Adherence in healthcare is though of as the extent to which a patient follows the advice of their provider. In the past, this was referred to as compliance. The difference between adherence and compliance is as follows:
If I tell you to do something (eg: take a drug) and you do it, then you are compliant. In other words, you are obeying orders.
If you decide to do something (eg: lose weight) and you do that, then you are adherent. You are doing what you want to do.
So the subtle difference between the two is whether the patient participated in designing the treatment plan. In most cases, compliance is likely to be less than adherence.
A strict definition of adherence is that it is the ratio of treatment that is followed divided by the treatment that has been recommended. This can range from 0% if no treatment is followed, all the way up to 100% if all of the treatment is followed. Most of the time the adherence is somewhere between 0 and 100%.
There are 2 reasons why a patient might follow or take a treatment: to relieve symptoms and to prevent a bad outcome. An example of the first is Strep throat. Most people who have a sore throat and who are culture positive for Strep are given an antibiotic to take for 10 days. Yet most will take it until they feel better plus 1 or 2 days for good measure. They seek treatment to relieve a symptom and once they are better they are no longer motivated to take the treatment.
On the other hand, hypertension is usually an asymptomatic condition. If I have high blood pressure and my doctor recommends that I take medicine, I will do it only if I believe that doing so will reduce my risk of having a bad outcome. In the case of blood pressure, I might get a stroke in the next 10-20 years if I don’t treat it so I have that motivation. Clearly it is less strong than relief of acute symptoms, but it is a motivation none the less.
For some conditions such as asthma, the risks and benefits are less clear. Patient generally are very adherent to taking their quick reliever when they have symptoms. Afterall, breathing is not optional. The treatment works quickly and reliably so the benefit is immediately obvious. For controllers the story is different. While the benefit may be obvious when symptoms resolve while taking them, they might not return immediately when the treatment is stopped. Long-term we don’t have any evidence that regular use of controllers alter the natural history of the disease. In fact, a study in children who either took a controller or placebo failed to show any difference later after the treatment was stopped.
Bottom line: patients will take precisely the amount of treatment they want to control their disease to the extent that they want it controlled. I may want it to be controlled better but they will do what they want. That is the weakness in surveys demonstrating that asthma is not well controlled. The patients might be perfectly satisfied with that and prefer having less-well controlled asthma to taking more treatment. It is their choice, afterall. Our job is to advise and guide.
If you have other ideas about this I would love to hear them.