In this course, we are characterizing early detection and screening as secondary prevention. Classic examples include mammography to detect breast cancer, Pap smears to detect cervical cancer, fasting blood glucose to detect diabetes, PSA's to detect prostate cancer, etc.

Let's look at Gordis' map of the natural history of disease, (from Gordis L. Epidemiology. Philadelphia: Saunders and Company, 1996). The biological onset of disease is followed by clinical symptoms, then diagnosis, and therapy until there is an outcome.

We can label phases in this process. From the onset of disease until clinical symptoms occur is the pre-clinical phase. The individual has the disease but doesn't know it. The clinical phase is the latter part of the process, from the occurrence of clinical symptoms through therapy.

Within the preclinical phase, there may be an interval between the onset of the disease and the occurrence of clinical symptoms during which disease can be detected with certain tests. This is called a detectable pre-clinical phase. If treatment is more effective when the disease is in the preclinical stage, screening for disease during the detectable pre-clinical phase offers an advantage.

The gain from screening for disease is the difference between the time a disease would have been diagnosed by clinical symptoms and when it is detected with a screening procedure. This is the lead time.

What is the Objective of Screening?

• To improve the quality of life or to reduce the morbidity and mortality for an individual, by applying effective treatment to disease or increased risk at an early stage, when treatment is more effective than if it were applied at a later stage.

To meet this objective, we seek to identify diseased or at-risk individuals at an asymptomatic stage. This helps us to separate individuals into populations such as:

• diseased vs non-diseased
• at-risk vs not at-risk

However, screening tests are not 100% accurate at classifying individuals. As a result, the distributions are not completely separated. For example, consider the distributions of blood sugar in diabetics and non-diabetics as depicted below. How would you set the cutpoint for a test of blood sugar to indicate diabetes? If you choose a low value, people with normal blood sugar will be included among the diabetics; if you select a high value, some diabetics will be included with the normals.

We must also recognize that screening is only useful when we are assured that treatment is more effective at the earliest stages of the disease.