EPIDEMIOLOGY

Custom Search

EPIDEMIOLOGY

EPIDEMIOLOGY - is the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to control health problems.

Uses of epidemiology:
1. identify the etiology of a disease and risk factors;
2. determine the extent of disease found in the community;
3. study the natural history and prognosis of disease;
4. evaluate new preventive & therapeutic measures and new modes of health care delivery;
5. provide a foundation for developing public policy and regulatory decisions relating to environmental problems

Some important milestones in the history of epidemiology:
1. 400 BC - Hippocratic concepts - "Whoever would study medicine aright…must consider the effect of the seasons…water…soil…the life(style) of the inhabitants themselves"
2. 1749 - Edward Jenner on cowpox exposure providing protection from smallpox and leading to the first vaccination
3. 1756 - James Lind on citric acid fruits as cure for survey
4. 1836 - Charles Alexander-Louis on the importance of statistical methods in medicine
5. 1837 - William Farr directed the Registrar-General's Office in London which provided statistical data for public health use
6. 1839 - William Budd on typhoid fever as a contagious diseases
7. 1849 - John Snow on water supply as the carrier of the agent cholera
8. 1915 - Joseph Goldberger and Edgar Sydenstricker on pellagra as due to deficiency of niacin
9. 1920s - Bradford Hill analyze statistics and later on contributed to the development of the randomized clinical trial for assessing efficacy of therapy
10. 1937 - Harold Dorn led the First National Cancer Survey which the predecessor of the current Cancer Surveillance




Applications of Epipdemiology:
1. infectious diseases 4. drug abuse
2. chronic diseases 5. suicide health serv. Research
3. accidents & injuries 6. epidemics

Statistical association - a health related condition is statistically associated to a certain factor if the population exhibiting the health condition is either significantly higher or lower in different categories of the factor than that which could be explained by chance alone.

Causal association - an association between a health-related event and a factor wherein an alteration in the frequency or quality of the factor is followed by a change in the frequency or quality of the event. It can be direct or indirect.

Secondary association - a non-causal association where two events may be secondarily associated if they are both associated with a third factor.

Causal and secondary associations could be distinguished by using 3 criteria:
1. time sequence
2. strength of association
3. consistency with existing knowledge

Cause of a disease could either be:
1. necessary cause - a cause that must be present for the disease to occur
2. sufficient cause - a cause that inevitably brings a certain consequence

Models of disease causation:
1. epidemiologic triangle - 3 components (host, agent, environment) must be analyzed for comprehension and prediction of patterns of disease
2. lever - 3 components with environment serving the fulcrum that balance the agent and the host
3. web of causation - effects develop as a result of linked chains of causation
4. wheel - man is found at the central hub, which is affected by the 3 sectors of the environment (biological, social and physical) in its circumference
Different stages of the natural history of disease:
1. Stage of susceptibility - disease not developed but groundwork has been laid by the presence of factors that favors its occurrence
2. Stage of pre-symptomatic disease - no manifestation of the disease but pathogenic changes have started to occur
3. Stage of clinical disease - presence of recognizable signs or symptoms of disease
4. Stage of disability - presence of residual defect of disease

Levels of disease prevention:
1. Primary prevention - general health promotion and specific protective measure
2. Secondary prevention - early detection and prompt treatment
3. Tertiary prevention - limitation of disability and rehabilitation

Criteria in classifying ill or disabled persons:
1. manifestational criteria - according to similarity with respect to several simultaneously present manifestations of illness ( e.g., s/s)
2. experiential criteria - accdg to similarity with respect to a specified type of experience begun at the time preceding the illness, and often said, loose to be the cause of the disease (e.g., difficult birth, inhalation of silica dust). Internalization of a microorganism (e.g., internalization of tubercle bacillus) is a special type of experiential criterion
3. simultaneous use of criteria - it is possible to identify or group the same persons using the 2 criteria but the overlap may not always be large

Steps in the epidemiologic approach:
1. examining existing facts and hypothesis
2. formulating a new or more specific hypothesis
3. additional information to test the acceptability of the new hypothesis
4. evaluating the new evidence and deriving appropriate conclusions








Considerations in evaluating the utility of a hypothesis:
1. the value of a hypothesis is inversely related to the number of acceptable alternatives
2. the utility of making a deliberate search for specific demographic information that may be relevant to the validity of the hypothesis
3. the possibility that a hypothesis is valid even if it is not consistent with all existing observations

Different epidemiologic study designs:
1. descriptive studies - case studies, case series, ecologic studies and some cross-sectional studies
2. observational studies - some cross-sectional studies, case-control studies and cohort studies
3. experimental studies - community trials and clinical trials
























Sources of data:
1. primary data - obtained by the investigator to answer specifically the questions he has in mind
2. secondary data - actually gathered by other individuals for purposes that may only be indirectly related to the investigator's investigational problem

Different levels of causes of death in the death certificate:
1. immediate cause of death - disease or condition directly leading to death
2. intervening antecedent cause - morbid conditions giving rise to the immediate cause
3. underlying cause - the basic condition which began the train of events that lead to the death. Is all causes are indicated, this is counted as the cause of death in statistical data.

Various sources of variation in data collection:
1. lack of constancy - blood pressure of an individual
2. lack of precision - defective instrument
3. lack of congruency - different weighing scales which were not standardized
4. lack of objectivity - biased data collector

Measures of validity and reliability of a given test :
1. Sensitivity - the proportion of diseased people who were correctly identified as such by the test (TP / TP + FN)
2. Specificity - the proportion of non-diseases people who were correctly labeled negative by the test (TN / TN + FP)
3. False positive - the proportion of non-diseased people who were incorrectly identified as positive by the test (FP / TN + FP)
4. False negative - the proportion of diseased people who were incorrectly called negative by the test (FN / TP + FN)
5. Positive predictive value - the proportion of people who tested positive that actually have the disease (TP / TP + FP)
6. Negative predictive value - the proportion of people who tested negative that actually do not have the disease (TN / TN + FN)



Factors that affect the number of individuals who have the disease in the population:
1. impact of the disease - greater impact of disease leads to more cases
2. size of the population - bigger population has more cases
3. period of observation - longer period of observation leads to more cases

Adjusted Rates - single summary measures that have been adjusted for the purpose of removing the effects of difference in the population (e.g., age differences). This can be applied to both the mortality and morbidity experiences of a population.
1. direct standardization - method of obtaining adjusted rates which uses the distribution of a standard population according to the variable of adjustment. Multiply the ASDR of the population being compared with the number of the standard population to obtain the number of deaths. Then obtain the total number of deaths and divide this by the total number of the standard population.
2. indirect standardization - method of obtaining adjusted rates which uses the specific rates of a standard population. Multiply the number of the population with the ASDR of the standard population to obtain the expected number of deaths. Obtain the total number of expected deaths then divide the actual number of deaths by the total number of expected deaths.















DESCRIPTIVE EPIDEMIOLOGY

Important associations between disease conditions and person variables:
1. age - ASDR higher at extremes of age due to weaker immunity, degenerative changes, etc.
2. gender - death rates in males higher than females except in areas of poor obstetric care; some diseases more common in one gender compared to the other
3. ethnic or racial - some races more prone to certain diseases
4. religion - religious practices may predispose to or protect from disease
5. family size - bigger family size correspond to higher chances of infection
6. marital status - disease condition may affect marital status and vice versa
7. occupation - certain occupations increase the exposure or risk for certain diseases
8. nutritional state and constitution - these person variables could affect puberty, pregnancy, susceptibility and resistance

Reasons for describing a disease condition according to place:
1. association of disease with a place implies that factors of greatest etiologic importance are present either in the inhabitants or the environment or both
2. in general, infectious disease spreads more rapidly in urban than in rural areas due to population density
3. differences in large geographical areas are generally due to variations in the distribution of reservoirs or in the etiologic requirements of the agents of infectious diseases

Important concepts pertaining to the distribution of disease according to time:
1. epidemic period - period during which the number of disease are clearly in excess of the expected or usual number for that period
2. months of the year - incidence of a disease may vary depending on the month of each year
3. consecutive years - incidence of a diseases may demonstrate a pattern when considered in the light of consecutive years


Descriptive study designs - constitute the beginning of epidemiologic research by serving as basis in formulating hypothesis regarding disease causation

1. Case reports - studies that document unusual medical occurrences in individual patients. The subjects are individual patients. They serve to alert investigators on the existence of an interesting or unusual or rare occurrence of a disease, provide early information on the existence of a new disease or an early stage of an epidemic, also provide initial suggestions of a connection between disease and some particular exposure
2. Case series - collections of individual case reports, which may occur within a fairly short period of time. Its subjects are still individual patients. Same uses as the case report
3. Ecologic studies - studies that utilize measures representing characteristics of entire populations to describe disease in relation to some factor of interest . Its subjects are population or groups of people. Frequently serves as a first step in investigating a possible exposure-disease relationship, which may be suggested by a case report or series, used to evaluate the value of interventions addressed to populations.

Cross-sectional studies - a study wherein exposure status and diseases status are measured at one point in time or for a short period of time

Uses of cross-sectional studies:
1. determination of disease magnitude
2. generation of hypothesis
3. evaluation of a particular treatment or intervention
4. establishment of baseline data
5. investigation of conditions which are quantitatively measured and which may vary over time
6. investigation of relatively frequent diseases with long duration

Advantages:
1. less costly than a cohort study
2. conducted more quickly than a cohort study
3. could provide both descriptive information and data on a disease
4. could identify early stages of a disease before it becomes clinically apparent
Disadvantages:
1. measures the combined effects of incidence of diseases and duration
2. do not define temporal sequence of exposure and disease
3. sensitive to exposure bias
4. prone to observation bias

Cohort studies - also called follow-up or incidence studies begin with a group of people (cohort) free of the disease, who are classified into subgroups according to exposure to a potential cause of disease or outcome.
1. prospective cohort study - individuals included in the study are free of the outcome of interest at the time of initiation of the study. The cohort may be fixed or dynamic
2. retrospective cohort - a group or groups of individuals who free of the disease of outcome of interest at some point in the past are identified , usually by means of record in terms of their exposure level. The cohort may be fixed or dynamic.

Advantages:
1. more clearly established temporal sequence between exposure and outcome
2. direct calculation of exposure-specific incidence rates
3. can look at multiple effects of a single exposure
4. suitable for studies of rare exposures
5. certain biases are minimized especially regarding ascertainment of exposure

Disadvantages:
1. generally more expensive
2. may require long follow-up periods
3. requires large sample sizes in rare diseases
4. susceptible to follow-up bias or attrition bias

Biases of a cohort study:
1. selection bias - may occur when disease status of potential subjects may affect their inclusion in the study, particularly in retrospective cohort
2. follow-up bias - may occur if those who are lost to follow-up among exposed and unexposed have different rates of outcome and losses to follow-up are associated with both exposure and the outcome
3. misclassification bias - may occur for both exposure and outcome
4. confounding - occurs when the effects of the exposure on the outcome of interest is mixed with the third factor