Evidence: Foundations of Clinical Medicine CMS100 PDF

Evidence Foundations of Clinical Medicine CMS100 Objectives 1. 2. 3. 4. Understand the concept of evidence from a Bayesian perspective Understand the components of likelihood ratios Evaluate the utility of likelihood ratios based on their numeric value Understand the relationship between pretest pro...

Evidence Foundations of Clinical Medicine CMS100 Objectives 1. 2. 3. 4. Understand the concept of evidence from a Bayesian perspective Understand the components of likelihood ratios Evaluate the utility of likelihood ratios based on their numeric value Understand the relationship between pretest probability and evidence 5. Avoid common errors in applying evidence 6. Locate diagnostic evidence in the literature 7. Determine likelihood ratios when they are absent in the literature Diagnostic Intuition “[A] physician in the early twentieth century … often had intuitions about patients who were about to develop typhoid. Unfortunately, he tested his hunch by palpating the patient's tongue, without washing his hands between patients. When patient after patient became ill, the physician developed a sense of clinical infallibility. His predictions were accurate – but not because he was exercising professional intuition!” - Thinking, Fast and Slow What Is Evidence? Is a pothole evidence? What Is Missing? An elderly Canadian man, a retired postal worker, recounts an episode in his youth when someone gave him an amulet which they claimed would protect him from tiger attacks. He remarks that, to this day, he has never been attacked by a tiger and that the amulet must therefore have worked. Likelihood Ratios Probability that you’d see some evidence if your hypothesis were true Probability that you’d see the exact same evidence if your hypothesis were false Likelihood Ratios A number representing the diagnostic usefulness of a test Range from 0 to infinity (can never be a negative number) LR = 1 is useless LR > 1 increases probability (the higher the better) LR < 1 decreases probability (the lower the better) “LR+” means the finding was present, not that it necessarily increases the probability “LR-” means that the finding was absent, not that it’s a negative number and not necessarily that it decreases the probability You can locate likelihood ratios (LRs) in the diagnostic research literature If you can’t find LRs, you can often calculate them (see later slide) Likelihood Ratios “The clinician who understands this evidence can then approach his [or her] own patients with the confidence and wisdom that would have developed had [s]he personally examined and learned from the thousands of patients reviewed in the studies of this book.” - Steven McGee Likelihood Ratios What test, if positive, most increases the probability of hyperthyroidism? What finding would most decrease the probability of hyperthyroidism? What test, if negative, most decreases the probability of hyperthyroidism? Finding LR+ LR- Pulse  90 bpm 4.4 0.2 Skin moist and warm 6.7 0.7 Enlarged thyroid 2.3 0.1 Eyelid retraction 31.5 0.7 Eyelid lag 17.6 0.8 Fine finger tremor 11.4 0.3 Wayne index 1) 1.6 0.59 3 3.3 Dimension (6mm) 2.3 0.17 2 2.6 Enlargement 11 0.18 1 1.5 0 0.07 Source Beware of Double-Counting Evidence Some findings are closely correlated or even included in other findings E.g. increased patellar reflex and increased ankle jerk reflex E.g. 4/5 findings on “ABCDE” score for melanoma and irregular borders for the lesion (the “B”) Avoid using more than one of the correlated findings (knowledge of physiology will help with this), otherwise your probability estimate will be too extreme When choosing between multiple options, pick the one that provides the best evidence Likelihood Ratios What tests should not both be included in calculations because they would represent doublecounting evidence? Finding LR+ LR- Pulse  90 bpm 4.4 0.2 Skin moist and warm 6.7 0.7 Enlarged thyroid 2.3 0.1 Eyelid retraction 31.5 0.7 Eyelid lag 17.6 0.8 Fine finger tremor 11.4 0.3 Wayne index