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A course for staff of the Global Center for Mass Medicine at the Ministry of Health by Dr. Omar Hasan Kasule MB ChB (MUK), MPH (Harvard), DrPH (Harvard) Professor of Epidemiology at King Fahad Medical City Riyadh
5.1 DISEASE CLASSIFICATION, DESCRIPTION, MEASUREMENT, DIAGNOSIS, and PROGNOSIS
5.1.1 Classification of Disease: Disease classification is useful for explanation & description, prediction of disease course, prognosis, planning treatment, and disease prevention. Classification is based on manifestational, causal, abstract, operational/pragmatic, etiologic agent, disease process, organ system, transmission, and portal of entry.
5.1.2 Disease Description: Disease description answers the what, why, when, how, where, and who of a disease. Natural history is the progression from susceptibility; sub-clinical disease; clinical disease; and recovery, disability, or death.
5.1.3 Disease description: Time intervals are induction (causal action to disease initiation), incubation, and latent periods (disease initiation to disease detection). Individual variation is by heredity, age, sex, SES, marital status, and ethnicity/race.
5.1.4 Disease description: Time-place interactions can be described as clustering, disease outbreaks, epidemics, endemics, and pandemics. epizootic is an epidemic disease in animal populations. An epizoodemic is an epidemic involving both human and animal populations.
5.1.5 Disease Measurement: Incidence rate (IR) = incident number/ total person-time. Cumulative incidence = incident number / susceptible population at the start. Prevalence proportion = # cases of illness at a particular time (old and new) / # of individuals in the population at the same time. Prevalence can be the point, period, and lifetime prevalence. Excess disease risk is measured as Odds Ratio. A common measure of disease impact is the years of potential life lost (YPLL).
5.1.6 Disease Diagnosis: Definition of the abnormal is based on four considerations: statistical, clinical, prognostic, and operational. Sensitivity, specificity, and predictive value are measures of validity (accuracy) of diagnostic tests.
5.1.7 Disease Prognosis: the concept of predetermination
5.2 DISEASE DETERMINANTS:
5.2.1 Concepts of Disease Causation: The causal triangle is environment, host, and disease. A risk factor is known empirically to be involved in disease causation. Risk factors (known) and risk indicators (not yet confirmed). Essential causal criteria: specificity, strength, time sequence, and biological plausibility. Back-up causal criteria: dose-effect relationship, repetition, consistency, evidence from intervention, and experimental evidence.
5.3.2 Concept of Exposure: An exposure is defined as a substance, phenomenon, or event that has a physiological effect, can cause or protect from disease. Exposures may be personal attributes or environmental agents.
5.2.3. Disease Determinants: Biological determinants are demographic or genetic. Environmental determinants are infections and physical agents such as heat, cold, and radiation. Social determinants are socioeconomic status, occupation, race, ethnicity, and medical care.
5.2.4 Variation of Disease Risk with Age: Highest risk at young and old age.
5.3 DISEASE CONTROL AND PREVENTION:
5.3.1 Control is the containment of the disease and includes prevention and other control measures. Eradication is the complete uprooting of a disease and its total elimination. Prevention can be primary, secondary, or tertiary.
5.3.2 Preventive Medicine: Legal Basis, wiqaya not reversing qadar
5.3.3. Preventive Medicine: Moderation, Balance, and Equilibrium
5.4 DISEASE SURVEILLANCE:
5.4.1 Definition: In 1968 The World Health Organization defined surveillance as the systematic collection and use of epidemiological information for planning, implementing, and assessing disease control
5.4.2 Objectives: Surveillance systems fulfill the following purposes: (a) identification of changes in disease incidence (b) epidemiological description of disease: incidence, causes, and associated factors (c) evaluation of disease control and prevention programs (c) assessment of the burden of disease to help health care delivery (d) planning of public health programs by the future projection of disease burden (e) using surveillance data for formulating public health policy (f) prediction of the occurrence of epidemics (g) provide information for researchers.
5.4.3 Methods: Surveillance can be based on disease notification systems, laboratory-based surveillance, disease registries, surveys, databases (eg vital records, immunization), sentinel events, and record linkage. In addition to health events, surveillance includes information on risk factors and preventive and curative services.
5.4.4 The immediate uses of surveillance are detection of epidemics, emerging diseases, changes in health practices, changes in demographic characteristics, and antibiotic resistance. Medium-term uses of surveillance are annual dissemination of information on the magnitude of health problems, assessing control activities, setting research priorities, testing hypotheses, facilitating planning, monitoring risk factors, and monitoring changes in health practices. Long-term uses include providing databases for describing the natural history of the disease, facilitating epidemiologic and laboratory research, validation of preliminary research, and setting research priorities.
5.4.3 Surveillance System: data collection, data analysis, data interpretation, and feedback or data dissemination.
5.4.4 The sources of surveillance data are: (a) mandatory morbidity and mortality notification systems (b) Health information systems: vital records (births and deaths), coroner reports, medical care records (integrated health information system & hospital discharge summary), insurance records, worker compensation records, and records of work-absence due to illness, school records (c) disease registries eg cancer registry (hospital-based, population-based, and exposure registers) (d) public health laboratory reports, (e) reports of disease outbreaks, epidemics, and individual case studies (f) vaccine utilization data, (g) records of hazard exposure surveillance (h) special surveys such as health interview surveys and other types of surveys (i) Sentinel surveillance that focuses on key health indicators for early warning. Sentinel events such as infant mortality and sentinel sites such as hospitals and clinics (j) Studies of animal reservoirs and vector distribution (k) study of biological markers (l) study of drug resistance (m) demographic and environmental data (n) media reports
5.5 DISEASE SCREENING:
5.5.1 Definition: Screening, a type of secondary prevention, is the identification of unrecognized disease by the application of tests, examinations, or other procedures which can be applied easily.
5.5.2 Objectives: early detection and treatment of disease. Its effectiveness is assessed by morbidity, mortality, survival, and quality of life.
5.5.3 Organization: Screening can be described as routine or episodic/Adhoc, individual or mass, selective or comprehensive.
5.5.4 Benefits: public (infectious disease), private (insurance screening), and individual (early treatment and reassurance)
5.5.5 Characteristics of a suitable Disease: 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.
5.5.6 Characteristics of a good screening test: 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).
5.5.7 Epidemiologic Evaluation of Screening Programs: Process parameters are accuracy, validity, reliability, and predictive value. Outcomes are health outcomes (reduction of morbidity, reduction of mortality, survival, and improvement in the quality of life) or economic outcomes. The correct interpretation of the outcome measures requires consideration of lead-time bias, length bias, selection bias, over-diagnosis bias, and over-treatment bias.
5.5.8 Cost-Benefit Analysis of Screening Programs: Cost-benefit analysis is used to decide on program initiation or continuation. The costs include the cost of screening, the cost of diagnosis and treatment, patient costs such as lost earnings, human emotions, and other costs. QUALY is used as a summary measure of benefits.
5.5.9 Ethical Issues: 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.