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Figure e2–6. Effect of pretest probability and test sensitivity and specificity on the posttest probability of disease. (See text for explanation.) (Reproduced, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.) Three depictions of effects of pretest probability on the line of disease probability.

Current Medical Diagnosis & Treatment 2025 > Use of Tests in Diagnosis & Management

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Figure e2–7. Nomogram for determining posttest probability from pretest probability and LRs. To figure the posttest probability, place a straightedge between the pretest probability and the LR for the particular test. The posttest probability will be where the straightedge crosses the posttest probability line. (Adapted from Fagan TJ. Nomogram for Bayes theorem. [Letter.] N Engl J Med. 1975;293:257.) A nomogram represents the pretest and posttest probability on the left and right of the likelihood ratio in the center.

Current Medical Diagnosis & Treatment 2025 > Likelihood Ratios & Odds-Probability Conversion in Medical Decision-Making

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Figure e2–10. Threshold approach applied to test ordering. If the contemplated test will not change patient management (as in scenario C), the test should not be ordered. (See text for explanation.) (Reproduced, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.) Three depictions of the effects of pretest probability on the band of disease probability with a treat or don't treat threshold.

Current Medical Diagnosis & Treatment 2025 > Likelihood Ratios & Odds-Probability Conversion in Medical Decision-Making

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Figure e2–11. Generic tree for a clinical decision where the choices are: (1) to treat the patient empirically, (2) to do the test and then treat only if the test is positive, or (3) to withhold therapy. The square node is called a decision node, and the circular nodes are called chance nodes. P, pretest probability of disease; Sens, sensitivity; Spec, specificity. (Reproduced, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.) A hierarchical tree represents treat and don't treat patients with or without tests.

Current Medical Diagnosis & Treatment 2025 > Likelihood Ratios & Odds-Probability Conversion in Medical Decision-Making

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