How changing definitions of disease affects us all

In 1967, the first trial showing the benefits of treating high blood pressure was published. This trial used a definition of high blood pressure that included only a tiny proportion of the population. By 2017, a new definition of high blood pressure in the US labelled 46% of adults as having the condition, increasing the prevalence overnight by 38%.

The trend to widen disease definitions is happening across all of medicine. Sometimes it happens because an expert body changes the definition, as happened with high blood pressure. Most new guidelines widen the definition of the disease they are considering. Another example is polycystic ovarian syndrome. The definition adopted by recent international guidelines increased the prevalence compared to a previous definition in Australian women aged 27 to 34 from about 9% to about 18%.

Sometimes the change happens because of the introduction of a new test that is able to detect milder cases of disease. The introduction of a new form of imaging to investigate suspected pulmonary embolism is thought to be the main factor behind the 45% increase in hospital admissions for this condition in Western Australia between 2002 and 2010, with some question over whether these newly detected cases benefit from treatment. Sometimes factors outside of medicine can be a driver as in the case of Autism. Helping children to access educational assistance seems to be one of the reasons for the increasing rates of autism diagnosis.

We often hear claims of “epidemics” of diseases, but often the apparent increase is due to increasing numbers of people being included in the definition, rather than the population becoming unhealthier. In the past 20 years, the prevalence of diabetes in Australia has increased from approximately 3% to 6%. However, since no tracking was conducted after a change in the definition of diabetes in 1997, it’s not known how much of this is due to lowering the threshold for diagnosis and how much is due to a true increase.

Survival rates of five years or more are frequently quoted as a measure of how successful health systems are in treating a disease. Measuring health outcomes such as survival rates is an unreliable way to assess changes over time or differences between regions. Survival rates will artificially look like they are improving when patients with milder or earlier forms of disease are included in the disease definition. These patients are less likely to die from their disease, but are included in the number of cases of disease, causing survival rates to artificially increase. This effect is especially obvious when screening programs are started. Survival rates and other measures using health outcomes per case diagnosed shouldn’t be used to assess the effectiveness of screening or other forms of testing, or to measure the success of health programs over time or between regions.

Part of the drive for widening disease definitions comes from the belief ‘prevention is better than cure’, and catching a disease early leads to better outcomes. In fact, most studies show the opposite. Patients with earlier and milder disease are less likely to benefit from treatment, but are just as likely as those with more advanced disease to suffer side effects from treatment, so the chances of overall harm are greater. This isn’t always true. Lowering the threshold used to define high blood pressure has clearly benefited many people, but the pendulum has swung too far and is now causing harm. We need better methods to ensure potential changes to disease definitions are evaluated rigorously by groups without academic and financial conflicts of interest.

Widening disease definitions affects us all because these changes are threatening the sustainability of healthcare systems worldwide. Treating patients with milder disease earlier takes resources and attention away from those with more severe illnesses. A Chinese study estimated treating additional patients affected by changes to the definition of diabetes, high cholesterol and high blood pressure, which happened around the year 2000, would have consumed 56% of the entire Chinese government’s total health expenditure.

In 1978, Susan Sontag wrote “Everyone who is born holds dual citizenship, in the kingdom of the well and in the kingdom of the sick. Although we all prefer to use the good passport, sooner or later each of us is obliged, at least for a spell, to identify ourselves as citizens of that other place.” We need to stop sending people to that other place unless we are certain we are doing them more good than harm.

Professor Jenny Doust is a Clinical Professorial Research Fellow with the School of Public Health.  Her new study titled: Potential consequences of changing disease classifications is published in the journal JAMA and analyses how changing disease definition can have harmful consequences.