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You will probably have heard a lot about the risk of something happening as 'a relative risk' or 'an absolute risk', particularly in the context of healthcare. These concepts can be very confusing, even for those individuals who work in the healthcare profession. Therefore, we asked
Dr Michael Schachter of Imperial College Medical School, London to help explain the basics behind these concepts.
What is the difference between relative and absolute risk?
The concept of risk is constantly in the news, and not just in medical contexts. However, it is not well understood by the public at large and even by the many professionals whose job it is to advise them. These ideas are vitally important not only for theoretical or research interest, but more importantly for their potential impact on patients, the apparently well and those who try to give them advice. One of the main areas of misunderstanding concerns the difference between relative and absolute risk, particularly in terms of increases in the level of risk that a particular disease imposes on an individual or decreases in the level of risk that may result from a change in lifestyle or a prescribed course of treatment.
Let's create a hypothetical headline as an example:
"Superstatin reduces coronary deaths by 40% in all patient groups."
At first glance, such a headline is highly impressive! It implies that whatever your cholesterol level, by reducing it with this 'superstatin' you will also reduce your risk of dying from a heart attack. In this example, the relative risk reduction is 40%. However, the key question that should be raised when considering the concept of risk is 40% of what?
Case studies
Case 1
Suppose a patient has a cholesterol level in his blood that is double the average for the population as a whole. As a consequence, he will have a higher than average risk of coronary heart disease. As a conservative estimate, we can say that over the next 10 years he has a one-in-three chance of having a heart attack, which may be fatal, or may require a procedure such as coronary bypass surgery. The one-in-three chance of this event (or 33%) is his absolute risk. If we reduce his risk by 40% then his chance of an event like this will be reduced from about one-in-three to one-in-five (20%), obviously a very important effect from his point of view.
Therefore, in Case 1, the absolute risk is reduced from 33% to 20% over 10 years.
Case 2
What of a man whose cholesterol level is half the average? He faces a much lower risk of a major problem such as a heart attack, and will have an absolute risk that will be as little as one-in-ten over the same 10-year period. However, this level of risk can still be reduced by 40% with the 'superstatin' intervention and will then only be about one-in-twenty.
Therefore, in Case 2, the absolute risk is reduced from 10% to 6% over 10 years.
Balancing benefits, risks and costs
In Case 2 above, the probability of a major event was already unlikely and has become even less so after treatment with the superstatin. Intervening here may still be worthwhile but there has to be a balance between the benefits, risk and costs of treatment. In Case 1, the benefit dominates, whilst in Case 2 the benefit is much less obvious. As such, in Case 1 the decision to intervene is clear and positive whilst in Case 2 the risk of side-effects and the cost of the drug to the NHS may be regarded by some as outweighing its advantages and therefore the decision about whether or not to intervene is less clear.
Relative risk vs absolute risk
Equally, of course, we can make a similar analysis of situations where risk is increased, or on the face of it appears to be. An example of this that you may have heard about in the news is the possible link between hormone replacement therapy (HRT) and breast cancer, with HRT use increasing the relative risk of breast cancer. However, as I have already pointed out, an alarming increase in relative risk may be deceptive in cases where the initial absolute risk is very low. This is why it is so important for us to understand the difference between relative and absolute risk and why you should always consult your doctor if you are concerned about your risk profile.
Conclusion
In summary, looking at relative risk in isolation can be very misleading and almost meaningless. As such, before we can properly interpret what we are told, we must always ask: what is the absolute risk and how has it changed? After all half of not very much is... not very much!
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