1. Diagnostics in EM; test thresholds and likelihood ratios Published courtesy of the CEM FOAMed Network http://www.cemfoamed.co.uk/portfolio/diagnostics- in-em/

2. Everything we do in a patient assessment is a test Including questions we ask

3. Test thresholds

4. Within the test thresholds; appropriate testing

5. Below a test threshold; more likely to harm than help a patient by testing

6. Above a test threshold; more likely to harm than help a patient by testing

7. Within a test threshold; appropriate testing

8. How do we use all of this information?? 100 % 0% Likelihood of diagnosis being correct Below test threshold Diagnosis considered

9. How do we use all of this information?? 0% Likelihood of diagnosis being correct Above test threshold Diagnosis considered 100 %

10. Above test threshold How do we use all of this information?? Below test threshold Appropriate testing Diagnosis considered 0% Likelihood of diagnosis being correct 100 %

11. Likelihood ratios

12. Positive Likelihood ratio; a high number is powerful >>1

13. Negative Likelihood ratio; a small number is powerful << 1

14. Likelihood ratioShift in probability >10Large (often conclusive) 5-10Moderate 2-5Small 1-2Minimal 1Nil 1-0.5Minimal 0.5-0.2Small 0.2-0.1Moderate <0.1Large (often conclusive)

15. Fagan’s nomogram

16. Application of serial tests 100 % 0% Finding 1 Finding 2 Finding 3 Likelihood of diagnosis being correct Above test threshold Below test threshold Diagnosis considered

17. Correct testing Correct test Correct circumstance Correct patient

18. Perry et al. Sensitivity of coputed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study. BMJ 2011; 343:d4277 doi. 10.1136/bmj.d4277

19. Han et al., The Role of Cardiac Risk Factor Burden in Diagnosing Acute Coronary Syndromes in the Emergency Department Setting. Ann Emerg Med. 2007; 49

20. Who not to test 100 % 0% Likelihood of diagnosis being correct Below test threshold Diagnosis considered

21. Who not to test but to treat 0% Likelihood of diagnosis being correct Above test threshold Diagnosis considered 100 %

22. Who should be tested Appropriate testing Diagnosis considered 0% Likelihood of diagnosis being correct 100 %

23. Correct testing Correct test Correct circumstance Correct patient