volume 182 - Issue 11
Genetic risk estimation by health care professionals
Authors: Benno Bonke, Aad Tibben, Dick Lindhout, Angus J Clarke and Theo Stijnen
Med J Aust 2005; 182 (11): 596-597. || doi: 10.5694/j.1326-5377.2005.tb06829.x
Published online: 6 June 2005
Published online: 6 June 2005
In reply: Geneticists and counsellors must be able to calculate risks according to professional standards, regardless of whether modified risks lead to decision changes. Does training in genetic risk calculation help? Only 21% of our respondents who had had such training recently (< 3 years ago) estimated all target risks correctly. In response to Kirk et al, calculating conditional risks need not be time-consuming in scenarios similar to our target pedigrees,1 and is often helpful when at-risk (grand)parents do not wish to be tested but their offspring do.
Given n children at 25% prior risk tested negative and no other (grand)children tested, the conditional risk for at-risk individuals in generation g (with g = 0 at 50% prior risk, g = 1 at 25% prior risk, etc) is 1/[2g(2n+1)]. Thus, in target #4 (n = 1), the father’s risk (g = 0) equals 1/[20(21+1)] = 0.33. In target #9 (n = 2), the unborn’s risk (g = 2) equals 1/[22(22+1)] = 0.05.
Similar formulas for more complicated scenarios are available upon request. In calculating risks, however, care must be taken that the pedigrees and target individuals are comparable to our scenarios. In target #7, for instance, the risk for the untested aunt does not increase simply because of the decreased risk for her brother (gambler’s fallacy).2