Diagnosis--what is the value?

In an earlier post, I wrote about the epistemology (or perhaps ontology---we never really did settle it) of disease. Defining what is disease is sometimes obvious, sometimes not. If you have HIV, you have HIV---a test is positive or negative, treatments are known. If you have high blood pressure, it's a little trickier. How do we know that 140 mmHg is "hypertension" and that 139 is not? Does it even matter? An essay in this week's Annals of Internal Medicine says "yes".

As stated above, some diseases/diagnoses are binary---you have chlamydia or you don't. Some are continuous---everybody has "blood pressure", but whose is "high", and what does it mean?

In his Annals article, Vickers argues that for "continuous" (vs. binary) diagnoses, we should be using a "risk assessment" approach to treatment, rather than the traditional model of diagnosis. This would involve using specific risk-assessment tools to evaluate each patient's risk of a particular outcome (for example, tools such as this one). Is this very different from what we already do? Yes. And no.

One of the main references we use for the diagnosis and treatment hypertension is the report of the Joint National Committee on the Prevention, Detection, Evaluation and Treatment of Hypertension (JNC). The committee uses evidence-based guidelines, evaluating the medical literature to make rational recommendations. When we say that hypertension is two sitting readings of systolic blood pressure above 140 mmHg, what we are really saying is that, according to the literature, people with an SBP > 140 have a significantly increased risk of cardiovascular events such as stroke and heart attack, and that lowering this number reduces risk. So, we actually do use risk assessment to diagnose and treat high blood pressure. Except that we don't.

The JNC recommendations have the advantage of using large studies to come up with risk assessments, but these numbers say less about an individual patient. Using tools such as the CV Risk Calculator linked to above, you can get a decent estimate of your individual risk of heart attack. For example, if I were a smoker, my 10-year risk of a severe cardiac event would be 17%. That's really high. Since I'm not a smoker, my risk is actually around 4%. Lowering my cholesterol and blood pressure just a bit reduces my risk to about 1%...rather acceptable. This type of tool is very useful for assessing individual risk when a patient is sitting in front of you.

Back to the question: is this very different from what we do now?

When we follow JNC recommendations, we are doing very much the same thing as using the risk calculator. We are using data from the same studies to develop a treatment plan, it's just that one source is a summary of what we know about people in the aggregate, and one more concretely uses an individual patient's data.

In practice, I often use both tools. For example, we know from the evidence that diabetics, in order to avoid heart attacks and strokes, need very tight blood pressure control. There is really no need to use a risk calculator as the risk of any elevation in blood pressure is unacceptably high. If I am looking at, for instance, a 40 year old male with few risk factors other than mild hypertension, it is useful to "plug in the numbers" to see what the individual risk is, and what advantage there may be to treating him medically. This gives the patient something to chew on, a basis to make wise decisions.

Physicians use risk-assessment every day to make treatment decisions, albeit not always explicitly. It seems reasonable to make this more explicit to aid in making treatment decisions, especially in situations where the value of risk-reduction may be minimal.