DISEASE CLASSIFICATION, DESCRIPTION, MEASUREMENT, DIAGNOSIS, and PROGNOSIS

Presented at an Interactive Workshop on Essentials of Epidemiology in Public Health at the Department of Social and Preventive Medicine University Malaya Kuala Lumpur Malaysia 19-25 October 2009 by Professor Omar Hasan Kasule Sr MB ChB (MUK), MPH (Harvard), DrPH (Harvard) Professor King Fahad Medical College Riyadh; Professor of Epidemiology and Islamic Medicine Institute of Medicine University Brunei Darussalam, Visiting Professor of Epidemiology University of Malaya

CLASSIFICATION OF DISEASE

In 1853 William Farr introduced standard nomenclature for causes of death. In 1946 WHO introduced an International Classification of Diseases, Trauma, and Cause of Death (ICD). Disease classification is useful for explanation & description, prediction of disease course, prognosis, planning treatment, and disease prevention. Manifestational, causal, abstract, operational/pragmatic criteria are used in disease classification. In classification is based on 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. Time can be described as calendar time (on the interval scale) or cohort time (on the ratio scale). Time trends are biorhythmic, periodic (monthly, annual, seasonal), linear, and curvilinear. 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. Time intervals are induction (causal action to disease initiation), incubation, and latent periods (disease initiation to disease detection). 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.

Time-place interactions can be described as clustering, disease outbreaks, epidemics, endemics, and pandemics. Clustering is excessive concentration of events at a point in time or a place. A disease outbreak is excessive disease occurrence of a lesser degree than an epidemic. An endemic disease has high prevalence in an area. An epidemic is excessive incidence over a brief period of time. A pandemic is an epidemic affecting several countries. A point source epidemic originates from one person or place. A common source epidemic has more than one origin. In epidemic progression new infections occur after the initial ones. An epidemic is visible when cases are few but the disease is rare, case number is large, disease is distinctive, swift shift from non-epidemic to epidemic status is swift, and disease duration is short. The occurrence of an epidemic can be ascertained by changes in trend over time, comparing incidence in epidemic and non-epidemic places, and comparing disease incidence among population sub-groups of the same country. Investigating an epidemic consists of establishing the diagnosis, case definition, determination is made whether an epidemic exists, characterizing the epidemic by place, time, and person, drawing a spot map showing cases, computing incidence rates by location, identifying clusters, developing hypotheses about the source and route of infection, testing the hypotheses laboratory and case control studies. Control measures are then instituted and may include sanitation, prophylaxis, diagnosis and treatment, and vector control. Surveillance is continued in the post epidemic period. An epizootic is an epidemic disease in animal populations. Epizootics can become epidemics in human populations. An enzootic is an endemic disease among animals. An epizoodemic is an epidemic involving both human and animal populations.

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 point, period, and lifetime prevalence. Prevalence proportion = incidence rate x average duration of disease. Prevalence is useful for administrative purposes. It is not used for etiological studies because the time sequence is not obvious. Prevalence changes due to changes in incidence and duration. Excess disease risk is measured as an absolute effect (Rate Difference or Risk Difference) or a relative effect (Relative Risk, Rate Ratio, Risk Ratio, Prevalence Ratio, Cumulative Incidence Ratio, Incidence density Ratio, Odds Ratio, and Standard Mortality Ratio). The following interpretations of the odds ratio and risk ratio are used – up to 0.3 strong benefit, 0.4 – 0.5 moderate benefit, 0.6 – 0.8 weak benefit, 0.9 – 1.1 no effect, 1.2 –1.6 weak hazard, 1.7 –2.5 moderate hazard, and >=2.6 strong hazard. OR values range from 0 to infinity. OR is a good estimator of risk ratio if the disease is rare and the cases and controls are randomly selected from the population. The proportion of disease due to a particular exposure is measured by various parameters of attributable rate (AR) that takes into consideration the population at risk. Proportional mortality studies are used to compare the proportion of deaths among the exposed to the proportion of deaths among the non-exposed. A common measure of disease impact is the years of potential life lost (YPLL). The Kaplan-Meier formula helps compute the survival proportion over several consecutive time intervals

DISEASE DIAGNOSIS

Disease identification is by symptoms, signs (clinical, laboratory, radiological), observation (direct & indirect, passive & induced), indirect marker of abnormality, and response to a therapeutic. Case definition uses clinical criteria, underlying pathology, epidemiologic, and logical criteria. Definition of the abnormal is based on four considerations: statistical, clinical, prognostic, and operational. There is observation error in symptoms and measurement error in signs. Signs can be pathognomonic, non-specific, qualitative, or quantitative. A syndrome is a complex of symptoms and signs. Tests are an extension of clinical examination for signs. Validity is when a test measures what it is supposed to measure. Sensitivity, specificity, and predictive value are measures of validity (accuracy). Sensitivity is a measure of the strength of association. Specificity measures the uniqueness of association. We can talk of tests as True positives; True negatives; False negative; and False positives. There is a trade-off between specificity and sensitivity. High sensitivity is associated with low specificity & vice versa. High specificity is associated with low sensitivity & vice versa. True/correct diagnosis is based on high specificity (with low or high sensitivity). Ability of a test to predict true diagnosis is measured as the positive predictive value (detection of disease) and the negative predictive value (correct indication of absence of disease). High prevalence of disease increases PV+ve. Reproducibility consists of repeatability, consistency, reliability, and stability.

DISEASE PROGNOSIS

Follow-up studies are used to study prognostic factors using case fatality rates, survival (5-year survival rates, the survival curve, median survival time, relative survival, and comparison of actual with expected survival), and life table analysis. The matching of discharge information with death certificates can also be used to study prognostic factors.

Key Words and Terms: Disease Classification, Disorder Classification, Epidemic Disease, Ethnicity, Etiology Of Disease, Heredity, Infection, Morbidity, Mortality, Nomeclature Of Disease, Nosology, Risk Ratio, Risk Difference, Incidence Rate Ratio, Incidence Rate Difference, Odds Ratio, Cumulative Incidence, Morbidity, Mortality.

DISEASE DETERMINANTS

CONCEPTS OF DISEASE CAUSATION

The concept of the causal triangle (environment, host, and disease) has been used for many years to simplify epidemiological reasoning. Disease risk is a probability. A risk factor is known empirically to be involved in disease causation. Risk indicators are likely to be causes but are not yet confirmed. Data on causes can be obtained from animal or human experiments/observations. Causes may be defined as causative or preventive. A risk factor is described as sufficient when its mere presence will trigger the disease concerned. In practice a sufficient cause refers to a constellation of 2 or more risk factors since most diseases are multi-causal. One disease normally has more than 1 sufficient cause. There are some risk factors that are always present in all sufficient causes of the disease. These are referred to as necessary causes. Causes may be weak or strong. Causes may interact either cooperatively in disease causation (synergy) or act against one another (antagonism). The causal chain or causal pathway is multi-stage. It is initiated by the main risk factor. The final stages are due to promotors.

Association of disease with a putative risk factor may be statistically or non-statistical. Statistical association can be causal or non-causal. One disease may have 2 or more co-factors. One disease may have 2 quite different independent causes. One cause leads to 2 different diseases. The criteria of causality are either essential criteria or back-up criteria. The essential causal criteria are four: specificity, strength, time sequence, and biological plausibility. The back-up causal criteria are five: dose-effect relationship, repetition, consistency, evidence from intervention, and experimental evidence.

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, defined by subjective or objective data, current or past exposures. Exposures can be dichotomous (exposed vs. unexposed), ranked according to importance, stratified. Categorization may be based on statistical distributions for example BMI. Exposures may be measured quantitatively or qualitatively. The following are instruments used to measure exposures: questionnaires, personal interviews, biochemical analyses of biological material, physical and chemical analysis of the environment. Measurement of an exposure involves three dimensions: nature of the exposure, the dose, and time. Differential errors in exposure measurement result in a biased odds ratio; the bias remains even of the sample size is increased. Non-differential errors make the odds ratio tend to the null value (attenuation of effect). Non differential error lowers study power and requires a larger sample size to detect a given difference. Measurement errors can be reduced by multiple assessments of the exposure such as repeat assessments of cholesterol. The effect measure can be adjusted to account for the effect of the error. The best approach is to use high quality control measures at the stage of data collection to minimize errors.

DISEASE DETERMINANTS

Biological determinants are demographic or genetic. Age and gender structure of a population have an impact on mortality and morbidity. Pre-disposition to many diseases is inherited. Some diseases are known to be genetically-caused while the genetic basis of others is being unraveled. 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.

Key words and Terms: Causal triangle, Risk factor, Risk indicator, Causality criteria, Exposure

DISEASE CONTROL and PREVENTION

Presented at an Interactive Workshop on Essentials of Epidemiology in Public Health at the Department of Social and Preventive Medicine University Malaya Kuala Lumpur Malaysia 19-25 October 2009 by Professor Omar Hasan Kasule Sr MB ChB (MUK), MPH (Harvard), DrPH (Harvard) Professor King Fahad Medical College Riyadh; Professor of Epidemiology and Islamic Medicine Institute of Medicine University Brunei Darussalam, Visiting Professor of Epidemiology University of Malaya

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. Prevention is pre-empting disease. Primary prevention is at the pre-pathogenic stage such as health promotion and health protection (specific & general). Secondary prevention is early detection and treatment. Tertiary prevention is disability limitation and rehabilitation. Curative medicine (for individuals) and preventive medicine (for the community) are synergistic. Preventive medicine has priority and is more effective with a bigger impact on disease over the past 100 years. Prevention strategies can be disease-oriented such as immunization and tobacco control, Environmental-oriented such as food quality control, or host-oriented such as immunization, nutrition, and medical care

PREVENTIVE MEDICINE: LEGAL BASIS

Preventive medicine, tibb wiqa’i, is a series of pro-active measures that subsumed under the Islamic concept of prevention, wiqayat. It is not reversing pre-determination, qadar. It is use of limited empirical knowledge to extrapolate, anticipate, and predict disease risk. It is modification, alleviation, or reversal of the effects of risk factors. All preventive has a legal basis under the 5 purposes of the Law, maqasid al shariat, and the 5 Principles of the Law, qawaid al shariat.

PREVENTIVE MEDICINE: MODERATION, BALANCE and EQUILIBRIUM

Disease prevention and control is covered in the concepts of moderation, balance, and equilibrium. Any human activity even a good one will harm health if done in excess. Humans strive to establish and maintain the optimal equilibrium.

Key Words and Terms: Disease control, Disease eradication, Disease prevention, Contact tracing, Disease notification, Mosquito control, Patient isolation, Pest control, Rodent control, Smoking cessation, Universal precautions, Vaccination, Vector control

DISEASE SURVEILLANCE

DEFINITION

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.

HISTORY OF SURVEILLANCE

Hippocrates mentioned observing, collecting, and analyzing facts to decide on a course of action. John Graunt carried out the first recorded surveillance activity in Europe by his analysis of the bills of mortality in London. William Farr as Registrar-General of England and Wales that lasted 1839-1879, developed modern surveillance concepts. Lemuel Shattuck (1839-1879) recommended collection of health data by age, gender, occupation, socio-economic level as well as locality. In its modern form, surveillance started as collection of morbidity data. Later mandatory reporting of notifiable diseases was instituted. Surveillance has developed to become a common tool in public health.

OBJECTIVES, METHODS, and SCOPE

Surveillance systems fulfills 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 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.

Surveillance can be based on disease notification systems, laboratory-based surveillance, disease registries, surveys, data bases (vital records, immunization registries, hospital data, insurance data, worker compensation), sentinel events, and record linkage. Surveillance covers infectious disease, occupational health, environmental health, injuries, maternal and child health, and non-communicable disease. In addition to health events, surveillance includes information on risk factors, use of preventive and curative services.

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 disease, facilitating epidemiologic and laboratory research, validation of preliminary research, and setting research priorities.

SURVEILLANCE SYSTEM

The essentials of a surveillance system are: data collection, data analysis, data interpretation, and feed-back or data dissemination. Before start of surveillance, the case definition and the target population must be defined. Appropriate personnel must be selected and trained. The logistics of data collection, analysis, and dissemination must be set up. Approvals from the relevant authorities must be obtained. A good surveillance system must be ongoing, practicable, uniform, frequent and rapid, sensitive, timely, representative, high predictive value, accurate, complete, simple, flexible, and acceptable. The surveillance system is evaluated using the following criteria: (a) attainment of objectives of surveillances: decrease of incidence and prevalence and decrease of mortality and case fatality (b) operational characteristics (sensitivity, timelinessness, representativeness, predictive value), acceptability, flexibility, simplicity, and cost.

DATA COLLECTION, ANALYSIS, and INTERPRETATION

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.

Data analysis is usually descriptive and usually consists of comparing with the baseline. Surveillance data is interpreted with the following in mind: identification of epidemics, identification of new syndromes, monitoring trends, evaluation of public policy, and projecting future needs. Observation of departures from usual disease distribution does not necessarily mean that there is a problem. Completeness of coverage of the surveillance system is an issue that may arise. Capture-recapture methods may be used to ascertain whether the surveillance system has adequate coverage. Surveillance data has limitations such as under reporting, unrepresentative case series, and inconsistent case definitions. Surveillance information must be disseminated through publications and the mass media. The message to be communicated must be packaged correctly and appropriately. Data dissemination need not wait until the surveillance system is fully operational; provisional surveillance data is still useful in public health.

Key Words And Terms: Surveillance, Disease Surveillance, Surveillance, Morbidity Surveillance, Disease Control, Active Surveillance, Passive Surveillance, Sentinel Surveillance, Laboratory-Based Surveillance, Disease Notification, Disease Registries, Vital Records, Surveys For Surveillance, Uses Of Surveillance Data, Surveillance System, Evaluation Of Surveillance.

DISEASE SCREENING

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.

Key Words and Terms: Screening, Breast Cancer Screening, Screening, Cervical Cancer Screening, Genetic Screening, Lead Time Bias, Length Bias, Selection Bias, Mammography, Mass Screening, Multi-Phasic Screening, Neonatal Screening, Pap Smear, Pre-Natal Screening, Vision Screening.

Integrated Medical Education Resources

091025P - ABSTRACTS FOR WORKSHOP ON ESSENTIALS OF EPIDEMIOLOGY IN PUBLIC HEALTH: MODULE ON DISEASE EPIDEMIOLOGY

DISEASE CONTROL and PREVENTION

DEFINITION, OBJECTIVES, ORGANIZATION, and BENEFITS