Disease Screening - Statistics Teaching Tools

What does screening mean?

Screening refers to the application of a medical procedure or test to people who as yet have no symptoms of a particular disease, for the purpose of determining their likelihood of having the disease. The screening procedure itself does not diagnose the illness. Those who have a positive result from the screening test will need further evaluation with subsequent diagnostic tests or procedures.

Why do we do screening?

The goal of screening is to reduce morbidity or mortality from the disease by detecting diseases in their earliest stages, when treatment is usually more successful.

Examples of Screening Tests:

Pap smear, mammogram, clinical breast exam, blood pressure determination, cholesterol level, eye examination/vision test, and urinalysis.

What are sensitivity and specificity?

Sensitivity and specificity are measures of a test's ability to correctly classify a person as having a disease or not having a disease. Sensitivity refers to a test's ability to designate an individual with disease as positive. A highly sensitive test means that there are few false negative results, and thus fewer cases of disease are missed. The specificity of a test is its ability to designate an individual who does not have a disease as negative. A highly specific test means that there are few false positive results. It may not be feasible to use a test with low specificity for screening, since many people without the disease will screen positive, and potentially receive unnecessary diagnostic procedures.

It is desirable to have a test that is both highly sensitive and highly specific. This is frequently not possible. Typically there is a trade-off. For many clinical tests, there are some people who are clearly normal, some clearly abnormal, and some that fall into the gray area between the two. Choices must be made in establishing the test criteria for positive and negative results.

What is predictive value?

The probability of having the disease, given the results of a test, is called the predictive value of the test. Positive predictive value is the probability that a patient with a positive (abnormal) test result actually has the disease. Negative predictive value is the probability that a person with a negative (normal) test result is truly free of disease. Predictive value is an answer to the question: If my patient's test result is positive, what are the chances that my patient does have the disease?

Predictive value is determined by the sensitivity and specificity of the test and the prevalence of disease in the population being tested. (Prevalence is defined as the proportion of persons in a defined population at a given point in time with the condition in question.) The more sensitive a test, the less likely an individual with a negative test will have the disease and thus the greater the negative predictive value. The more specific the test, the less likely an individual with a positive test will be free from disease and the greater the positive predictive value.

When the prevalence of preclinical disease is low, the positive predictive value will also be low, even using a test with high sensitivity and specificity. For such rare diseases, a large proportion of those with positive screening tests will inevitably be found not to have the disease upon further diagnostic testing. To increase the positive predictive value of a screening test, a program could target the screening test to those at high risk of developing the disease, based on considerations such as demographic factors, medical history or occupation. For example, mammograms are recommended for women over the age of forty, because that is a population with a higher prevalence of breast cancer.

What criteria should be considered for an effective screening program?

1. Life-threatening diseases, such as breast cancer, and those known to have serious and irreversible consequences if not treated early, such as congenital hypothyroidism, are appropriate for screening.
2. Treatment of diseases at their earlier stages should be more effective than treatment begun after the development of symptoms. For example, cancer of the uterine cervix develops slowly, taking more than a decade for the cancer cells to progress to a phase of invasiveness. During this preinvasive stage, the cancer is usually asymptomatic but can be detected by screening using the Pap smear. Treatment is more effective during this stage than when the cancer has become invasive. On the other hand, lung cancer has a poor prognosis regardless of the stage at which treatment is initiated. Early diagnosis and treatment appear to prolong life little more than therapy after symptoms have developed. Screening to detect early stage lung cancer using currently available techniques would not be beneficial.
3. The prevalence of the detectable preclinical phase of disease has to be high among the population screened. This relates to the relative costs of the screening program in relation to the number of cases detected and to positive predictive value. The expenditure of resources on screening must be justifiable in terms of eliminating or decreasing adverse health consequences.

   A screening program that finds diseases that occur less often could only benefit few individuals. Such a program might prevent some deaths. While preventing even one death is important, given limited resources, a more cost-effective program for diseases that are more common should be given a higher priority, because it will help more people.

   In some cases though, screening for low prevalence diseases is also cost effective, if the cost of screening is less than the cost of care if the disease is not detected early. For example, phenylketonuria (PKU) is a rare disease but has very serious long-term consequences if left untreated. PKU occurs in only 1 out of every approximately 15,000 births, and if left untreated can result in severe mental retardation that can be prevented with dietary intervention. The availability of a simple, accurate and inexpensive test has lead many states, including New York State, to require PKU screening for all newborns.

4. A suitable screening test must be available. Suitability criteria includes adequate sensitivity and specificity, low cost, ease of administration, safe, imposes minimal discomfort upon administration, and is acceptable to both patients and practitioners.
5. There must also be appropriate follow-up of those individuals with positive screening results to ensure thorough diagnostic testing occurs.