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Statins, NNTs and risk

 

Clinical bottom line

Using BNF definitions of cardiovascular risk and extrapolated 10-year efficacy, the NNT of statins is 6 and below for risks of 40% and above, and 11 falling to 6 at risks of 20-40%.


The evidence is that statins are effective in reducing heart attacks and stroke, both fatal and non-fatal.

This reduction occurred consistently in large randomised trials with event rates for the combined outcome of fatal or nonfatal heart attack or stroke over five years of 10 to 35%. This corresponds to a 10-year risk of 20 to 70%.

If we are able to assume a consistent degree of risk reduction at different levels of risk, then we can calculate what the number needed to treat should be at all levels of risk. The importance of this is to help determine which patients to treat.

Assumptions

In the calculations below, Bandolier made the following assumptions:

  1. That the effect of statins was the same at all levels of risk (that is, the relative risk, and the relative risk reduction would the same).
  2. That the five-year benefit found in statin trials could be extrapolated to double the effect for a 10-year benefit.
  3. That the levels of risk found in the trials were of the same nature as the levels of risk used in calculating cardiovascular and coronary heart disease risk in, for instance, the British National Formulary.

Results

The results are shown in Table 1 and Figure 1 using the relative risk of 0.78 for statins for the outcome of "all bad things", encompassing all death, and nonfatal heart attacks and strokes. Using BNF definitions of cardiovascular risk and extrapolated 10-year efficacy, the NNT of statins is 6 and below for risks of 40% and above, and 11 falling to 6 at risks of 20-40%.

Table 1: NNTs for preventing any death, or nonfatal heart attack or stroke at different levels of baseline cardiovascular disease risk

10-year risk of cardiovascular event (percent)
NNT using 5-year statin efficacy
NNT using 5-year statin efficacy extrapolated to 10 years

5

91

45

10

45

23

15

30

15

20

23

11

25

18

9

30

15

8

35

13

6

40

11

6

45

10

5

50

9

4

The BNF says that high risk individuals are defined as those whose 10-year CHD risk exceeds 15% (equivalent to a cardiovascular risk of 20% over the same period). At a minimum those with the highest risk (over 30% CHD risk, equivalent to 40% cardiovascular risk) should be targeted and treated now, and as resources allow others with risks above 15% should be progressively targeted.

Figure 1: NNTs for preventing any death, or nonfatal heart attack of stroke at different levels of baseline cardiovascular disease risk

The results in Table 2 use the relative risk of 0.75 for statins for the outcome of fatal and nonfatal heart attacks and strokes. Using BNF definitions of cardiovascular risk and extrapolated 10-year efficacy, the NNT of statins is 5 and below for risks of 40% and above, and 10 falling to 5 at risks of 20-40%.

Table 2: NNTs for fatal or nonfatal heart attacks or strokes at different levels of baseline cardiovascular disease risk

10-year risk of CHD event (percent)

NNT using 5-year statin efficacy

NNT using 5-year statin efficacy extrapolated to 10 years

5

77

38

10

38

19

15

26

13

20

19

10

25

15

8

30

13

6

35

11

5

40

10

5

45

9

5

50

8

4

BNF says that high risk individuals are defined as those whose 10-year CHD risk exceeds 15% (equivalent to a cardiovascular risk of 20% over the same period). At a minimu those with the highest risk ((over 30% CHD risk) should be targeted and treated now, and as resources allow others with risks above 15% should be progressively targeted.

Comment

These calculations show just how effective statins can be in high risk individuals, if taken correctly over long periods. The assumptions made in these calculations are probably justified. The difficult part is determining at what level of risk treatment should start.