Instructions for the Nomogram
In both cases, remember that e = 0.2 and f = 0.6, and that the pretest probability is 55%.
For the positive test result, first draw a straight line from A to B between the appropriate points labeled "e and f for LR+" on the outside scale on the blue parts of ellipse. You should obtain an intersection point D at 2.2 on the blue LR+ horizontal scale in the middle. Going to the pretest probability scale on the lower part of the circle, we start at 55% (point C) and draw a line through D at LR+ = 2.2. Extending that line to the upper part of the circle, we find that the posttest probability at E is 73%.
For the negative test result, we draw a straight line from F to G between the appropriate points labeled "e and f for LR-" on the outside scale on the red parts of the ellipse. The intersection point is H at 0.27 on the red LR- horizontal scale in the middle. Starting with the same 55% on the pretest probability scale on the lower part of the circle, point C, we draw a line through H at LR- = 0.27. Extending that line to the upper part of the circle, we find that the posttest probability at J is about 25%.
The four lines described above are indicated on the diagram, see Figure 4.
If you would like to obtain a full-size rendering of the nomogram without the
lines, click here.
Figure 4: Post test probability predictor
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Appendix
