📚 Park's Screening Chapter - Complete Study Guide

1. CONCEPT OF SCREENING

"The search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals."

• Annual health exams were too costly and time-consuming for large populations
• Screening conserves physician-time - technicians administer tests, physician only interprets
• Original programmes: TB, syphilis, antenatal mothers, school children, occupational groups

1. Capable of wide application
2. Relatively inexpensive
3. Requires little physician-time (physician not needed to administer - only to interpret)

2. SCREENING vs. DIAGNOSTIC TESTS (Table - Very Exam Important)

3. CASE-FINDING (Important distinction)

4. USES OF SCREENING (4 main uses)

a. Case Detection (Prescriptive Screening)

• Presumptive identification of unrecognized disease
• Does NOT arise from a patient's request
• People screened primarily for their own benefit
• Examples: neonatal screening, bacteriuria in pregnancy, breast cancer, cervical cancer, PKU, pulmonary TB, haemolytic disease of newborn

b. Control of Disease (Prospective Screening)

• People examined for the benefit of others
• Examples: screening immigrants for TB and syphilis to protect the home population; screening for streptococcal infection to prevent rheumatic fever
• Can reduce spread of infectious disease and/or mortality

c. Research Purposes

• To study the natural history of diseases whose course is not fully known (e.g., cancer, hypertension)
• Initial screening = prevalence estimate
• Subsequent screening = incidence figure
• Investigator has ethical obligation to treat those found positive

d. Syndromic Surveillance

• Screening populations to detect disease outbreaks early (e.g., influenza-like illness trends)

5. TYPES OF SCREENING (Very Important for MCQs)

a. Mass Screening

• Screening of whole population regardless of risk
• Example: Universal newborn screening for PKU/hypothyroidism
• Advantage: No one is missed
• Disadvantage: Expensive, many low-risk individuals screened unnecessarily

b. Selective / High-Risk Screening

• Screening only the high-risk group (a subgroup of the population)
• More cost-effective than mass screening
• Example: Screening only smokers for lung cancer; screening only hypertensives for target organ damage
• Also called "targeted screening"

c. Multiphasic Screening

• Application of two or more screening tests simultaneously to a large population
• Example: Measuring BP + blood glucose + cholesterol + vision in a single visit
• Advantage: More diseases detected in one contact; cost-effective per person
• Used extensively in health camps and industrial health programmes

6. CRITERIA FOR SCREENING (Wilson & Jungner Criteria - Most Important)

1. The condition sought should be an important health problem (significant morbidity/mortality)
2. There should be an accepted treatment for patients with recognized disease
3. Facilities for diagnosis and treatment should be available
4. There should be a recognizable latent or early symptomatic stage
5. A suitable test or examination should be available - acceptable to the population
6. The natural history of the condition should be adequately understood
7. There should be an agreed policy on whom to treat as patients
8. The cost of case-finding (including diagnosis and treatment) should be economically balanced in relation to possible expenditure on medical care as a whole
9. Case-finding should be a continuing process and not a "once and for all" project

7. VALIDITY OF SCREENING TESTS

The 2×2 Table (Must memorise)

                    DISEASE
                 Present    Absent
TEST    Positive    a         b       (a+b)
        Negative    c         d       (c+d)
                  (a+c)     (b+d)     Total

• a = True Positives (TP)
• b = False Positives (FP)
• c = False Negatives (FN)
• d = True Negatives (TN)

Sensitivity

"The ability of a test to correctly identify those WHO HAVE the disease."

• Sensitivity = TP / (TP + FN) × 100
• A highly sensitive test has few False Negatives
• Used when missing a case is dangerous (e.g., HIV, TB, cancer)
• "Snout" mnemonic: SnNout - Sensitive test, Negative result rules OUT disease

Specificity

"The ability of a test to correctly identify those WHO DO NOT have the disease."

• Specificity = TN / (TN + FP) × 100
• A highly specific test has few False Positives
• Used when a false positive causes harm (e.g., unnecessary surgery, stigma)
• "Spin" mnemonic: SpPin - Specific test, Positive result rules IN disease

Key Trade-off

• Sensitivity and specificity are inversely related
• Raising the cut-off point → ↑ Specificity, ↓ Sensitivity
• Lowering the cut-off point → ↑ Sensitivity, ↓ Specificity

8. PREDICTIVE VALUE (Very High-Yield)

Positive Predictive Value (PPV)

"The probability that a person who tests positive ACTUALLY HAS the disease."

Negative Predictive Value (NPV)

"The probability that a person who tests negative ACTUALLY DOES NOT HAVE the disease."

Effect of Prevalence on Predictive Value

• When prevalence is HIGH → PPV increases, NPV decreases
• When prevalence is LOW → PPV decreases (many false positives), NPV increases
• Same test in a high-prevalence population has a higher PPV than in a low-prevalence population

9. RELIABILITY (REPRODUCIBILITY / PRECISION)

Sources of variation reducing reliability:

• Intra-observer variation: Same observer gets different results on repeat measurement
• Inter-observer variation: Two different observers get different results
• Occurs in: reading X-rays, interpreting cervical smears, auscultation of BP
• Minimized by: standardization of procedures, intensive training, using ≥2 independent observers

• Changes in the parameter itself: e.g., cervical smear may be normal one day, abnormal another; BP varies through the day
• Variation in how patients perceive/answer: recall errors, deliberate concealment
• Regression to the mean: Extreme values tend to move toward the average on re-measurement. Important when evaluating drug therapy (e.g., antihypertensives) - improvement may be partly regression to mean, not true drug effect

• Defective instruments, erroneous calibration, faulty reagents
• Test itself may be inappropriate or unreliable

10. DISTRIBUTION OF VARIABLES - The Cut-Off Point

Bimodal Distribution

• Two separate curves: one for "normal" population, one for "diseased"
• Clear separation with a "borderline" zone where the curves overlap
• Cut-off point set at point E (where distributions intersect) minimizes both false positives and false negatives
• Example: Phenylketonuria (PKU)

Unimodal Distribution

• Single continuous curve (e.g., blood pressure, blood sugar, serum cholesterol)
• No sharp dividing line between normal and diseased
• Cut-off point is arbitrary
• Cutting at level C/A (lower) → Higher sensitivity, more false positives
• Cutting at level D/B (higher) → Higher specificity, more false negatives
• Example: Diabetes (blood glucose cut-off)

11. EVALUATION OF SCREENING PROGRAMMES

1. Yield - number of new cases detected in the screened population
2. Sensitivity and Specificity of the test used
3. Predictive value in the target population
4. Cost-effectiveness - cost per case detected
5. Compliance - uptake rate in the target population
6. Follow-up - proportion of screen-positives who receive definitive diagnosis and treatment
7. Impact - reduction in morbidity/mortality in the screened group vs. unscreened

• Prevalence of unrecognized disease in the population
• Sensitivity of the test
• Number of persons tested

12. BIAS IN SCREENING (High-Yield Exam Topic)

Lead Time Bias

• Screening detects disease earlier in its natural history
• Survival appears longer from diagnosis - but this is just because diagnosis was moved earlier, not because the patient actually lived longer
• This can make a screening programme appear effective when it is not

Length Bias

• Screening tends to detect slower-progressing (less aggressive) cases, because they spend more time in the detectable pre-clinical phase
• Rapidly progressive (aggressive) cases pass through the detectable phase quickly and are missed
• Result: Screened cases appear to have better outcomes than unscreened cases, even if screening has no real benefit

Selection Bias (Volunteer Bias)

• People who participate in screening tend to be more health-conscious, and thus have better outcomes regardless of screening
• Makes screening appear more effective than it is

13. SOME SPECIFIC SCREENING TESTS (from Table 8 in Park's)

14. CANCER SCREENING (from Park's block 6)

• Cancer screening is most useful when a detectable pre-clinical phase exists
• Must have an effective treatment that improves outcomes when applied early
• Most evidence-based cancer screening programmes:
  • Cervical cancer: Pap smear (every 3 years from age 21, or HPV co-testing every 5 years)
  • Breast cancer: Mammography (annual/biennial from age 40-50 depending on guideline)
  • Colorectal cancer: Faecal occult blood test (FOBT), colonoscopy
  • Oral cancer: Visual inspection with naked eye / acetic acid (VIA)

15. NEONATAL SCREENING

• Congenital hypothyroidism (CHT) - most common
• Phenylketonuria (PKU)
• G6PD deficiency
• Congenital adrenal hyperplasia (CAH)
• Sickle cell disease

• CHT: Thyroxine supplementation (prevents cretinism)
• PKU: Phenylalanine-restricted diet (prevents intellectual disability)

QUICK REVISION FLASHCARDS

HIGH-YIELD MCQ TRAPS

1. Screening is not a diagnostic test - screen positives need further workup
2. A highly sensitive test is preferred for serious diseases where missing a case is dangerous
3. A highly specific test is preferred where a false positive causes harm (unnecessary surgery, stigma)
4. Reliability ≠ Validity - a test can be reliable (reproducible) but invalid (consistently wrong)
5. PPV depends heavily on prevalence - the same test has a lower PPV in low-prevalence settings
6. Multiphasic screening is the most cost-effective strategy per person screened
7. The Guthrie test is the screening test for PKU
8. Pap smear is the screening test for cervical cancer (not diagnosis)
9. Lead time bias overestimates benefit of screening by shifting apparent survival without real benefit
10. In bimodal distribution, cut-off at intersection point (E) minimizes misclassification

📚 Park's Chapter 2: Concept of Health and Disease - Complete Study Guide

PART A: CONCEPT OF HEALTH

1. CHANGING CONCEPTS OF HEALTH

1. Biomedical Concept

• Oldest and most traditional view: "health = absence of disease"
• Basis: Germ theory of disease (dominated early 20th century)
• Viewed the body as a machine, disease = machine breakdown, doctor = mechanic
• Criticism: Ignores environmental, social, psychological, cultural determinants
• Failed to solve problems like malnutrition, chronic disease, drug abuse, mental illness, pollution

2. Ecological Concept

• Health = dynamic equilibrium between man and his environment
• Disease = maladjustment of the human organism to environment
• Dubos' definition: "Health implies the relative absence of pain and discomfort and a continuous adaptation and adjustment to the environment to ensure optimal function"
• Raises two issues: imperfect man and imperfect environment
• Improvement in human adaptation to natural environments can lead to longer life, even without modern health services

3. Psychosocial Concept

• Health is NOT only a biomedical phenomenon
• Influenced by social, psychological, cultural, economic, and political factors
• Health is both a biological AND social phenomenon

4. Holistic Concept (Current - most comprehensive)

• Synthesis of all three above concepts
• Recognizes strength of social, economic, political, environmental influences
• Health = unified/multidimensional process involving well-being of the whole person in the context of their environment
• Corresponds to the ancient view: "sound mind, in a sound body, in a sound family, in a sound environment"
• All sectors of society have an effect on health

2. WHO DEFINITION OF HEALTH (Must memorise verbatim)

"Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity."

• Adopted in the Preamble to the Constitution of WHO (1948)
• Three dimensions: Physical + Mental + Social
• Later, a 4th dimension was proposed: Spiritual health
• Some also add: Vocational health

1. The word "complete" is unrealistic - no one can be in complete health all the time
2. It is not measurable (no objective criteria)
3. It is static - doesn't account for dynamic nature of health
4. Ignores economic and political dimensions

3. DIMENSIONS OF HEALTH

4. POSITIVE HEALTH vs NEGATIVE HEALTH

5. HEALTH AS A RELATIVE CONCEPT

• Health is NOT universal - it is culturally, geographically, and socio-economically relative
• A newborn in India weighs 2.8 kg on average vs 3.5 kg in developed countries - yet both are healthy in their context
• Height/weight standards vary between countries and socio-economic groups
• Many "normal" people have heart murmurs, enlarged tonsils, X-ray shadows yet show no ill health
• Implication: Each country should define its own norms based on prevailing ecological conditions, rather than universal health standards

6. HEALTH AS A HUMAN RIGHT

• 1946: WHO Constitution - health as a fundamental human right
• 1977: 30th World Health Assembly - "Health for All by 2000" (HFA 2000)
• 1978: Alma-Ata Declaration on Primary Health Care
• 1979: UN adopted health as integral part of socio-economic development
• 1986: Ottawa Charter for Health Promotion
• 2000: Millennium Development Goals (MDGs) - health centrally positioned
• 2015: Sustainable Development Goals (SDGs) - 2030 Agenda
• Right to health recognized in at least 115 countries' constitutions

7. DETERMINANTS OF HEALTH

1. Biological Determinants

• Genetic make-up at conception (cannot be altered after)
• Chromosomal anomalies, errors of metabolism, mental retardation, some types of diabetes
• Role of medical genetics: genetic screening, gene therapy

2. Behavioural and Socio-cultural Determinants

• Lifestyle, habits, customs, behaviours (smoking, diet, exercise, substance use)
• Social norms, cultural practices, education level
• Lifestyle diseases now dominate: obesity, CVD, type 2 DM, lung cancer

3. Environment

• Physical: climate, housing, water, sanitation
• Social: social support networks, social cohesion
• Biological: microorganisms in the environment
• All diseases reflect, to some degree, failure of the organism to successfully adapt to the environment

4. Socio-economic Conditions

• Poverty is the single most important determinant of ill-health
• Education, occupation, income strongly correlate with health outcomes
• Social class gradients in health are seen globally

5. Health Services

• Availability, accessibility, affordability, acceptability
• Quality and coverage of preventive, promotive, curative services

6. Ageing of Population

• Changing demographic structure brings shift in disease pattern
• Rise of non-communicable diseases, degenerative conditions

8. RESPONSIBILITY FOR HEALTH

• Health is primarily an individual responsibility - cannot be given by others
• Self-care definition: "Health activities including promotion, maintenance, treatment care and health-related decision making, carried out by individuals and families"
• Includes: diet, sleep, exercise, weight control, avoiding tobacco/alcohol/drugs, personal hygiene, immunization, early reporting when sick, family planning

• Things individuals cannot achieve alone - achieved through collective action
• Community involvement in health planning, resource mobilization, implementing programmes
• Social support systems, group behaviours

• Create conditions for health through legislation, policy, resource allocation
• Health system governance, social protection, sanitation laws

PART B: CONCEPT OF DISEASE

1. DEFINITIONS OF DISEASE

• Webster: "A condition in which body health is impaired, a departure from a state of health, an alteration of the human body interrupting the performance of vital functions"
• Oxford: "A condition of the body in which its functions are disrupted or deranged"
• Ecological: "A maladjustment of the human organism to the environment"
• Simple: Disease = opposite of health = any deviation from normal functioning

2. DISEASE vs. ILLNESS vs. SICKNESS (Susser's Distinction - Very Exam Important)

• You can have disease without feeling ill (e.g., asymptomatic hypertension, subclinical diabetes)
• You can be ill without physical impairment (e.g., hypochondria, psychosomatic illness)
• The clinician sees people who are ill - not the disease - which he must diagnose and treat

3. CONCEPT OF CAUSATION OF DISEASE

Old Theories (Pre-Pasteur)

• Supernatural theory
• Theory of humors (body fluids out of balance - shared by ancient Indians and Greeks)
• Concept of contagion
• Miasmatic theory (bad air causes disease)
• Theory of spontaneous generation

Germ Theory of Disease (Pasteur era, 19th century)

• A single microbe causes a specific disease (one-to-one relationship)
• Disease model: Disease agent → Man → Disease
• Limitation: Not everyone exposed to TB develops TB; exposure + susceptibility (host factors) both matter
• Modern medicine has moved AWAY from strict germ theory

Epidemiological Triad (Agent-Host-Environment)

4. AGENTS OF DISEASE (Classification)

• Bacteria, viruses, fungi, parasites, rickettsia, prions

• Protein-energy malnutrition (PEM), anaemia, goitre, obesity, vitamin deficiencies

• Excessive heat, cold, humidity, pressure, radiation, electricity, sound

• Endogenous: Produced in body due to dysfunction - urea (uraemia), serum bilirubin (jaundice), ketones (ketosis), uric acid (gout), calcium carbonate (kidney stones)
• Exogenous: Allergens, metals, fumes, dust, gases, insecticides - acquired by inhalation, ingestion, or inoculation

• Chronic friction and mechanical forces causing crushing, tearing, sprains, dislocations, death

• Chemical: hormones (insulin, oestrogens), enzymes
• Structural: absence of thymus, cardiac defects
• Chromosomal: Mongolism (Down syndrome), Turner's syndrome
• Immunological: agammaglobulinaemia

• Poverty, smoking, drug/alcohol abuse, unhealthy lifestyles, social isolation, maternal deprivation
• Modern concept of "agent" is very broad - includes both living and non-living agents

5. HOST FACTORS (Intrinsic)

• (i) Demographic: Age, sex, ethnicity
• (ii) Biological: Genetic factors, biochemical blood levels (cholesterol), blood groups, enzymes, immunological factors, physiological function (BP, FEV)
• (iii) Social/economic: Socio-economic status, education, occupation, stress
• (iv) Lifestyle: Diet, exercise, smoking, alcohol, drug use

6. ENVIRONMENTAL FACTORS

• Physical: Climate, geography, housing, water supply, sanitation
• Biological: Flora, fauna, vectors, reservoirs of infection
• Social: Overcrowding, population density, social norms, cultural practices
• Economic: Poverty, income inequality, employment

7. SPECTRUM OF DISEASE

• Different manifestations reflect individuals' different states of immunity and receptivity
• In infectious diseases, also called "gradient of infection"
• Leprosy is the classic example of a spectral disease
• Exception: Rabies - almost always fatal (no spectrum of severity)
• The spectrum can be interrupted at any point by early diagnosis, treatment, or preventive measures

8. ICEBERG OF DISEASE (Closely related to Spectrum)

• Floating tip (above waterline) = Clinical cases - what the physician sees
• Submerged portion (below waterline) = Hidden mass of disease: latent, inapparent, presymptomatic, undiagnosed cases, and carriers
• Waterline = Demarcation between apparent and inapparent disease

• Hypertension, diabetes, anaemia, malnutrition, mental illness

9. CONCEPT OF RISK (Risk Factors and At-Risk Groups)

• Age groups (infants, toddlers, elderly)
• Sex (females in reproductive age)
• Physiological state (pregnancy, high cholesterol, high BP)
• Genetic factors (family history)
• Existing disease/unhealthy behaviour

• Rural/urban slums
• Overcrowding, poor living conditions
• Proximity to industries, poor water supply

• Low social class
• Ethnic/cultural group
• Family disruption, poor education, poor housing
• Smoking, lack of exercise, overeating, drug addiction
• Poor access to health services

PART C: INDICATORS OF HEALTH (Measurement of Health)

1. Mortality indicators
2. Morbidity indicators
3. Disability rates
4. Nutritional status indicators
5. Health care delivery indicators
6. Utilization rates
7. Indicators of social and mental health
8. Environmental indicators
9. Socio-economic indicators
10. Health policy indicators
11. Indicators of quality of life
12. Other indicators

• Valid - measures what it is supposed to measure
• Reliable and objective - same answer regardless of who measures
• Sensitive - picks up changes in the situation
• Specific - reflects changes only in that specific situation
• Feasible - data can actually be obtained
• Relevant - contributes to understanding the phenomenon

QUICK REVISION FLASHCARDS

HIGH-YIELD MCQ TRAPS

1. WHO defined health in 1948 in the Preamble to its Constitution - but has NOT defined disease
2. The holistic concept is the synthesis of all three other concepts - not just biomedical
3. Dubos gave the ecological concept definition - remember his name
4. Germ theory = one-to-one (agent → disease). Epidemiological triad = multifactorial
5. Disease = physiological dysfunction; Illness = subjective; Sickness = social dysfunction
6. Rabies is the exception to the spectrum of disease (almost no mild cases)
7. Floating tip of iceberg = clinical cases (visible to physician); submerged = hidden mass
8. Leprosy = best example of spectrum (from tuberculoid to lepromatous)
9. Regression to the mean - extreme values move toward average on re-measurement (trap in evaluating drug therapy)
10. The waterline in the iceberg = the cut-off between apparent and inapparent disease
11. Positive health - Dubos said it is always a mirage (not achievable in reality)
12. Self-care = activities by individuals and families WITHOUT professional assistance
13. The right to health is recognized in at least 115 countries' constitutions