Figure 1

Nomogram for likelihood ratios. The pre-test and post-test probabilities and likelihood ratios of any diagnostic test, including a genetic test, can be visualized using a nomogram familiar to most physicians and medical students. The nomogram shown is derived from the Fagan nomogram [14], and modified from one generated using a web-based tool [28]. The left side of the figure indicates a hypothetical pre-test probability of disease of 27%. Three lines represent the three possible genotypes, from top to bottom: homozygous risk alleles with a likelihood ratio of 1.61, heterozygous alleles with a likelihood ratio of 1.26, and homozygous protective alleles with a likelihood ratio of 0.83. The right side of the figure indicates three possible post-test probabilities resulting from the three genotypes. Multiple such tests can be 'chained' together serially, if they describe independent risks and cover the same pre-test assumptions.