The resulting curves are shown in Figure 1. The pretest probability is shown on the abscissa, the post- test probability on the ordinate. As an example, suppose a physician estimated that a nonsmoker had a 50 percent likelihood of asthma on the basis of history, physical examination, and laboratory information. If a methacholine challenge test showed a PD20 of 49.5 CBU, the curve for nonsmokers at a pretest probability of 0.5 shows that the post-test likelihood of asthma is approximately 90 percent.
FIGURE 1. Curves for post-test probability of asthma constructed from the data of Casale et al. The numbers along the curves represent the cumulative breath units at which the test became positive. Solid lines represent positive tests, the single dashed line in each panel represents a negative test at 224 cumulative breath units.