Symptoms are a lagging indicator of disease

Graphic source: Centers for Disease Control and Prevention. Principles of epidemiology, 2nd ed. Atlanta: U.S. Department of Health and Human Services;1992.

In our continued series on “what didn’t make it into the book”, I thought I’d cover the stages of disease today - and what that means for early testing. 

I talked about this concept in the book, but I wanted to share a visual that I think clarifies things even further. In the graphic above from the CDC, you can see the natural progression of a disease without intervention. 

As you can see, there is a progression through main four stages from susceptibility to subclinical disease then clinical disease and ultimately resolution, disability or death. 

Currently, “The onset of symptoms marks the transition from subclinical to clinical disease. Most diagnoses are made during the stage of clinical disease.” I want to focus on this time period from subclinical to clinical disease because that’s where I think we are missing the opportunity to intervene earlier. 

“After the disease process has been triggered, pathological changes then occur without the individual being aware of them. This stage of subclinical disease, extending from the time of exposure to onset of disease symptoms, is usually called the incubation period for infectious diseases, and the latency period for chronic diseases. During this stage, disease is said to be asymptomatic (no symptoms) or inapparent. This period may be as brief as seconds for hypersensitivity and toxic reactions to as long as decades for certain chronic diseases.”

Let’s focus on chronic diseases for this discussion. For most people, symptoms emerge late in the disease process. At that point, you have already experienced pathologic changes (i.e., structural or functional changes to your cells and organs) - but you don’t know it because you don’t “feel” it yet. And, this period can last for years!

The CDC goes on to say, “Although disease is not apparent during the incubation period, some pathologic changes may be detectable with laboratory, radiographic, or other screening methods. Most screening programs attempt to identify the disease process during this phase of its natural history, since intervention at this early stage is likely to be more effective than treatment given after the disease has progressed and become symptomatic.”

Basically, we can test using different body fluids (e.g., blood, urine) or using imaging (e.g., MRI) to detect these physiologic changes before they progress to symptomatology. This is where we can “test, not guess”. 

Essentially, the point is - why wait? If we know that this is how disease progression works, why wait to experience symptoms to determine what’s going on in your body? It’s clearly a lagging indicator. Why wait to move through further stages of disease before taking action? The later someone is in the disease process, the harder and more extensive / expensive the interventions tend to be. The earlier you catch something (whether that is a nutrient deficiency or thyroid dysfunction or insulin resistance), the more options you have to address it and the easier it is to address.