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Lecture in the Basic Epidemiology Lecture Series Department of Ophthalmology, RIPAS Hospital, Bandar Seri Begawan, Brunei on 7th October 2008 by Professor Omar Hasan Kasule MB ChB (MUK), MPH (Harvard), DrPH (Harvard).
DEFINITION, OBJECTIVES, ORGANIZATION, and BENEFITS
Screening, a type of secondary prevention, is identification of unrecognized disease by the application of tests, examinations or other procedures which can be applied easily. Screening can be described as routine or episodic/adhoc, individual or mass, selective or comprehensive. Its effectiveness is assessed by morbidity, mortality, survival, and quality of life. Its objectives are achieved through early detection and treatment of disease. Its benefits may be public (infectious disease), private (insurance screening), and individual (early treatment and reassurance). Its disadvantages are longer morbidity for untreatable screen-detected cases, over-treatment of borderline cases, false reassurance of false negatives, unnecessary treatment of false positives, risks and costs of the screening tests.
The most successful screening programs are breast and cervical cancer. Breast cancer screening is by mammography screening every 1-2 years is recommended in women above 50 years. There is no proof that BSE and mammography are useful in women below 50 years of age. Mammography for women below 50 years is done only if there is a family history of breast cancer. Cervical cancer screening is by use of the PAP smear test is very popular and is recommended for women above 20 years every 3 years.
CHARACTERISTICS OF DISEASE & SCREENING TESTS
A disease suitable for screening must be definable clearly, with known natural history and a relatively long detectable pre-clinical phase, common (high prevalence), serious, and effectively treatable if detected early. The screening test must be simple, cheap and cost-effective, acceptable, safe, and perform optimally (high sensitivity, high specificity, low false positive, suitable cut-off level, and reliability).
EPIDEMIOLOGIC EVALUATION OF SCREENING PROGRAMS
Process parameters of screening program effectiveness are accuracy, validity, reliability, and predictive value. The outcome parameters of a screening program are health outcomes (reduction of morbidity, reduction of mortality, survival, and improvement in the quality of life) or economic outcomes. Correct interpretation of the outcome measures requires consideration of lead-time bias, length bias, selection bias, over diagnosis bias, and over-treatment bias. The effectiveness of the screening program is affected by the test used, the attendance or coverage, the screening interval, and success of referral for diagnostic confirmation. Outcome assessment can be by pre and post screening comparisons of the same population or comparison of morbidity and /or mortality in the screened and non-screened using the case control or random allocation designs. Apparent lack of benefit from a screening program could be due to two the disease having no detectable pre-clinical phase or an ineffective intervention. Screening programs can be improved by selective screening (by age, gender, and high risk), optimal screening frequency, multi-phase screening, and sequential screening.
COST BENEFIT ANALYSIS OF SCREENING PROGRAMS
Cost benefit analysis is used to decide on program initiation or continuation. The costs include cost of screening, the cost diagnosis and treatment, patient costs such as lost earnings, human emotional and other costs. QUALY is used as a summary measure of benefits.
ETHICAL ISSUES
Ethico-legal considerations in screening programs are: the benefit of screening must outweigh the harm, the efficacy of screening must be proved in a proper trial, confidentiality must be maintained, and informed consent must be obtained.