NLE Community Health Nursing — Epidemiology & BiostatisticsRevision Notes

Exam Tips

• When an NLE item asks 'what does epidemiology study?' — look for the words distribution, determinants, and populations in the options.
• Items framed as 'The nurse is conducting a community diagnosis' or 'analyzing mortality data' are testing your epidemiologic knowledge.
• Person-place-time is always the correct answer when asked how to DESCRIBE an outbreak.

Key Points

• Epidemiology is defined as the study of the DISTRIBUTION and DETERMINANTS of health-related states or events in POPULATIONS, and the application of this study to the CONTROL of health problems.
• The community health nurse uses epidemiology to identify who is sick (person), where disease occurs (place), and when it occurs (time) — the classic epidemiologic triad of description.
• PERSON variables include age, sex, occupation, ethnicity, lifestyle, and socioeconomic status.
• PLACE variables include geographic location, rural vs. urban settings, proximity to water sources, and environmental conditions.
• TIME variables include seasonality (e.g., dengue peaks during rainy season in the Philippines), secular trends, and epidemic periods.
• Purposes of epidemiology: (1) describe community health status, (2) identify causes and risk factors, (3) evaluate health programs and interventions, (4) guide health policy and resource allocation.
• Epidemiology shifts focus from the INDIVIDUAL (clinical nursing) to the POPULATION (community nursing) — this philosophical shift is tested in NLE item stems.

Definitions

Section Title

1. Epidemiology: Definition, Purpose, and Descriptive Framework

Common Mistakes

• Confusing epidemiology (population-focused) with clinical medicine (individual-focused).
• Forgetting that 'person, place, time' is the framework for DESCRIBING a health event — not for explaining it.
• Mixing up 'distribution' (descriptive — who, where, when) and 'determinants' (analytic — why and how).

Exam Tips

• NLE items often ask: 'The nurse conducts fogging — this addresses which component of the epidemiologic triangle?' Answer: ENVIRONMENT (vector control).
• Immunization addresses the HOST component (increases host resistance/immunity).
• Treating a patient with antibiotics addresses the AGENT (reducing infectious load).
• Philippine endemic diseases and their vectors are high-yield: dengue = Aedes aegypti; malaria = Anopheles; schistosomiasis = Oncomelania snail; filariasis = Culex mosquito.

Key Points

• The EPIDEMIOLOGIC TRIANGLE (also called the ecologic triangle) explains communicable disease causation through THREE interacting elements: AGENT, HOST, and ENVIRONMENT.
• AGENT: the biologic (bacteria, virus, parasite), chemical (toxins, pollutants), or physical (radiation, trauma) cause of disease. Key agent factors: infectivity, pathogenicity, virulence, and antigenicity.
• HOST: the susceptible human or animal. Key host factors: age, sex, genetic makeup, nutritional status, immune status, and health behavior.
• ENVIRONMENT: external conditions that facilitate or inhibit agent–host interaction. Includes physical (climate, geography), biologic (vectors, reservoir animals), and social (crowding, sanitation, socioeconomic) factors.
• A VECTOR (e.g., Aedes aegypti mosquito for dengue, Anopheles mosquito for malaria, snail Oncomelania for schistosomiasis) is often added as the vehicle carrying the agent to the host.
• Disease occurs when the triangle is IN IMBALANCE — i.e., the agent is virulent, the host is susceptible, and the environment is favorable for transmission.
• Breaking ANY point of the triangle interrupts disease — the basis of all infection control measures (e.g., treating the agent with antibiotics, immunizing the host, improving sanitation in the environment).
• The WEB OF CAUSATION (Brian MacMahon's model) explains NON-COMMUNICABLE diseases (e.g., hypertension, diabetes) by recognizing MULTIPLE interacting risk factors rather than a single agent — more appropriate for chronic disease epidemiology.
• In the Philippine context: dengue (Aedes aegypti vector), malaria (Anopheles vector, endemic in Palawan and parts of Mindanao), and schistosomiasis (snail vector, endemic in Leyte, Mindanao) are classic triangle examples.

Definitions

Section Title

2. The Epidemiologic Triangle and Web of Causation

Common Mistakes

• Applying the epidemiologic triangle to NON-COMMUNICABLE diseases — use the web of causation instead.
• Forgetting the VECTOR as an additional component of the triangle in vector-borne diseases.
• Confusing PATHOGENICITY (ability to cause any disease) with VIRULENCE (ability to cause severe disease).
• Not linking nursing interventions to which point of the triangle they address (e.g., fogging targets the ENVIRONMENT/VECTOR, immunization targets the HOST).

Exam Tips

• Memorize the 6 links in order: Agent → Reservoir → Portal of Exit → Transmission → Portal of Entry → Host.
• NLE favorite question: 'The nurse isolates a TB patient — which link is being broken?' Answer: RESERVOIR (preventing agent escape) or PORTAL OF EXIT.
• For TB: Agent = M. tuberculosis; Reservoir = infected human; Portal of Exit = respiratory tract (coughing); Transmission = AIRBORNE; Portal of Entry = respiratory tract; Host = immunocompromised individual.
• Immunization (e.g., BCG for TB, MMR for measles) targets Link 6 — SUSCEPTIBLE HOST.

Key Points

• The CHAIN OF INFECTION describes the sequential links by which a communicable disease spreads from one person to another. There are SIX links.
• Link 1 — INFECTIOUS/ETIOLOGIC AGENT: the pathogen (e.g., Mycobacterium tuberculosis for TB, dengue virus for dengue).
• Link 2 — RESERVOIR: the place where the agent lives and multiplies (human reservoir: TB patient; animal reservoir: rabid dog; environmental reservoir: soil for tetanus).
• Link 3 — PORTAL OF EXIT: how the agent leaves the reservoir (respiratory tract — coughing/sneezing for TB; GI tract — feces for cholera/typhoid; blood — HIV, hepatitis B; skin/mucosa — wound secretions).
• Link 4 — MODE OF TRANSMISSION: how the agent travels to the new host. Direct contact (touching, sexual contact, droplet spray within 1 meter); Indirect contact (fomites, vehicle-borne via contaminated food/water/blood, vector-borne); Airborne (droplet nuclei traveling >1 meter, e.g., TB measles).
• Link 5 — PORTAL OF ENTRY: how the agent enters the new host (respiratory tract, GI tract, genitourinary tract, skin break, mucous membranes, transplacental).
• Link 6 — SUSCEPTIBLE HOST: a person lacking adequate immunity or resistance (determined by age, nutritional status, immune status, genetic factors).
• Nursing interventions are designed to BREAK THE CHAIN at any link: handwashing breaks transmission (Link 4); immunization reduces host susceptibility (Link 6); isolation reduces reservoir exposure (Link 2).
• Standard precautions in healthcare settings target multiple links simultaneously.

Definitions

Section Title

3. Chain of Infection

Common Mistakes

• Confusing DROPLET transmission (>5 microns, short range, surgical mask) with AIRBORNE transmission (<5 microns, long range, N95 mask, negative pressure).
• Forgetting that RESERVOIR ≠ source of infection always; the reservoir is where the agent normally LIVES, not necessarily where infection comes from in each case.
• Missing that HANDWASHING primarily breaks Link 4 (mode of transmission) and Link 3 (portal of exit/contamination).
• Confusing portal of EXIT with portal of ENTRY — they can be different organ systems.

Exam Tips

• Classic NLE trap: A disease affecting many people in a community where it is always present = ENDEMIC, not epidemic.
• One confirmed case of polio in the Philippines = EPIDEMIC (because the expected number is zero — Philippines was certified polio-free).
• COVID-19 = pandemic; dengue outbreaks in a barangay = epidemic/outbreak; malaria in Palawan = endemic.

Key Points

• SPORADIC: a disease that occurs occasionally, irregularly, and without predictable pattern (e.g., a few isolated cases of rabies scattered across different provinces with no temporal connection).
• ENDEMIC: the CONSTANT or USUAL presence of a disease at an EXPECTED level in a given geographic area or population group (e.g., malaria is endemic in Palawan; schistosomiasis is endemic in Leyte and parts of Mindanao).
• EPIDEMIC (OUTBREAK): the occurrence of cases CLEARLY IN EXCESS OF THE EXPECTED number in a community or region over a given period. Note: even 2–3 cases of a rare disease (e.g., polio) may constitute an epidemic.
• PANDEMIC: an epidemic that has spread across several COUNTRIES or CONTINENTS, affecting a large number of people (e.g., COVID-19 pandemic of 2020; 1918 influenza pandemic).
• HYPERENDEMIC: persistently high levels of disease occurrence, higher than in neighboring areas.
• HOLOENDEMIC: high level of disease occurrence beginning early in life; most of the population has been exposed.
• NATURAL HISTORY OF DISEASE follows two stages: (1) PREPATHOGENESIS — before disease begins; the person is susceptible but not yet infected; (2) PATHOGENESIS — from initial infection through subclinical (inapparent) stage, clinical stage, and resolution (recovery, disability, or death).
• INCUBATION PERIOD: the time between exposure to the agent and the appearance of first symptoms (important for quarantine/isolation duration).
• LATENT PERIOD: the time from infection to becoming infectious (relevant for communicable disease control).

Definitions

Section Title

4. Levels of Disease Occurrence and Natural History of Disease

Common Mistakes

• Equating 'epidemic' with 'many people sick' — epidemic is defined by EXCESS OVER EXPECTED, not by absolute numbers.
• Confusing ENDEMIC (constant presence) with EPIDEMIC (excess above expected).
• Forgetting that PANDEMIC is a geographic/spread descriptor, not a severity descriptor — a pandemic can have low mortality.

Exam Tips

• Memory tip: 1°Prevention = BEFORE (protect); 2°Prevention = DETECT (screen); 3°Prevention = DEFECT (rehabilitate).
• NLE item: 'The nurse teaches a hypertensive patient to take medications regularly to prevent stroke' = SECONDARY prevention (preventing complications = disability limitation).
• NLE item: 'The nurse administers BCG vaccine to a newborn' = PRIMARY prevention (specific protection).
• NLE item: 'The nurse refers a stroke patient to physiotherapy' = TERTIARY prevention (rehabilitation).
• NLE item: 'The nurse conducts Pap smear screening' = SECONDARY prevention (early diagnosis).

Key Points

• Prevention is mapped onto the NATURAL HISTORY OF DISEASE and occurs at three levels: PRIMARY, SECONDARY, and TERTIARY.
• PRIMARY PREVENTION targets the PREPATHOGENESIS stage — before disease begins. Goal: prevent disease occurrence entirely. Activities: (a) Health Promotion (general measures not specific to a disease — nutrition, sanitation, health education, exercise) and (b) Specific Protection (targeted to a specific disease — immunization, use of helmets, fluoridation of water, chemoprophylaxis for malaria).
• SECONDARY PREVENTION targets EARLY PATHOGENESIS — disease has started but may be subclinical. Goal: detect early and treat promptly to prevent progression. Activities: Early Diagnosis and Prompt Treatment (screening programs, case-finding, contact tracing) and Disability Limitation (treatment before complications occur).
• TERTIARY PREVENTION targets LATE PATHOGENESIS / RESIDUAL DEFECT — disease has caused damage. Goal: minimize disability and restore function. Activities: Rehabilitation (physical, occupational, vocational, social therapy) and Disability Limitation at the clinical stage.
• In the Philippine healthcare system, the Rural Health Unit (RHU) and Barangay Health Centers primarily deliver PRIMARY and SECONDARY prevention. Tertiary hospitals manage tertiary prevention.
• The Expanded Program on Immunization (EPI) is the flagship primary prevention program in the Philippines.
• Screening (secondary prevention) examples in the Philippines: newborn screening (RA 9288), cervical cancer screening (IVA/Pap smear), Philhealth TB-DOTS program.

Definitions

Section Title

5. Levels of Prevention

Common Mistakes

• Placing immunization at SECONDARY prevention — immunization is PRIMARY (specific protection) because it prevents disease BEFORE it occurs.
• Confusing disability limitation (which can be both secondary and tertiary) — at secondary level it prevents complications from early disease; at tertiary level it manages existing disability.
• Placing rehabilitation at secondary prevention — rehabilitation is TERTIARY.
• Saying treatment is ONLY tertiary — treatment can be primary (prophylaxis), secondary (prompt treatment of early disease), or tertiary (managing chronic disability).

Exam Tips

• If an NLE item asks about the national disease reporting system in the Philippines = PIDSR.
• If asked about routine community health program data (immunization rates, prenatal visits) = FHSIS.
• The community nurse's role in surveillance includes: case finding, timely reporting, contact tracing, and documentation.
• Know the two surveillance types: Passive (routine, everyday, lower sensitivity) vs. Active (deliberate case-finding, higher sensitivity, used in outbreaks).

Key Points

• EPIDEMIOLOGIC SURVEILLANCE is the ONGOING, SYSTEMATIC collection, analysis, interpretation, and dissemination of health data for PUBLIC HEALTH ACTION.
• Key word: ONGOING and SYSTEMATIC — surveillance is a continuous process, not a one-time activity.
• In the Philippines, surveillance is coordinated by the DOH EPIDEMIOLOGY BUREAU (formerly called the Epidemiology and Biostatistics Division).
• PIDSR (Philippine Integrated Disease Surveillance and Response): The national system for reporting notifiable diseases upward from health facilities → city/municipal health office → provincial health office → regional health office → DOH central.
• ESUs (Epidemiology and Surveillance Units): established at regional, provincial, and city/municipal health offices to collect, analyze, and respond to surveillance data.
• PASSIVE SURVEILLANCE: routine, facility-based reporting — health facilities submit regular reports without active case-finding. Most common type. Relies on patients seeking care.
• ACTIVE SURVEILLANCE: health workers actively go to communities or health facilities to find cases — more complete and accurate but resource-intensive. Used during outbreaks or for high-priority diseases.
• SENTINEL SURVEILLANCE: monitoring conducted at selected representative sentinel sites (not all facilities) to detect trends at lower cost.
• Notifiable diseases in the Philippines are classified into IMMEDIATELY NOTIFIABLE (within 24 hours — e.g., cholera, rabies, measles, AFP, HFMD cluster) and WEEKLY NOTIFIABLE (e.g., dengue, typhoid, leptospirosis, malaria).
• The FHSIS (Field Health Service Information System) collects routine service delivery data from RHUs and Barangay Health Centers — the primary source of morbidity and maternal/child health program data at the community level.
• Reporting notifiable diseases is a LEGAL AND PROFESSIONAL DUTY of the nurse under RA 9173 and the DOH notifiable disease policies.

Definitions

Section Title

6. Epidemiologic Surveillance and the Philippine PIDSR System

Common Mistakes

• Confusing PASSIVE surveillance (routine reporting) with ACTIVE surveillance (active case-finding) — passive is the norm; active is used in special situations/outbreaks.
• Forgetting that PIDSR and FHSIS are DIFFERENT systems — PIDSR is for disease surveillance/outbreak response; FHSIS is for routine service delivery data.
• Not knowing that the DOH EPIDEMIOLOGY BUREAU (not the CHO or RHU) coordinates national surveillance.

Exam Tips

• NLE items may show an epidemic curve and ask what type of outbreak it represents. Single peak = point source; multiple waves = propagated.
• The first step in outbreak investigation is always VERIFY THE DIAGNOSIS — not immediately quarantine or treat.
• Attack Rate (AR) is computed during outbreak investigations to measure the probability of disease in those exposed — AR = cases/exposed × 100.
• Remember: Describing the outbreak (person, place, time) ALWAYS comes before formulating the hypothesis.

Key Points

• Outbreak investigation is a SYSTEMATIC, MULTI-STEP process conducted when an epidemic is suspected.
• Step 1: ESTABLISH/VERIFY THE DIAGNOSIS — confirm that cases truly have the suspected disease (clinical, laboratory, pathologic confirmation); compare current case numbers with expected (baseline) numbers to confirm outbreak.
• Step 2: DEFINE AND IDENTIFY CASES — create a CASE DEFINITION (clinical and/or laboratory criteria for who counts as a case); conduct case-finding to identify all cases.
• Step 3: DESCRIBE THE OUTBREAK — characterize by PERSON (who is affected), PLACE (where), and TIME (when); construct an EPIDEMIC CURVE (histogram of cases plotted against time of onset).
• Step 4: FORMULATE AND TEST A HYPOTHESIS — propose a source and mode of transmission based on descriptive data; test by analytical methods (case-control or cohort study).
• Step 5: IMPLEMENT CONTROL AND PREVENTION MEASURES — do not wait for step 4 to be completed; control measures should begin as soon as feasible to prevent further cases.
• Step 6: COMMUNICATE FINDINGS / WRITE THE REPORT — disseminate results to stakeholders, continue surveillance to ensure the outbreak is controlled.
• THE EPIDEMIC CURVE: A histogram plotting number of cases (Y-axis) against time of onset (X-axis). The shape reveals the type of outbreak: POINT SOURCE = single sharp peak (all cases exposed to same source at same time, e.g., contaminated food at a fiesta); PROPAGATED/PERSON-TO-PERSON = successive waves of increasing then decreasing cases (e.g., measles spreading person-to-person); CONTINUOUS COMMON SOURCE = sustained plateau with gradual decline (e.g., contaminated water supply).
• A CASE DEFINITION includes: (a) clinical criteria (symptoms and signs), (b) laboratory criteria (if available), (c) epidemiologic criteria (time, place, person linkage), and (d) classification (confirmed, probable, suspected).

Definitions

Section Title

7. Outbreak Investigation

Common Mistakes

• Thinking control measures must wait until the hypothesis is CONFIRMED — control measures should begin as soon as there is reasonable evidence, even while investigation continues (Steps 5 and 4 often occur simultaneously).
• Confusing a POINT SOURCE epidemic curve (single sharp peak) with a PROPAGATED curve (successive waves) — the shape is critical for identifying the source.
• Forgetting that Step 1 is VERIFYING THE DIAGNOSIS first — you must confirm cases actually have the disease before declaring an outbreak.

Formulas

Exam Tips

• MEMORIZE the denominators: 'LB LB LB LB = live births for IMR, NMR, MMR, FDR; MP MP = midyear pop for CBR, CDR, cause-specific rate.'
• Fetal Death Rate denominator = Live Births + Fetal Deaths (total births) — this is the most commonly missed formula.
• PMR denominator = TOTAL DEATHS (not population) — PMR is a proportion, NOT a rate.
• If the NLE item gives you a multiplier different from the standard, USE THEIR MULTIPLIER.
• Always set up the formula before solving: write it out, plug in numbers, then multiply by the constant.

Key Points

• VITAL STATISTICS are data on vital events: births, deaths, fetal deaths, marriages, and divorces — registered under the civil registration system (Philippine Statistics Authority, PSA).
• RATE = (Number of events / Population at risk) × Constant (multiplier). The multiplier (1,000; 10,000; 100,000) is stated in the formula.
• RATIO: comparison of two quantities where the numerator is NOT necessarily part of the denominator (e.g., maternal mortality ratio).
• PROPORTION: numerator IS part of the denominator; always between 0 and 1 (expressed as percentage).
• MIDYEAR POPULATION is used as the denominator for CRUDE rates (CBR, CDR, cause-specific death rate).
• LIVE BIRTHS is used as the denominator for INFANT MORTALITY RATE, NEONATAL MORTALITY RATE, MATERNAL MORTALITY RATE/RATIO, and FETAL DEATH RATE.
• CRUDE BIRTH RATE (CBR) = (Total live births in a year / Midyear population) × 1,000
• CRUDE DEATH RATE (CDR) = (Total deaths in a year / Midyear population) × 1,000
• INFANT MORTALITY RATE (IMR) = (Deaths under 1 year of age / Total live births) × 1,000 — a key indicator of community health status and quality of maternal-child care.
• NEONATAL MORTALITY RATE = (Deaths under 28 days of age / Total live births) × 1,000
• POST-NEONATAL MORTALITY RATE = (Deaths from 28 days to <1 year / Total live births) × 1,000
• MATERNAL MORTALITY RATE/RATIO (MMR) = (Maternal deaths during pregnancy or within 42 days of termination of pregnancy / Total live births) × 100,000 (some Philippine DOH materials use × 1,000 or × 10,000 — ALWAYS read the item's stated constant).
• FETAL DEATH RATE = (Fetal deaths / Total live births + Fetal deaths) × 1,000 — NOTE: denominator includes BOTH live births AND fetal deaths.
• CAUSE-SPECIFIC DEATH RATE = (Deaths from a specific cause / Midyear population) × 100,000
• PROPORTIONATE MORTALITY RATE (PMR) = (Deaths from a specific cause / Total deaths from all causes) × 100 — tells you the PROPORTION of all deaths attributed to a specific cause; NOT a true rate.

Definitions

Section Title

8. Vital Statistics: Formulas and Computation

Common Mistakes

• Using MIDYEAR POPULATION as denominator for IMR, NMR, or MMR — these use LIVE BIRTHS as denominator.
• Using LIVE BIRTHS ONLY as denominator for FETAL DEATH RATE — the denominator must include BOTH live births AND fetal deaths.
• Applying multiplier of 1,000 to MMR — it is typically × 100,000 (though read the item carefully).
• Confusing PMR (proportion of deaths) with cause-specific death rate (deaths per population) — PMR cannot measure risk.

Formulas

Exam Tips

• KEY NLE DISTINCTION: Incidence = NEW cases / population at RISK (risk over time); Prevalence = ALL cases / total population (burden at a point).
• INCIDENCE ↑ → PREVALENCE ↑ (more new cases); SHORTER disease duration → LOWER prevalence; LONGER disease duration → HIGHER prevalence.
• CFR denominator = CASES (not population) — do not confuse with cause-specific death rate (denominator = midyear population).
• Attack Rate is ALWAYS used in OUTBREAK context; always expressed as a PERCENTAGE (× 100).

Key Points

• INCIDENCE RATE measures the rate of occurrence of NEW cases in a population over a defined time period. It measures RISK of developing the disease.
• PREVALENCE RATE measures ALL EXISTING cases (new + old) in a population at a specific point or period. It measures the BURDEN or LOAD of disease.
• INCIDENCE vs PREVALENCE: Think of a bucket — incidence = water flowing IN (new cases); prevalence = total water IN the bucket at any given time (all existing cases). Prevalence = Incidence × Duration of disease.
• ATTACK RATE (AR): a SPECIAL incidence rate used during OUTBREAKS. Expressed as a percentage (× 100). Measures the probability of disease among those EXPOSED.
• SECONDARY ATTACK RATE (SAR): proportion of susceptible CONTACTS of primary cases who develop disease — measures communicability within a household or defined group.
• CASE FATALITY RATE (CFR): measures the SEVERITY or LETHALITY of a disease — proportion of diagnosed cases who die. NOT a true rate; it is a PROPORTION.
• Prevalence is HIGHER than incidence for diseases with LONG DURATION (e.g., diabetes, TB, HIV); incidence and prevalence are SIMILAR for short-duration diseases (e.g., cholera, measles).
• In a screening program, increasing SENSITIVITY increases FALSE POSITIVES; increasing SPECIFICITY increases FALSE NEGATIVES detection (see Section 10 for screening metrics).

Definitions

Section Title

9. Morbidity Formulas: Incidence, Prevalence, Attack Rate, and Case Fatality Rate

Common Mistakes

• Using ALL cases (instead of only NEW cases) in the incidence numerator.
• Using population at risk as incidence denominator but total population (including those already sick) as prevalence denominator — both must be specified correctly.
• Applying the incidence formula to attack rate items — attack rate uses × 100 (percent), not × 1,000.
• Confusing CFR (deaths per CASES) with cause-specific DEATH RATE (deaths per POPULATION).

Formulas

Exam Tips

• Memory aid: SnNout — high Sensitivity, Negative result rules OUT disease; SpPin — high Specificity, Positive result rules IN disease.
• For SCREENING (finding all sick people) → maximize SENSITIVITY (accept more false positives).
• For CONFIRMATORY testing (diagnosing true cases) → maximize SPECIFICITY (minimize false positives).
• NLE item: 'A screening test has high PPV but low NPV — what does this suggest?' → Disease prevalence in the tested population is HIGH.

Key Points

• SCREENING is the application of a test to apparently HEALTHY individuals in a population to detect disease or risk factors EARLY (SECONDARY PREVENTION).
• A good screening program requires: (1) the disease is IMPORTANT and common; (2) it has a detectable EARLY/PRESYMPTOMATIC stage; (3) EFFECTIVE TREATMENT is available; (4) the test is SAFE, ACCEPTABLE, SIMPLE, and COST-EFFECTIVE.
• VALIDITY of a screening test is measured by SENSITIVITY and SPECIFICITY.
• SENSITIVITY = the proportion of TRUE POSITIVES correctly identified by the test (among those WITH the disease). Formula: TP / (TP + FN) × 100. A sensitive test has few FALSE NEGATIVES. A highly sensitive test RULES OUT disease when NEGATIVE (SnNout = Sensitivity, Negative test, rules OUT).
• SPECIFICITY = the proportion of TRUE NEGATIVES correctly identified by the test (among those WITHOUT the disease). Formula: TN / (TN + FP) × 100. A specific test has few FALSE POSITIVES. A highly specific test RULES IN disease when POSITIVE (SpPin = Specificity, Positive test, rules IN).
• FALSE POSITIVE (Type I error / Alpha error): test says positive but person does NOT have the disease. High false positives → unnecessary anxiety and follow-up costs.
• FALSE NEGATIVE (Type II error / Beta error): test says negative but person DOES have the disease. High false negatives → missed cases and continued disease spread.
• POSITIVE PREDICTIVE VALUE (PPV): the probability that a person WITH A POSITIVE TEST actually HAS the disease. Formula: TP / (TP + FP) × 100. PPV is greatly influenced by the PREVALENCE of the disease in the population tested — higher prevalence = higher PPV.
• NEGATIVE PREDICTIVE VALUE (NPV): the probability that a person with a NEGATIVE TEST truly DOES NOT have the disease. Formula: TN / (TN + FN) × 100.
• TRADE-OFF: Increasing sensitivity DECREASES specificity (and vice versa). Screening tests are usually designed to be highly SENSITIVE to catch all cases; confirmatory tests are highly SPECIFIC to rule in true cases.
• Philippine screening examples: Newborn screening (RA 9288) — mandatory for 6 conditions; IVA/Pap smear for cervical cancer; blood glucose screening for diabetes.

Definitions

Section Title

10. Screening and Test Validity

Common Mistakes

• Confusing SENSITIVITY (ability to detect DISEASE) with SPECIFICITY (ability to detect HEALTH).
• Forgetting that PPV is influenced by PREVALENCE — a highly sensitive and specific test can still have low PPV if the disease is RARE in the population being screened.
• Mixing up false positive (healthy person tests positive) with false negative (sick person tests negative).
• Thinking that high sensitivity and high specificity can always be achieved simultaneously — there is always a trade-off.

Formulas

Exam Tips

• Memory tip: COHORT = C for Concurrent (forward) = C for Comparative Incidence = RR; CASE-CONTROL = looks BACK = OR.
• For RARE disease → Case-control study is most efficient.
• For establishing CAUSALITY → RCT (strongest), then Cohort.
• NLE item with outliers in data: 'Which measure of central tendency is most appropriate?' → MEDIAN.
• NLE item with symmetrical data: either mean or median — prefer MEAN for computation items unless outliers are mentioned.

Key Points

• DESCRIPTIVE STUDIES describe the frequency and distribution of disease by person, place, and time WITHOUT testing associations. Types include: case reports (single case), case series, cross-sectional (prevalence) surveys.
• ANALYTIC STUDIES test ASSOCIATIONS between exposures and outcomes. Two main types: OBSERVATIONAL (cohort, case-control) and EXPERIMENTAL (randomized controlled trials).
• COHORT STUDY (Prospective study): follows a group of EXPOSED and UNEXPOSED persons FORWARD in time to compare incidence of disease. Yields RELATIVE RISK (RR). Best for COMMON diseases. Limitation: expensive, takes long time.
• CASE-CONTROL STUDY (Retrospective study): identifies persons WITH disease (cases) and WITHOUT disease (controls), then LOOKS BACK at past exposures. Yields ODDS RATIO (OR). Best for RARE diseases and RARE exposures. Faster and cheaper than cohort.
• RELATIVE RISK (RR) = Incidence in Exposed / Incidence in Unexposed. RR > 1 = increased risk; RR < 1 = protective; RR = 1 = no association.
• CROSS-SECTIONAL STUDY: measures exposure and disease STATUS at the SAME POINT in time. Gives prevalence. Cannot establish temporal causality. Cheap and quick.
• RANDOMIZED CONTROLLED TRIAL (RCT): the investigator randomly ASSIGNS participants to intervention or control group. Strongest evidence for causality. Gold standard for evaluating treatments.
• HIERARCHY OF EVIDENCE (highest to lowest): Systematic review/Meta-analysis > RCT > Cohort study > Case-control study > Cross-sectional study > Case report.
• MEAN: arithmetic average. Best for normally distributed, symmetrical data. Sensitive to OUTLIERS (extreme values).
• MEDIAN: the MIDDLE value when data are arranged in order. Best for SKEWED data or when outliers are present.
• MODE: the most FREQUENTLY OCCURRING value. Used for nominal/categorical data.
• In a NORMAL DISTRIBUTION: Mean = Median = Mode. In a POSITIVELY SKEWED distribution: Mode < Median < Mean. In a NEGATIVELY SKEWED distribution: Mean < Median < Mode.

Definitions

Section Title

11. Study Designs and Measures of Central Tendency

Common Mistakes

• Saying cohort studies produce ODDS RATIO — cohort studies produce RELATIVE RISK; case-control studies produce ODDS RATIO.
• Confusing PROSPECTIVE (cohort — goes FORWARD) with RETROSPECTIVE (case-control — looks BACK).
• Using the MEAN for skewed data — use the MEDIAN for skewed distributions (e.g., income data, which is almost always right-skewed).
• Thinking cross-sectional studies can establish causality — they only provide a SNAPSHOT and cannot determine which came first (exposure or disease).

Exam Tips

• Census = PSA; Civil registration = PSA/LCRO; Surveillance = DOH Epidemiology Bureau (PIDSR); Routine services = FHSIS.
• If an NLE item asks 'What is the primary source of vital statistics in the Philippines?' → CIVIL REGISTRATION SYSTEM.
• The community nurse contributes to data quality through ACCURATE, TIMELY reporting and recording in FHSIS and PIDSR forms.

Key Points

• CENSUS: a complete, periodic count of the population conducted by the PHILIPPINE STATISTICS AUTHORITY (PSA). Conducted every 5 years in the Philippines. Provides population denominators for all rate calculations.
• CIVIL REGISTRATION SYSTEM: the PRIMARY source of VITAL STATISTICS in the Philippines. Registration of births, deaths, fetal deaths, marriages, and divorces is done at the Local Civil Registry Office (LCRO) and consolidated by the PSA.
• FHSIS (Field Health Service Information System): routine health service information collected from RHUs and Barangay Health Centers. Primary source of morbidity data, immunization coverage, maternal care indicators, and nutrition data at the local level.
• PIDSR (Philippine Integrated Disease Surveillance and Response): primary system for notifiable disease data and outbreak detection.
• NATIONAL DEMOGRAPHIC AND HEALTH SURVEY (NDHS): periodic nationally representative survey providing health status, fertility, maternal and child health, and nutrition data. Conducted by PSA in partnership with DOH.
• ANNUAL POVERTY INDICATORS SURVEY (APIS) and other PSA surveys provide socioeconomic data relevant to social determinants of health.
• COMMUNITY DIAGNOSIS: the process of identifying the health status, needs, and problems of a defined community using various data sources — census, FHSIS data, community surveys, key informant interviews. A core NCM/CHN nursing activity.
• Accurate DOCUMENTATION and RECORDING by the community nurse directly impacts the quality of all downstream health statistics — inaccurate records = misleading data = poor health planning.

Definitions

Section Title

12. Sources of Health Data in the Philippines

Common Mistakes

• Saying the DOH conducts the census — the PSA (under NEDA) conducts the census.
• Confusing FHSIS (routine service data) with PIDSR (notifiable disease surveillance).
• Forgetting that civil registration data quality depends on COMPLETENESS of registration — many rural areas still have under-registration of births and deaths.