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Presented at a course for diploma of cardiovascular diseases by Professor Omar Hasan Kasule Sr. MB ChB (MUK), MPH (Harvard), DrPH (Harvard) Professor of Epidemiology at King Fahad Medical City Riyadh
CLASSIFICATION OF DISEASE
• WHO International Classification of Diseases, Trauma, and Cause of Death (ICD) is useful for explanation & description, prediction of disease course, prognosis, planning treatment, and disease prevention.
• Criteria of disease classification: manifestational, causal, abstract, operational/pragmatic criteria, etiologic agent, disease process, organ system, transmission, and portal of entry.
DISEASE DESCRIPTION
• Disease description answers the what, why, when, how, where, and who of a disease.
• Diseases are acute (<3 months), sub-acute, and chronic >3 months).
• Natural history is progression from susceptibility; sub-clinical disease; clinical disease; and recovery, disability, or death.
• Disease is described by place (as rural, urban, sub-urban, and slums/shacks (septic fringe), boundaries (political and natural), institution (hospital, home, school, factory, farm, and outer space). Individual variation is by heredity, age, sex, SES, marital status, and ethnicity/race.
• Disease clustering, disease outbreaks, epidemics, endemics, and pandemics.
DISEASE MEASUREMENT
• Incidence rate (IR) = incident number/ total person-time.
• Prevalence proportion = # cases of illness at a particular time (old and new) / # of individuals in the population at the same time.
• Excess disease risk is measured as an absolute effect (Rate Difference or Risk Difference) or a relative effect (Risk Ratio, Odds Ratio).
TESTS FOR DIAGNOSING AND SCREENING FOR DISEASE
• Disease identification is by symptoms and signs (clinical, laboratory, radiological)
• A syndrome is a complex of symptoms and signs.
• Tests are an extension of clinical examination for signs.
• Sensitivity is a measure of the strength of association.
• Specificity measures the uniqueness of association.
• Tests can be True positives; True negatives; False negative; and False positives.
• Ability of a test to predict true diagnosis is measured as the predictive value
• Reproducibility consists of repeatability, consistency, reliability, and stability.
DISEASE DETERMINANTS
• Biological determinants are demographic or genetic.
• Behavioral determinants are lifestyle and nutrition.
• Environmental determinants are infections and physical agents such as heat, cold, and radiation.
• Social determinants are the socio-economic status, occupation, race, ethnicity, and medical care.
CONCEPTS OF CONTROL, ERADICATION, and PREVENTION
• Control is a containment of disease and includes both prevention and control measures.
• Eradication is complete uprooting of a disease and its total elimination.
• Primary prevention is at the pre-disease stage such as vaccination, good nutrition, clean water, clean environment
• Secondary prevention is early detection and treatment such as antibiotics and surgery
• Tertiary prevention is disability limitation and rehabilitation.
DISEASE SURVEILLANCE
• In 1968 The World Health Organization defined surveillance as systematic collection and use of epidemiological information for planning, implementing, and assessing disease control.
• In active surveillance mechanisms are set up to actively look for and identify disease conditions.
• Passive surveillance does not set up any special monitoring mechanisms but relies on the existing systems to report disease occurrence.
DISEASE SCREENING
• 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.
• Effectiveness of screening is assessed by morbidity, mortality, survival, and quality of life.
• Benefit of screening is early detection and treatment of disease.
• Disadvantage of screening is 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.
EVALUATION OF SCREENING PROGRAMS
• The most successful screening programs are breast and cervical cancer.
• 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.
• 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.
• 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.