Diagnosis

1. Diagnosis Appearances to the mind are of four kinds. Things either are what they appear to be; Or they neither are, nor appear to be; Or they are, and do not appear to be; Or they are not, yet appear to be. Rightly to aim in all these cases Is the wise man’s task. Epictetus 2nd Century AD

2. We spend a lot of time diagnosing but receive little training in interpretation of diagnostic tests

3. Data Nominaldata are categorical data where the order of the categories is arbitrary • nominal Ordinaldata are categorical data where there is a logical ordering to the categories • ordinal Intervaldata is continuous data • interval

4. Data • nominal Ordinaldata are often simplification of interval data Eg low, normal and raised IOP • ordinal • interval

5. Data Complex data are frequently reduced to a simple dichotomy Present/absent Abnormal/normal Diseased/well

6. Accuracy of a test result Four interpretations of a test result

7. Accuracy of a test result

8. Accuracy of a test result Should know all boxes for each test we use

9. Accuracy of a test result Requires a Gold Standard Often elusive, more costly, less feasable (expensive, dangerous tests)

10. Accuracy of a test result Lack of information on negative tests Clinicians do tests if something suspected but don’t do aggressive work-up if negative test

11. Accuracy of a test result For diseases that ordinarily become overt in a matter of a few years after they are first suspected, the results of the follow-up can serve as the Gold Standard

12. Accuracy of a test result Lack of objective standards for a disease Some diseases do not have hard and fast criteria for diagnosis

13. Accuracy of a test result Danger in circular reasoning • test validity by comparing with • careful clinical history and examination • test then used to validate clinical • diagnosis gained from history & exam!

14. Accuracy of a test result Consequence of imperfect standards • Imperfect test may be only one available • New test may be better but seem worse

15. Accuracy of a test result Consequence of imperfect standards • Imperfect test may be only one available • New test may be better but seem worse

16. Accuracy of a test result Consequence of imperfect standards • Imperfect test may be only one available • New test may be better but seem worse An example is oral cholecystography. Could only be confirmed by surgery. Along came ultrasound and identified more with stones. ?subject to surgery for negative old Gold Standard.

17. Sensitivity & specificity

18. Disease column Specificity Sensitivity

19. True positive Sensitivity Specificity Sensitivity Diseasepresent

20. Use a sensitive test when there is an important penalty for missing a disease …….also early work-up ‘rule out’ Most helpful for a negative result Rare Life/sight threatening

21. Specificity Sensitivity True negative Disease absent

22. Use a specific test when need to confirm a diagnosis ie false positive bad Most helpful for a positive result Physically Financially Emotionally