Vanderbilt Sports Medicine How to practice and teach EBM Chapter 3 May 3, 2006

Diagnosis and Screening 3 questions to consider about tests –Is the evidence about accuracy of a test valid –Does evidence show test can distinguish patients who do/don’t have disorder –How can I apply test to a specific patient If the evidence is valid, is it important –Ie: clinically worthwhile

Validity of evidence Measurement (most important) –Was there an independent, blind comparison with a reference gold standard Representative –Appropriate spectrum of patients Ascertainment –Was reference standard ascertained regardless of diagnostic test result

Importance Does evidence demonstrate ability of test to distinguish patients w and w/o disorder Sensitivity –pts with disorder who have positive test Specificity –Pts without disorder who have negative test

Importance Likelihood ratio - LR(+) Prob of positive test in presence of disorder Prob of positive test in absence of disorder –LR+ = sensitivity/(1-specificity) –Positive test is more likely in pt w disorder LR(-) = (1-sensitivity)/specificity –Negative test more likely in absence of disord

Importance SnNout –Test with high sensitivity (Sn) –Negative result (N) –Rules OUT diagnosis SpPin –Test with high specificity (Sp) –Positive result (P) –Rules IN diagnosis

Applying test to a patient Is test available, affordable, accurate, and precise in our setting –Some tests have higher LR in florid disease and lower LR in mild –As patients are referred to specialists, some tests loose power because patients displaying symptoms will include more false-positives

Applying test to a patient Can we generate an estimate of pre-test probability –From experience, prevalence studies, practice databases, pre-test probability studies Does post-test probability affect mgmt and help patient –Does result cross threshold that causes us to stop testing (-) test that r/o likelihood of diagnosis (+) test that r/in diagnosis –If not, perform additional tests to pursue diagnosis

Test treatment thresholds May not cross threshold until several tests are performed If tests are independent, can chain LR together for each test to increase overall likelihood ratio –Ex: LR test 1 (0.06) x LR test 2 (13) x LR test 3 (11) = 9.13 –Cvt LR to prob: 9.13/10.13 = 90% prob More than any single test alone

Multilevel LR Expressing LR for two levels (+/-) is helpful, but giving LR for multiple levels allows more distinction –Ex: 5 levels from extremely negative – mod negative – neutral – mod pos – extr pos –Extremely positive more definitive than if only had two levels (pos/neg), so result is more useful

Multiple Tests Multiple tests grouped together –Considered a “cluster” –Combination of results helps make diagnosis –When validated in second independent group “Clinical prediction guideline”

Critically Appraised Topic (CAT) Standardized, one page summary of evidence on one topic –Shows bottom line result –Show scenario and clinical question –Show studies and gold standard –Presents evidence and results –References

Screening/Case Finding Screening –Making early diagnosis of pre- symptomatic disease among well general public Case Finding –Making early diagnosis of pre- symptomatic disease in patients who presented for an unrelated disorder

Screening/Case Finding Implicitly recruit pts with promise that they will live longer or better if they are tested Requires that evidence shows pts are better off in long run with early diagnosis –Labels all pts (may be harmful) –False + test is only harmful (no benefit) –Test must not only be accurate, but treatment must be efficacious

Screening/Case Finding Does RCT evidence show early diagnosis leads to improved survival/quality of life –RCT type 1 – screen vs no screen –RCT type 2 – (+) screen test – treat vs no treat Will early diag pts cooper with treatment –If not, no benefit How do benefits/harms compare Does frequency/severity of disorder warrant the effort

Screening/Case Finding Bias –Early detection always appears to improve survival (may in fact not) - “pt sicker longer” –Pts who volunteer for testing are more cooper with health advice Have better outcomes anyway –Early tests ID pts with slower/more benign dz –Is follow-up adequate Early detection is not always a good thing!