How to Analyze Therapy in the Medical Literature (part 2)

1. How to Analyze Therapy in the Medical Literature(part 2) Akbar Soltani. MD. Tehran University of Medical Sciences (TUMS) Shariati Hospital www.soltaniebm.com

2. Objectives • Review different concepts such as • Relative Risk , Absolute Risk Reduction, Relative Risk Reduction, Number Needed to Treat • Measures of precision of effects such as P value or confidence interval? • Principles of application of the results to patient care?

3. Three Step Guide in Using an Article to Assess Therapy • Are the results of the study valid? • What are the results?What measures of precision of effects were reported (CIs, p-values)? • How can I apply these results to patient care?

4. Measuring Risk: Relative Risk Relative Risk (RR) = rate in exposed = 0.04 = 0.67 rate in nonexposed 0.06 SHEP. JAMA. 1991;265:3255-3264

5. Communicating risk: 40% RRR=? ARR=? 20% 20% 10% 10% 5% placebo

6. Measuring Risk: Relative Risk 6% 4% Relative Risk (RR) = rate in exposed = 0.04 = 0.67 rate in nonexposed 0.06 SHEP. JAMA. 1991;265:3255-3264

7. Measuring Risk: ARR? 6% 2% 4%

8. Measuring Risk:Absolute Risk Reduction Absolute Risk Reduction (ARR)is the absolute difference in event rates between the experimental and control patients. Calculated by: ARR = CER - EER = 0.06 - 0.04 = 0.02 In its decimal form the ARR is not easy to use! Converted to a percentage - there is an absolute risk reduction of 2%

9. Measuring Risk: RRR? 6% 2% 4%

10. Measuring Risk:Relative Risk Reduction Relative Risk Reduction (RRR)is the proportional reduction in event rates between the experimental and control patients. Two ways to calculate: RRR = (1 - RR) = (1 - 0.67) = 0.33 OR RRR = CER - EER = 0.06 - 0.04 = 0.33 CER 0.06 Therefore, treatment reduced the stroke rate by 33% OR a RRR of 33% means that the new treatment reduced the risk of death by 33% relative to that occurring among control patients CER = control event rate EER = experimental event rate

11. Usefulness of the ARR:Number Needed to Treat Number Needed to Treat (NNT)is the number of patients a clinician needs to treat in order to prevent one additional adverse outcome. NNT is for dichotomous outcomes. Calculated by: NNT = 1/ARR = 1/0.02 = 50 Therefore, you would have to treat 50 hypertensive patients to prevent one stroke.

12. Number Needed to Treat CER=0.06=100----------6 EER=0.04=100----------4 SO: 100-----------2 X------------1 50-----------1 Calculated by: NNT = 1/ARR = 1/0.02 = 50 Therefore, you would have to treat 50 hypertensive patients to prevent one stroke.

13. Why not just use RRR?PTH trial CER = 6 % Age =70 + 7 EER = 3 % ARR = CER – EER = 3% NNT = 1/ARR = 1/ 0.03 = 33 CER = 1/1000 Age = 55 + 5 EER = 1/2000 ARR=1/1000-1/2000=1/2000 NNT = 1/ARR= 2000

14. Why not just use RRR? RRR remains the same despite differences in absolute rate of events.

15. Why not just use RRR? ARRs reflect underlying susceptibility of patients and provides more complete information.

16. Why not just use RRR? NNTs provide a useful measure of the clinical effort that must be expended to avoid bad events.

17. Randomised trial of cholesterol lowering in 4,444 patients with CHD: the Scandinavian Simvastatin Survival Study (4S) Lancet 1994: 344; 1383-1389 • 4,444 patients recruited as a sample • inclusion criteria • CHD and cholesterol 5.5 - 8 mmol/l • exclusion criteria • planned cardiac surgery, HF, child bearing potential • simvastatin Vs. placebo • double blind • Outcomes • mortality, major coronary events, admissions for acute CHD, incidence of revascularisation procedures

18. 4S Study: self evaluation • Median follow up 5.4 y • analyse by intention to treat • Significant reduction in all cause mortality • 11.5% placebo Vs. 8.2% simvastatin • ARR = • RRR = • NNT = patients with CHD and cholesterol 5.5 - 8 need to treated with simvastatin (20 mg) for 5.4 years to save one life

19. 4S Study Cont’d • Median follow up 5.4 y • analyse by intention to treat • Significant reduction in all cause mortality • 11.5% placebo Vs. 8.2% simvastatin • ARR = 11.5 - 8.2 = 3.3% • RRR = (11.5 - 8.2)/11.5 = 29% • NNT = 1/ARR = 30 • 30 patients with CHD and cholesterol 5.5 - 8 need to treated with simvastatin (20 mg) for 5.4 years to save one life

20. Why NNT is not enough • NNT for continuous outcome is difficult to calculate • Time can be added to calculations cautiously • We can not compare NNTs from different studies easily • Example: 2 RCT (Risedronate) 2 NNT! Can we say Risedronate is better than Risedronate!

21. Three Step Guide in Using an Article to Assess Therapy • Are the results of the study valid? • What are the results?What measures of precision of effects were reported (CIs, p-values)? • How can I apply these results to patient care?

22. P Value or CI?

23. P Value or CI?

24. P Value or CI?

25. P Value or CI?

26. P Value or CI?

27. P Value or CI?

28. Confidence Intervals for Small Numerators • Example: A new drug is given to 60people. It seems to work, and has no serious adverse effects. • The authors conclude it is "safe and effective." The upper limit for the 95% CI for any serious adverse effect is, or 5%. • 95% confident of adverse event = 0/n – 3/n

29. Another example from textbooks! • Based on one observational study in 94 women, no one report adverse reactions. • Rule of 3: 3/94=upper limit of adverse effects that may not have been seen.

30. Three Step Guide in Using an Article to Assess Therapy • Are the results of the study valid? • What are the results?What measures of precision of effects were reported (CIs, p-values)? • How can I apply these results to patient care?

31. How can I apply the results to patient care? • Were the study patients similar to my patient? • Were all clinically important outcomes considered? • Are the likely treatment benefits worth the potential harm and costs?

32. Thank you