SCREENING FOR DISEASE

Learning Objectives Definition of screening; Principles of Screening.

Definitions 1. Screening program -- comprehensive disease control activity based on the identification and treatment of persons with either unrecognized disease or unrecognized risk factors for disease. 2. Screening test -- specific technology (survey questionnaire, physical observation or measurement, laboratory test, radiological procedure, etc.) used to help identify persons with unrecognized disease or unrecognized risk factors for disease.

Definitions 3. Primary prevention -- disease control approach based on the elimination or reduction of risk factors for disease. Primary prevention aims to prevent the occurrence of disease. Primary prevention may use screening tests to identify persons with risk factors.

Definitions 4. Secondary prevention -- disease control approach based on the active identification and treatment of persons with unrecognized disease. Secondary prevention aims to prevent the occurrence of adverse outcomes from disease (such as fatal outcomes), without necessarily reducing the occurrence of disease. Secondary prevention must screen to identify persons with unrecognized disease.

What is Screening Screening is the testing of apparently healthy populations to identify undiagnosed diseases or people at high risk of developing a disease. Screening aims to detect early disease before it becomes symptomatic. Screening is an important aspect of prevention, but not all diseases are suitable for screening.

The Principles of Screening The choice of disease for which to screen; The nature of the screening test or tests to be used; The availability of a treatment for those found to have the disease; The relative costs of the screening.

Disease The disease must be an important health problem. There should be a recognizable latent or early symptomatic stage. The natural history of the disease, including latent to declared disease, should be adequately understood.

There should be a suitable test or examination. The test should be acceptable to the population.

Important properties of a screening test 1. Screening is simple, inexpensive, and easily diffused through the population. 2. The act of screening is safe and acceptable. 3. The screening test is reliable. 4. The screening test is accurate.

There should be an acceptable treatment for the patients with recognized disease. There should be facilities for diagnosis and treatment should be available. There should be an agreed policy on whom to treat as patients.

The cost of case finding (including diagnosis and treatment of patients diagnosed) should be economically balanced in relation to possible expenditure on medical care as a whole. Case finding should be a continuing process and not a "once for all" project.

Summary Screening is the testing of apparently healthy populations to identify previously undiagnosed diseases or people at high risk of developing a disease. Principles of Screening: disease, test, treatment and cost. What is the next step? Define the validity of the screening test and

Comparison of screening test results against gold standard 1. A screening test may produce only a categorical (positive or negative) result. Important concepts include disease prevalence, test sensitivity, test specificity, positive predictive value, negative predictive value, proportion test positive, proportion test negative, and disease yield.

THREE KEY MEASURES OF VALIDITY of the screening test 1. SENSITIVITY 2. SPECIFICITY 3. PREDICTIVE VALUE

True Disease Status Screening Test PositiveNegativeTotal Positive True Positives (TP) False Positives (FP) TP+FP Negative False Negatives (FN) True Negatives (TN) FN+TN Total TP+FNFP+TNTP+FP+FN+TN Outcomes of a Screening Test

SENSITIVITY Sensitivity tells us how well a positive test detects disease. It is defined as the fraction of the diseased who test positive. Its complement is the false negative rate, defined as the fraction of the diseased who test negative. Sensitivity and false negative rate add up to one.

SENSITIVITY AND THE FALSE NEGATIVE RATE ARE COMPLEMENTARY N who test positive + N who test negative = 1 All with disease SENSITIVITY + FALSE NEGATIVE RATE = 1

SPECIFICITY Specificity tells us how well a negative test detects non-disease. It is defined as the fraction of the non- diseased who test negative. Its complement is the false positive rate, defined as the fraction of the non-diseased who test positive. Specificity and the false positive rate add up to one.

SPECIFICITY AND THE FALSE POSITIVE RATE ARE COMPLEMENTARY N who test negative + N who test positive = 1 All without disease SPECIFICITY + FALSE POSITIVE RATE = 1

DENOMINATORS OF THESE RATES Note that all the denominators of the four rates so far defined (sensitivity, specificity and the false + and false – rates) are DISEASE STATES The denominators of specificity and the false positive rate is PEOPLE WITHOUT DISEASE

Net effects from screening 1. True negatives are exposed to the costs, inconvenience, and hazards of screening. True negatives may be reassured by knowledge of a negative screening test result. 2. False positives are exposed to the costs, inconvenience, and hazards of screening and follow-up diagnostic evaluations. The falsely positive screening test result may cause psychological and emotional distress.

Net effects from screening 3. False negatives are exposed to the costs, inconvenience, and hazards of screening. False negatives may be falsely reassured by knowledge of a negative screening test result. False negatives represent lost opportunities to prevent adverse outcomes from disease. 4. True positives are exposed to the costs, inconvenience, and hazards of screening, follow- up diagnostic evaluations, and therapeutic interventions. Only true positives have an opportunity to benefit from medical therapy.