Communicating CV risk to patients
Helping patients fully understand the risks and benefits of treatment can be a challenge. GP Dr Christopher Cates explains how a piece of software he’s developed can help when starting therapy for increased CV risk
Helping patients fully understand the risks and benefits of treatment can be a challenge. GP Dr Christopher Cates explains how a piece of software he's developed can help when starting therapy for increased CV risk
So what does absolute risk mean? In order to make sense of this it is important to distinguish between relative and absolute measures of risk.
Relative measures of risk
Reports from controlled clinical trials and systematic reviews are usually presented in terms of the relative effect of treatment. This is a measure of the ratio of the risk with treatment in comparison to a group of untreated patients.
In the case of statins, the cardiovascular risk of patients who take statins is about three-quarters of the risk in untreated patients. This could also be expressed as a risk ratio of 0.75 or a relative risk of 75%.
The same relative risk could also be described as a relative risk reduction of 25%, to indicate that the statins reduce the risk by one quarter.
So already there are four different ways of describing exactly the same ratio – so if you are confused what chance do patients have?
Why are relative measures of risk reported in papers?
The main advantage of relative risks – or risk ratios, which are the same thing – is that they tend to be stable for both high risk and low-risk patients.
Overall, the reduction in risk is one quarter with statins, however high or low the patient's risk is in the first place.
This is handy when reporting trial data, but not too helpful for the patient, unless they know what their own level of risk is.
Happily, many GP computing systems will calculate patients' risks of CVD by using data entered into the system on age, sex, smoking status, blood pressure, lipids and the presence or absence of diabetes.
The result is presented as a baseline risk of developing CVD over the next 10 years, and is stated as a percentage.
So if the baseline risk is 20% and 100 identical patients were followed over the next 10 years, we would expect 20 of them to develop CVD and 80 not to. Of course the individual patient does not know for sure which group they will be in. This is rather like knowing the risk of being called for jury service in the next year, but not whether you will be called until it happens.
What are absolute risks?
When we try to apply the results of clinical research to individual patients we need to combine the information from the trials (risk ratio) and the baseline risk of the patients. So in this case we have a risk ratio of 0.75 and this must be multiplied by the patient's baseline risk (which came out as 20% or 0.2).
This calculation will indicate how much the patient risk will fall if they are treated with statins. Taking a statin will reduce the initial 20% risk to three-quarters of that level, or 15%.
In absolute terms this represents a reduction of 5% rather than the 25% reduction in relative risk. What is more, this absolute reduction will be twice as large for a patient who starts with a 40% risk and reduces this to 30% on statins, leading to a 10% absolute risk reduction.
In order to try to make this somewhat complicated set of calculations simpler, we have created a freely accessible computer program to show pictures of these risks.
To generate the pictures you need to enter the baseline risk of CVD for the individual patient from their 10-year risk (as a percentage).
The relative risk needs to be entered as 0.75 (use the drop-down menu next to odds ratio), and if you click on the Calculate button a picture of the baseline risk appears. This shows 20 red faces who will suffer CVD over the next 10 years and 80 green faces who will not.
Clicking Next will generate a second picture (see right) that shows five of the red faces have turned yellow, indicating that 5% of the treated patients will be saved from CVD when all 100 are given statins.
The number needed to treat (NNT) is also shown. In this case for every 20 patients given a statin for 10 years, one will be saved from a CVD event, so the NNT is 20.
You can use the program to generate calculations for a baseline risk of 40%. In this case, there are 10 yellow faces for every 100 treated with statins, giving an absolute risk reduction of 10% and an NNT of 10.
The software is free and can either be downloaded onto your PC or used online. I recommend trying it and seeing if it helps in your next relevant consultation.
Dr Christopher Cates is a GP in Hertfordshire and is currently working for the Cochrane Airways Group
Competing interests: None declaredNICE Cates plot for 20% CVD risk Cates plot for 20% CVD risk