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# Diagnostic Thinking

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### Diagnostic Thinking

1. 1. Diagnostic Thinking
2. 2. Steps <ul><li>Start with pre-test probability </li></ul><ul><li>Add Ruling-in power/Ruling-out power of the test </li></ul><ul><li>Get new, post-test probability </li></ul>
3. 3. 0.1 Normal V/Q PE 0.5 20 > 40 pack years COPD 0.2 40 Compression USG DVT in hospital patient LR - LR + Test Target Disorder
4. 4. Fagan Nomogram
5. 5. Likelihood ratios <ul><li>LR around 2 are minimally powerful </li></ul><ul><li>LR around 5 are moderately powerful </li></ul><ul><li>LR above 10 are highly powerful </li></ul><ul><li>LR- of 0.5 minimally powerful </li></ul><ul><li>LR- of 0.2 is moderately powerful </li></ul><ul><li>LR- below 0.1 are highly powerful </li></ul>
6. 6. Ruling-in Power <ul><li>If a minimally powerful test is positive, adds 15% to pre-test probability </li></ul><ul><li>If a moderately powerful test is positive, adds 30% to pre-test probability </li></ul><ul><li>If a highly powerful test is positive, adds 45% to pre-test probability </li></ul><ul><li>NOTE THAT ITS MULTIPLES OF 15 </li></ul>
7. 7. Ruling-out Power <ul><li>If a minimally powerful test is negative, subtracts 15% from pre-test probability </li></ul><ul><li>If a moderately powerful test is negative, substracts 30% from pre-test probability </li></ul><ul><li>If a highly powerful test is positive, subtracts 45% from pre-test probability </li></ul><ul><li>NOTE THAT ITS MULTIPLES OF 15 </li></ul>
8. 8. Tips on testing <ul><li>Ordering tests when pretest prob is high or low unlikely to lead to changed post test prob, often confuses picture, unexpected results usually are false pos/neg. </li></ul><ul><li>Tests are most useful when the pretest prob is 50% </li></ul><ul><li>Use tests in sequence so the posttest prob of the first test becomes the pretest prob for the next one: tests build on each other this way </li></ul>