Illustrative lines are drawn using data from the UTI test in . Line A represents a positive test result, drawn from a pre-test probability of 0.3 through an LR + of 4.73 to a post-test value of slightly 0.7, similar to the calculated probability of 0.67. Line B represents a negative test result drawn from pre-test probability of 0.3 through an LR - of 0.34 to a post-test value slightly > 0.1, similar to the calculated probability of 13%. LR + = likelihood ratio for a positive result; LR - = LR for a negative result. Adapted from Fagan TJ. Letter: Nomogram for Bayes theorem. New England Journal of Medicine 293:257, 1975.