General Epidemiology

General Epidemiology- PHO Loksewa

General Epidemiology

In-depth exploration of disease distribution, determinants, methods, and public health indicators.

3.1 Definition, Concepts, Approaches, Aims, and Uses

Epidemiology is the fundamental science of public health. It provides the methodological backbone for studying how diseases affect populations rather than individuals. This section breaks down the foundational aspects of the discipline.

Extensive Breakdown of Core Principles (50 Key Points)

Etymology: Derived from Greek: Epi (upon), Demos (people), and Logos (study).
Classic Definition (John M. Last): The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.
“Study”: Implies a highly quantitative, systematic, and data-driven scientific discipline.
“Distribution”: Refers to the frequency and pattern of health events in a population.
Frequency: Involves quantifying disease via numbers, rates, and risks.
Pattern: Refers to occurrence by time, place, and person (Descriptive Epidemiology).
“Determinants”: Causes and other factors that influence the occurrence of disease.
Risk Factors: Behaviors, exposures, or genetic traits associated with disease occurrence.
“Health-related states”: Includes diseases, injuries, birth defects, maternal health, and occupational health.
Wellness: Epidemiology also studies behaviors promoting positive health (e.g., seatbelt use).
“Specified populations”: The focus is on communities or groups, distinguishing it from clinical medicine (which focuses on the individual).
“Application”: The ultimate goal is public health intervention and policy-making.
Historical Origin: Hippocrates first recognized the association of disease with environment, but modern epidemiology began in the 19th century.
John Snow: The “Father of Field Epidemiology,” famous for tracing the 1854 Broad Street cholera outbreak.
William Farr: Pioneer of vital statistics, compiled mortality data in mid-1800s Britain.
Ignaz Semmelweis: Used epidemiological methods to discover handwashing reduced puerperal fever.
Doll and Hill (1950s): Landmark cohort and case-control studies linking smoking to lung cancer.
Framingham Heart Study: Prototypical longitudinal cohort study defining CVD risk factors.
Concept of Disease Causation: Moved from single-cause (germ theory) to multi-factorial etiology.
Epidemiological Triad: The traditional model for infectious disease (Agent, Host, Environment).
Web of Causation: Model for non-communicable diseases showing complex, interacting risk factors.
Wheel Model: Emphasizes genetic core surrounded by biological, physical, and social environment.
Iceberg Phenomenon: Clinical cases represent only the visible “tip” of the disease burden; hidden cases form the massive base.
Approaches – Asking Questions: Epidemiologists ask “Who, What, When, Where, Why, and How?”
Approaches – Making Comparisons: Comparing diseased vs. non-diseased, or exposed vs. unexposed.

Descriptive Approach: Generates hypotheses by organizing data by Time, Place, and Person.
Analytical Approach: Tests hypotheses using control/comparison groups to determine causation.
Aim 1: Describe: Detail the magnitude and distribution of health problems.
Aim 2: Explain: Identify etiologic factors and determinants of disease.
Aim 3: Predict: Forecast the future incidence and distribution of disease based on trends.
Aim 4: Control/Prevent: Implement strategies to prolong life and improve health status.
Morris’s 7 Uses of Epidemiology: (Points 33-39 cover these)
Use 1: Historical Study: To study the history of the health of populations (e.g., rise and fall of infectious diseases).
Use 2: Community Diagnosis: To assess the health, morbidity, and mortality of a community.
Use 3: Planning & Evaluation: To study the working of health services for improvement.
Use 4: Estimating Risks: To calculate individual risks of disease on average.
Use 5: Identifying Syndromes: To describe clinical syndromes (e.g., identifying AIDS before HIV was discovered).
Use 6: Completing Clinical Picture: To understand the natural history of a disease from onset to resolution.
Use 7: Searching for Causes: To identify causes of health and disease for prevention.
Concept – Natural History of Disease: The progression of a disease process in an individual over time, in the absence of treatment.
Stage of Susceptibility: Disease has not developed, but risk factors exist.
Stage of Subclinical Disease: Pathological changes occur without symptoms (Incubation/Latency).
Stage of Clinical Disease: Time of diagnosis and symptom onset.
Stage of Recovery, Disability, or Death: The final outcome of the disease process.
Concept – Endemic: Constant presence of a disease within a geographic area.
Concept – Epidemic: Occurrence of disease in excess of normal expectancy.
Concept – Pandemic: An epidemic spreading over multiple countries or continents.
Concept – Sporadic: Disease occurring irregularly and infrequently.
The “Person” Variable: Age, sex, race, marital status, socio-economic status, occupation.
The “Place” Variable: Natural boundaries, political boundaries, urban/rural differences.
The “Time” Variable: Secular trends, periodic changes, seasonal variations, epidemic curves.

The Epidemiological Triad

3.2 Epidemiological Methods, Incidence, Prevalence, Causation & Prevention

Comprehensive Methodology Breakdown (50 Key Points)

Observational Studies: The investigator measures but does not intervene. Includes Descriptive and Analytic.
Experimental Studies: The investigator actively assigns an intervention/exposure.
Case Report: Detailed report of symptoms, signs, and diagnosis of a single patient.
Case Series: Describes characteristics of a group of patients with a similar disease.
Ecological Study: Unit of analysis is populations, not individuals (correlational study).
Ecological Fallacy: Erroneously attributing population-level associations to individuals.
Cross-Sectional Study: “Snapshot” study. Measures exposure and outcome simultaneously.
Cross-Sectional Strength: Good for determining prevalence and generating hypotheses.
Case-Control Study: Retrospective. Starts with disease (cases) vs. no disease (controls) and looks back for exposure.
Odds Ratio (OR): The measure of association in case-control studies. (ad/bc).
Cohort Study: Prospective (usually). Starts with exposure vs. no exposure and follows forward for disease.
Relative Risk (RR): Measure of association in cohort studies. Incidence in exposed / Incidence in unexposed.
Randomized Controlled Trial (RCT): Gold standard of experimental design. Reduces confounding via randomization.
Blinding: Concealing treatment allocation to reduce observer/patient bias (Single, Double, Triple).
Incidence: Number of new cases in a population over a specific time period.
Cumulative Incidence: Proportion of a fixed population that becomes diseased.
Incidence Density (Rate): New cases divided by person-time at risk.
Incidence Use: Best measure to study disease etiology and risk.
Prevalence: Number of existing (old + new) cases at a specific point or period in time.
Point Prevalence: Disease frequency at a specific, single point in time.
Period Prevalence: Disease frequency over a defined period (e.g., a year).
P = I x D: Prevalence = Incidence × Average Duration of disease.
High Prevalence causes: High incidence, prolonged duration, improved survival (without cure).
Low Prevalence causes: High case-fatality, rapid recovery, low incidence.
Association: Statistical relationship between two variables. Can be spurious, indirect, or direct.

Spurious Association: False association often due to selection bias or chance.
Indirect Association: Due to a confounding variable (e.g., coffee drinking and lung cancer, confounded by smoking).
Confounding: A third variable associated with both exposure and outcome, not on the causal pathway.
Direct (Causal) Association: Altering the exposure directly alters the outcome.
Bradford Hill Criteria: Guidelines for assessing causation in epidemiology.
Temporality (Hill): The exposure MUST precede the disease (the only absolute criterion).
Strength (Hill): High relative risk/odds ratio indicates stronger likelihood of causation.
Consistency (Hill): Repeated observation of association in different populations under different circumstances.
Specificity (Hill): One cause leads to one specific effect (often invalid for NCDs).
Biological Gradient (Hill): Dose-response relationship; more exposure = higher risk.
Plausibility (Hill): Biological mechanism exists to explain the association.
Coherence (Hill): Does not conflict with generally known facts of the natural history of the disease.
Experiment (Hill): Reversibility; removing the exposure reduces the risk.
Analogy (Hill): Similar exposures cause similar effects.
Levels of Prevention: Interventions applied at different stages of the disease’s natural history.
Primordial Prevention: Preventing the emergence of risk factors in a population (e.g., banning tobacco advertising).
Primordial Target: Directed at underlying health determinants (social, economic, environmental).
Primary Prevention: Action taken prior to disease onset to remove the possibility of disease occurrence.
Primary Methods: Health promotion (education) and Specific protection (immunization).
Secondary Prevention: Action that halts the progress of a disease at its incipient stage.
Secondary Methods: Early diagnosis (Screening) and prompt treatment (Pap smears, mammograms).
Secondary Goal: Prevents complications and reduces prevalence by shortening duration.
Tertiary Prevention: Interventions in late clinical stage to reduce impairments and disabilities.
Tertiary Methods: Disability limitation and Rehabilitation (Physical, psychological, vocational).
Population Strategy vs High-Risk Strategy: Shifting the whole population distribution curve vs focusing only on those at highest clinical risk.

Feature	Incidence	Prevalence
Numerator	Number of new cases	Number of existing (old + new) cases
Denominator	Population at risk (disease-free at start)	Total population (sick + healthy)
Focus	Whether the event is a new occurrence (Risk)	Presence or absence of a disease (Burden)
Best for	Acute diseases, studying etiology	Chronic diseases, healthcare planning

3.3 Communicable & Non-Communicable Diseases (NCDs)

Extensive Concepts (50 Key Points)

Communicable Disease: Illness caused by an infectious agent or its toxins, transmitted from infected host to susceptible host.
Agent Factors: Biological properties of the pathogen (bacteria, virus, fungi, parasite, prion).
Infectivity: Ability of agent to invade and multiply in host.
Pathogenicity: Ability of agent to produce clinical disease in infected hosts.
Virulence: Severity of the disease produced (measured by case-fatality rate).
Antigenicity/Immunogenicity: Ability to induce a specific immune response.
Host Factors: Demographic, biological, and behavioral determinants determining susceptibility.
Environmental Factors: Physical (climate), biological (vectors), and social (crowding) surroundings.
Chain of Infection: A cyclical sequence of events required for disease transmission.
Link 1: Infectious Agent: The pathogen itself.
Link 2: Reservoir: Habitat where agent normally lives, grows, and multiplies (humans, animals, environment).
Link 3: Portal of Exit: Path by which agent leaves reservoir (respiratory, GI, blood).
Link 4: Mode of Transmission: How the agent is conveyed (direct contact, droplets, vectors).
Link 5: Portal of Entry: Path agent enters susceptible host (mucous membranes, skin breaks).
Link 6: Susceptible Host: Individual lacking immunity or resistance to the agent.
Breaking the Chain: Public health interventions aim to break at least one link to stop outbreaks.
Host Defenses: Skin, mucous membranes, gastric acid, cilia, cellular immunity, humoral immunity.
Active Immunity: Host produces own antibodies (via infection or vaccination). Long-lasting.
Passive Immunity: Pre-formed antibodies transferred to host (maternal to fetal, or immunoglobulins). Short-lasting.
NCD Definition: Non-infectious, chronic medical conditions progress slowly over a long duration.
Epidemiologic Transition: Shift in morbidity/mortality from infectious to non-communicable diseases.
Causes of Transition: Aging populations, urbanization, lifestyle changes, improved infectious control.
The “Big Four” NCDs: Cardiovascular diseases, Cancers, Chronic respiratory diseases, Diabetes.
Shared Modifiable Risk Factors (NCDs): Tobacco use, unhealthy diet, physical inactivity, harmful use of alcohol.
Metabolic Risk Factors: Raised blood pressure, raised blood glucose, raised cholesterol, overweight/obesity.

Non-Modifiable Risk Factors (NCDs): Age, sex, genetics/family history.
Web of Causation: Concept highly applicable to NCDs; diseases have complex, interconnected roots, not a single ‘bug’.
Synergism in NCDs: Multiple risk factors occurring together multiply the overall risk (e.g., smoking + asbestos for lung cancer).
NCD Burden: Accounts for over 70% of all deaths globally.
Premature Mortality: High concern in NCDs is death occurring before age 70.
DOHaD Concept: Developmental Origins of Health and Disease; early life/fetal exposures influence adult NCD risk.
Secondary Prevention for NCDs: Massive reliance on mass and targeted screening (BP, blood sugar, mammograms).
Tertiary Prevention for NCDs: Essential to prevent amputations in diabetes, strokes in hypertension.
WHO “Best Buys”: Cost-effective public health interventions for NCDs (e.g., taxing tobacco, reducing salt intake).
Mental Health: Often grouped with NCDs; heavily influenced by social determinants.
Injuries: Another major non-communicable public health issue (RTAs, falls, self-harm).
Double Burden of Disease: Developing nations suffering from both infectious diseases AND rising NCDs simultaneously.
Surveillance differences: Communicable uses rapid/syndromic surveillance; NCDs rely on registries and population surveys (e.g., STEPS).
Control of Communicable: Focus on isolation, quarantine, contact tracing, vaccination, sanitation.
Control of NCDs: Focus on policy, lifestyle modification, chronic care models, environment design.
Reservoir vs Source: Reservoir is the natural habitat; source is the specific object/person from which agent is immediately transferred.
Zoonosis: Infection transmissible under natural conditions from vertebrate animals to humans (e.g., Rabies).
Epizootic: An epidemic in an animal population.
Enzootic: Endemic disease in an animal population.
Eradication: Complete global termination of the disease agent (e.g., Smallpox).
Elimination: Reduction to zero of incidence of disease in a defined geographic area.
Control: Reducing disease incidence/prevalence to locally acceptable levels.
Notification: Statutory requirement to report specific infectious diseases to public health authorities.
Isolation: Separation of infected persons.
Quarantine: Restriction of activities of exposed, healthy persons for the duration of the maximum incubation period.

Flowchart: The Chain of Infection

Infectious Agent

Reservoir

Portal of Exit

Mode of Transmission

Portal of Entry

Susceptible Host

3.4 Dynamics of Disease Transmission

Modes, Timelines, & Immunity Concepts (50 Key Points)

Clinical Case: Individual with clinical disease and apparent symptoms.
Subclinical Case: Individual with infection but no overt clinical signs (diagnosed via lab).
Latent Infection: Agent is present but not multiplying actively; no symptoms.
Carrier: Infected person harboring agent without symptoms, but capable of transmitting to others.
Incubatory Carrier: Sheds agent during incubation period before symptoms start (e.g., Measles).
Convalescent Carrier: Sheds agent during recovery phase (e.g., Salmonella, Diphtheria).
Healthy Carrier: Experiences subclinical infection but sheds agent (e.g., “Typhoid Mary”).
Chronic Carrier: Sheds agent for long periods, sometimes lifetime (e.g., Hepatitis B).
Direct Transmission: Immediate transfer of infectious agent.
Direct Contact: Skin-to-skin, kissing, sexual intercourse (e.g., STIs, Scabies).
Droplet Spread: Large, short-range aerosols (<1 meter) from coughing/sneezing (e.g., Pertussis).
Contact with Soil: Direct exposure to environmental reservoirs (e.g., Hookworm, Tetanus).
Bite/Inoculation: Rabies bite.
Transplacental/Vertical: Mother to fetus during pregnancy or childbirth (e.g., Syphilis, HIV).
Indirect Transmission: Requires a vehicle or vector.
Vehicle-borne: Transmission via inanimate objects/substances (Water, Food, Blood).
Fomites: Inanimate objects harboring agents (doorknobs, clothing, surgical instruments).
Airborne Transmission: Dissemination of droplet nuclei (residues of dried droplets <5 microns) or dust.
Airborne distances: Can remain suspended in air for long periods and travel long distances (e.g., TB, Measles).
Vector-borne Transmission: By living organisms (usually arthropods like mosquitoes, ticks).
Mechanical Vector: Simple physical carriage on vector’s body/legs (e.g., flies carrying cholera).
Biological Vector: Agent undergoes multiplication or developmental changes inside the vector.
Propagative (Vector): Agent multiplies inside vector but doesn’t change form (e.g., Plague in fleas).
Cyclo-developmental (Vector): Agent changes form but doesn’t multiply (e.g., Filaria in mosquitoes).
Cyclo-propagative (Vector): Agent both multiplies and changes developmental form (e.g., Malaria in Anopheles).

Incubation Period: Time interval from initial infection to onset of clinical symptoms.
Latency Period (Infectiousness): Time from infection to onset of infectiousness.
Generation Time: Time interval between receipt of infection by a host and maximal infectivity of that host.
Serial Interval: Time gap between symptom onset in primary case and symptom onset in secondary case.
Communicable Period: Time during which an infectious agent may be transferred directly/indirectly.
Basic Reproduction Number (R0): Average number of secondary cases produced by one infected individual in a totally susceptible population.
R0 > 1: Disease will spread into an epidemic.
R0 = 1: Disease will become endemic.
R0 < 1: Disease will eventually die out.
Effective Reproduction Number (R): R0 adjusted for the actual proportion of susceptible individuals in the population.
Herd Immunity: Resistance of a group/community to an infectious agent based on high proportion of immune individuals.
Herd Immunity Threshold: Percentage of population that needs immunity to stop spread. Formula: 1 – (1/R0).
Highly Contagious (e.g., Measles R0~15): Requires ~94% herd immunity.
Mechanisms of Herd Immunity: Interrupts the chain of transmission; protects those unable to be vaccinated.
Exceptions to Herd Immunity: Diseases not transmitted person-to-person (e.g., Tetanus from soil).
Secondary Attack Rate (SAR): Number of exposed contacts who develop disease / Total number of exposed contacts.
SAR usefulness: Best measure of the infectivity/transmissibility of a disease.
Source of Infection vs Reservoir: Contaminated food (source) vs. Animal GI tract (reservoir).
Asymptomatic Transmission: Major challenge in public health (e.g., COVID-19, Polio).
Seasonality: Many diseases have predictable temporal transmission patterns (e.g., Flu in winter, Malaria post-monsoon).
Socio-behavioral dynamics: Overcrowding, migration, and cultural practices dramatically alter transmission rates.
Iatrogenic transmission: Caused by medical examination or treatment (e.g., unsterile needles).
Nosocomial Infection: Hospital-acquired infection (appears >48 hours after admission).
Window Period: Time between infection and when a test can reliably detect that infection (e.g., HIV antibodies).
Extrinsic Incubation Period: Time taken for pathogen to develop inside the vector before it becomes infective.

3.5 Investigation of an Epidemic

Outbreak Steps & Epicurves (50 Key Points)

Outbreak vs Epidemic: Often used interchangeably, but outbreak usually refers to a localized area.
Step 1: Prepare for Field Work: Assemble team, supplies, lab equipment, and establish administrative rules.
Step 2: Establish Existence of Outbreak: Compare observed cases with expected cases (using surveillance data).
Step 3: Verify the Diagnosis: Review clinical findings and laboratory results to ensure accuracy.
Step 4: Define and Identify Cases: Create a standard working case definition.
Case Definition Components: Clinical criteria, limitations of Time, Place, and Person.
Confirmed Case: Laboratory confirmed.
Probable Case: Typical clinical features but no lab confirmation.
Possible/Suspect Case: Fewer typical clinical features.
Step 5: Perform Descriptive Epidemiology: Characterize outbreak by Time, Place, and Person.
Spot Map: Geographical plotting of cases to identify clusters (Place).
Epidemic Curve: Histogram of cases by time of onset of illness (Time).
Step 6: Develop Hypotheses: Formulate theories on source, agent, and mode of transmission.
Step 7: Evaluate Hypotheses: Compare hypotheses with established facts, or conduct analytical studies.
Analytical Study in Outbreak: Often a Case-Control study (if illness is rare) or Retrospective Cohort (if population is defined, e.g., a wedding feast).
Step 8: Refine Hypotheses / Additional Studies: Environmental investigations or lab testing of food/water.
Step 9: Implement Control and Prevention Measures: Primary goal! Often done concurrently with other steps.
Control measure targets: Destroy reservoir, interrupt transmission, protect susceptible host.
Step 10: Communicate Findings: Written report for health authorities, oral briefings, public messaging.
Point-Source Epidemic: Exposure of group to a common source simultaneously (e.g., contaminated food at a picnic).
Point-Source Curve Shape: Sharp upward slope, single distinct peak, gradual downward slope.
Point-Source Duration: All cases fall within one incubation period of the disease.
Continuous Common-Source: Exposure is prolonged over days/weeks (e.g., contaminated municipal water supply).
Continuous Curve Shape: Rapid rise followed by a plateau rather than a sharp peak.
Propagated Epidemic: Person-to-person transmission (e.g., Measles, COVID-19).

Propagated Curve Shape: Series of progressively taller peaks, roughly one incubation period apart.
Index Case: The very first case that comes to the attention of the investigator.
Primary Case: The person who first introduces the disease into the population.
Secondary Case: People infected by the primary case.
Identifying Source via Epicurve: Look backward from peak by median incubation period.
Line Listing: A rectangular database table where each row is a case, columns are variables (symptoms, demographics).
Attack Rate: (Sick / Total Exposed) × 100. Actually a proportion, not a true rate.
Food-Specific Attack Rate: Crucial for foodborne outbreaks to identify the culprit dish.
Case Fatality Rate (CFR): (Deaths from disease / Total cases of disease) × 100. Measures virulence.
Proportional Mortality Ratio: (Deaths from specific cause / Total deaths from all causes).
Surveillance: Continuous, systematic collection, analysis, and interpretation of health data.
Active Surveillance: Health departments proactively contact providers/labs for case reports.
Passive Surveillance: Routine reporting by providers (cheaper, but under-reporting is common).
Sentinel Surveillance: Using selected institutions/groups to monitor trends (e.g., specific STD clinics).
Syndromic Surveillance: Tracking symptom clusters (e.g., sales of cold medicine) before clinical diagnosis.
False Alarm: Epidemic exists artificially due to changes in reporting, new lab tests, or population influx.
Recall Bias in Outbreaks: Sick people are more likely to remember strange foods they ate than healthy controls.
Cluster vs Outbreak: Cluster is aggregation of cases in time/space without necessarily more than expected; outbreak implies an excess.
Environmental sampling constraints: Agents might have cleared from food/water by the time investigation starts.
Role of Public Health Lab: Critical for agent identification, serotyping, and genomic sequencing.
Genomic Epidemiology: Using pathogen DNA/RNA to trace exact transmission pathways.
Contact Tracing: Identifying and isolating individuals exposed to a confirmed case.
Ring Vaccination: Vaccinating all contacts around a confirmed case (used in Smallpox eradication).
Post-Exposure Prophylaxis (PEP): Treatment given after exposure to prevent disease (e.g., Rabies vaccine).
Morbidity vs Mortality: Investigation aims to reduce morbidity (disease) and mortality (death).

3.6 Screening of Diseases

Criteria, Validity, & Bias (50 Key Points)

Screening Definition: Presumptive identification of unrecognized disease in apparently healthy, asymptomatic individuals.
Purpose: Sort out those who probably have the disease from those who probably do not.
Diagnostic Test vs Screening: Screening is applied to asymptomatic groups; diagnosis is applied to individuals with symptoms (or positive screens) to confirm.
Mass Screening: Screening whole population (or large groups) regardless of risk status.
High-Risk (Targeted) Screening: Screening only groups with known high risk (e.g., BRCA mutation testing for specific families).
Multiphasic Screening: Using two or more screening tests together.
Wilson & Jungner Criteria (1968): The gold standard rules for deciding if screening is appropriate.
Criterion 1 (Disease): Condition must be an important public health problem.
Criterion 2 (Disease): There must be a recognizable latent or early symptomatic stage.
Criterion 3 (Disease): Natural history of the condition must be adequately understood.
Criterion 4 (Treatment): An accepted, effective treatment must exist for patients with recognized disease.
Criterion 5 (Treatment): Facilities for diagnosis and treatment should be available.
Criterion 6 (Treatment): There should be an agreed policy on whom to treat as patients.
Criterion 7 (Test): There must be a suitable test or examination.
Criterion 8 (Test): The test must be acceptable to the population (non-invasive, painless).
Criterion 9 (Economic): Cost of screening/diagnosis must be economically balanced against overall medical costs.
Criterion 10 (Process): Screening should be a continuous process, not a “once and for all” project.
Evaluation of Screening: Based on Validity, Reliability, and Yield.
Validity (Accuracy): Ability of the test to separate those who have disease from those who don’t.
Sensitivity: Ability to correctly identify those WITH the disease (True Positives / All Diseased).
Specificity: Ability to correctly identify those WITHOUT the disease (True Negatives / All Healthy).
False Negative (FN): Sick person wrongly told they are healthy. (Tragic, delays treatment).
False Positive (FP): Healthy person wrongly told they are sick. (Causes anxiety, unnecessary procedures).
Sensitivity vs Specificity Trade-off: Lowering the cutoff point increases sensitivity but decreases specificity (and vice versa).
Predictive Value Positive (PPV): Proportion of positive tests that are truly diseased (TP / All Positives).

Predictive Value Negative (NPV): Proportion of negative tests that are truly healthy (TN / All Negatives).
Prevalence Effect: PPV and NPV depend heavily on disease prevalence. Sensitivity/Specificity do NOT.
High Prevalence: Increases PPV, decreases NPV.
Low Prevalence: Decreases PPV (most positives will be false positives).
Reliability (Precision/Reproducibility): Ability of a test to give consistent results on repeated trials.
Factors affecting reliability: Biological variation (BP changes by time of day), observer error, technical error of instrument.
Kappa Statistic: Measures agreement between two observers, adjusting for agreement due to chance.
Yield: The amount of previously unrecognized disease diagnosed and treated as a result of screening.
Lead Time: The time between early detection by screening and the time clinical diagnosis would have normally occurred.
Lead Time Bias: Survival appears longer in screened patients merely because diagnosis was made earlier, not because death was delayed.
Length Bias (Prognostic Bias): Screening tends to disproportionately detect slow-growing, less aggressive forms of a disease (because they have a longer latent period).
Selection Bias (Volunteer Bias): People who volunteer for screening tend to be healthier and more health-conscious than those who don’t.
Overdiagnosis Bias: Screening detects pseudodisease—abnormalities that meet diagnostic criteria but would never cause symptoms or death in the patient’s lifetime.
Overdiagnosis harm: Leads to overtreatment (surgery, radiation for harmless lesions).
Parallel Testing: Doing multiple tests at once; if ANY is positive, patient is considered positive. Increases Sensitivity.
Series Testing: Doing tests sequentially; test 2 only done if test 1 is positive. Increases Specificity.
Gold Standard: The definitive diagnostic test to which the screening test is compared (e.g., biopsy for cancer).
Screening Examples: Pap smear (Cervical cancer), Mammogram (Breast cancer), Colonoscopy/FOBT (Colorectal cancer), TSH (Congenital hypothyroidism).
Opportunistic Screening: Offering screening when a patient presents to healthcare for an unrelated reason (e.g., checking BP at a dental clinic).
Ethical constraints: Do not screen if no treatment exists (e.g., untreatable Huntington’s, unless for family planning).
Borderline cases: The threshold for “abnormal” is often an arbitrary statistical cutoff point in continuous variables (like BP or blood sugar).
Cost-benefit analysis in screening: Evaluating money saved by preventing advanced disease vs. money spent on mass testing and false-positive workups.
Screening interval: Time between tests (e.g., 3 years for Pap, 10 years for colonoscopy) depends on disease natural history.
Interval Cancers: Cancers that arise and become symptomatic between scheduled screening rounds (often highly aggressive).
Number Needed to Screen (NNS): How many people must be screened to prevent one death or adverse event.

Standard 2×2 Table for Test Evaluation

	Disease Present (Gold Standard)	Disease Absent (Gold Standard)	Total
Test Positive	True Positive (a)	False Positive (b)	a + b
Test Negative	False Negative (c)	True Negative (d)	c + d
Total	a + c	b + d	N (Total Pop)

Sensitivity = a/(a+c) | Specificity = d/(b+d) | PPV = a/(a+b) | NPV = d/(c+d)

3.7 Health Indicators

Categories & Metrics (50 Key Points)

Health Indicator Definition: Variables that help measure changes directly or indirectly in a given health situation.
Purpose: To compare health status between countries/regions, assess needs, allocate resources, monitor progress.
Ideal Indicator – Valid: Measures what it is supposed to measure.
Ideal Indicator – Reliable: Objective and gives the same results when measured by different people.
Ideal Indicator – Sensitive: Changes quickly when the situation changes.
Ideal Indicator – Specific: Reflects changes only in the situation it is concerned with.
Ideal Indicator – Feasible: Easy and inexpensive to measure with available data.
Ideal Indicator – Relevant: Contributes to understanding the phenomenon of interest.
Mortality Indicators: Traditional, widely used because death is a definite event and legally registered.
Crude Death Rate (CDR): Deaths per 1,000 mid-year population. (Lacks comparability due to age structures).
Life Expectancy: Average number of years a person is expected to live based on current mortality rates. (Best overall indicator of health status).
Infant Mortality Rate (IMR): Deaths of infants <1 year per 1,000 live births. Highly sensitive indicator of socio-economic development.
Child Mortality Rate: Deaths in children 1-4 years per 1,000 children in that age group. (Reflects nutrition/environmental sanitation).
Under-5 Mortality Rate: Deaths <5 years per 1,000 live births. Key WHO/UNICEF metric.
Maternal Mortality Ratio (MMR): Maternal deaths per 100,000 live births. Indicates quality of obstetric care.
Disease-Specific Mortality: Deaths from specific diseases per 100,000 population.
Morbidity Indicators: Describe the burden of illness, since mortality doesn’t capture chronic suffering.
Incidence Rate: Indicates outbreaks and risk.
Prevalence Rate: Indicates overall burden on health system.
Attendance Rates: Out-patient visits per capita.
Disability Rates: Measures the impact of disease on functioning.
Event-Type Disability: Number of days restricted activity, bed disability days, work-loss days.
Person-Type Disability: Limitation of mobility or limitation of major activity.
Sullivan’s Index: Life expectancy minus probable duration of bed disability and inability to perform major activities.
HALE (Health-Adjusted Life Expectancy): Equivalent number of years expected to live in full health.

DALY (Disability-Adjusted Life Year): One DALY = One lost year of “healthy” life. (YLL + YLD).
YLL (Years of Life Lost): Due to premature mortality.
YLD (Years Lost due to Disability): Weighted by severity of the condition.
Nutritional Indicators: Key for pediatric and developing-world health.
Anthropometrics: Weight-for-age, Height-for-age (Stunting), Weight-for-height (Wasting).
Low Birth Weight (LBW) %: Percentage of babies born <2500g. Indicates maternal malnutrition.
Health Care Delivery Indicators: Measure provision of services.
Doctor-Population Ratio: Usually expressed as 1 doctor per X population.
Bed-Population Ratio: Hospital beds per 1,000 population.
Utilization Rates: Measure actual use of services, not just availability.
Immunization Coverage: % of children fully immunized.
ANC Coverage: % of pregnant women receiving >4 Antenatal care visits.
Institutional Delivery %: Births occurring in health facilities vs home.
Social and Mental Health Indicators: Indirect measures of societal health.
Examples of Social Indicators: Suicide rates, homicide rates, substance abuse rates, domestic violence incidence.
Environmental Indicators: Measure physical surroundings impacting health.
Water and Sanitation: % of population with access to safe drinking water and adequate sanitary facilities.
Air Quality: Levels of PM2.5 or PM10 in urban areas.
Socio-economic Indicators: Direct determinants of population health.
GDP/GNP per capita: Gross Domestic Product.
Literacy Rate: Specifically female literacy, which strongly correlates with lower IMR.
Family Size/Dependency Ratio: Ratio of dependents (children + elderly) to working-age population.
Health Policy Indicators: % of GNP spent on healthcare.
Quality of Life Indicators: Subjective well-being. Includes PQLI and HDI.
Physical Quality of Life Index (PQLI): Consolidates IMR, life expectancy at age 1, and literacy (Scale 0-100).
Human Development Index (HDI): Combines life expectancy at birth, education index, and GNI per capita (Scale 0-1).

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