Epidemiology psm

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Epidemiology PSM: Screening & Natural History of Disease

Source: Park's Textbook of Preventive and Social Medicine, 24th ed.

NATURAL HISTORY OF DISEASE

Natural history of disease refers to the course a disease takes in an individual from its earliest stage until its final outcome - without medical intervention. It is divided into two major phases:

Phase 1: Pre-pathogenesis Phase

Refers to the period before the disease agent enters the host
The disease agent has not yet interacted with man, but favourable factors (Agent + Host + Environment) already exist
This situation is called "man in the midst of disease" or "man exposed to risk"
We are all, potentially, in the pre-pathogenesis phase of many diseases
The three interacting factors form the Epidemiological Triad:
- Agent - biological, nutrient, chemical, physical, or social
- Host - age, sex, immunity, genetics, behaviour
- Environment - physical, biological, social
Their interaction determines whether disease starts, and whether it becomes sporadic, endemic, or epidemic

Level of prevention: PRIMARY PREVENTION Modes of intervention: Health Promotion + Specific Protection

Phase 2: Pathogenesis Phase

Begins when the disease agent enters the susceptible host.

In Infectious Diseases:

Agent multiplies → tissue and physiological changes → incubation period → early pathogenesis → late pathogenesis
Final outcome: Recovery, Disability, or Death
Host response is not always predictable - infection may be clinical or subclinical, typical or atypical, or the host may become a carrier (e.g., diphtheria, Hepatitis B)

In Chronic Diseases (e.g., hypertension, CAD, cancer):

Early pathogenesis is the presymptomatic phase - pathological changes are below the "clinical horizon" (not yet apparent clinically)
Disease becomes clinically apparent only in the late pathogenesis phase when signs/symptoms appear, by which time the disease is already well advanced

Progression through pathogenesis phase (from the Leavell & Clark model):

Stage	Description
Early pathogenesis	Agent established, tissue changes beginning
Discernible early lesions	Sub-clinical changes detectable by tests
Advanced disease	Clinical signs and symptoms appear (above clinical horizon)
Convalescence/Outcome	Recovery, Defect, Disability, Chronic state, or Death

Level of prevention: SECONDARY PREVENTION (early detection) + TERTIARY PREVENTION (late disease) Modes of intervention: Early Diagnosis & Treatment → Disability Limitation → Rehabilitation

CONCEPT OF LEAD TIME

Lead time is the time by which screening advances the diagnosis - the period between when a screening test detects the disease and when it would ordinarily have been diagnosed clinically.

Only diseases with a significant time lag between biological onset and clinical appearance are worth screening
Screening has no value in diseases whose onset is immediately obvious
The goal is to intervene during the presymptomatic phase (below the clinical horizon) before irreversible damage occurs

SCREENING

Definition

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

Screening is a preventive care function - it is not diagnostic. Positive results are referred for formal diagnostic work-up.

Screening vs. Diagnostic Tests

Feature	Screening Test	Diagnostic Test
Applied to	Apparently healthy	Sick / with indications
Scope	Groups	Individual patients
Accuracy	Less accurate	More accurate
Cost	Less expensive	More expensive
Result	Not final - needs confirmation	Basis for treatment
Initiative	From investigator / agency	From patient with complaint
Cut-off	Single criterion / cut-off point	Multiple symptoms, signs, labs

Note: Some tests serve both purposes (e.g., GTT, anaemia test).

Types of Screening

Type	Description
Mass screening	Applied to the entire population without selection
Selective (high-risk) screening	Targets a defined high-risk group (e.g., antenatal mothers, industrial workers)
Multiphasic screening	Two or more tests applied simultaneously to a large group at one time

Multiphasic screening: RCT evidence from the UK and USA showed no benefit in mortality or morbidity reduction and increased healthcare costs without observable benefit. Its utility has therefore been questioned.

Uses of Screening

Early detection and treatment of disease
Epidemiological research - estimating prevalence, distribution
Education of public and health professionals
Completing the natural history of disease

CRITERIA FOR SCREENING (Wilson & Jungner Criteria)

Two sets of criteria must be satisfied before initiating a programme: criteria related to the disease and the test.

A. Criteria Related to the Disease

The condition must be an important health problem (generally, high prevalence)
There must be a recognizable latent or early asymptomatic stage
The natural history must be adequately understood - including when the process becomes irreversible
A test must exist that can detect disease before signs/symptoms appear
Facilities for confirmation of diagnosis must be available
There must be an effective treatment available
There must be an agreed policy on whom to treat (e.g., borderline hypertension, pre-diabetes)
Good evidence that early detection + treatment reduces morbidity and mortality
Expected benefits must exceed risks and costs

B. Criteria Related to the Screening Test

The test must satisfy:

Acceptability - must be acceptable to the target population; painful, embarrassing, or uncomfortable tests (e.g., rectal/vaginal exams) reduce compliance in mass campaigns
Repeatability (Reliability/Precision) - consistent results on repeated application under same conditions
Validity - ability to correctly identify those with and without disease, measured by:
- Sensitivity - ability to correctly identify true positives (TP / TP+FN × 100)
- Specificity - ability to correctly identify true negatives (TN / TN+FP × 100)
Yield - amount of previously unrecognized disease detected
Simplicity - can be administered by non-physicians
Safety - minimal risk to the population
Rapidity - results available quickly
Low cost

Sensitivity & Specificity (2×2 Table)

	Disease Present	Disease Absent
Test Positive	True Positive (TP)	False Positive (FP)
Test Negative	False Negative (FN)	True Negative (TN)

Sensitivity = TP / (TP + FN) × 100 → "Ability to detect disease when it IS present"
Specificity = TN / (TN + FP) × 100 → "Ability to rule out disease when it is ABSENT"
Positive Predictive Value (PPV) = TP / (TP + FP) × 100 → depends heavily on prevalence
Negative Predictive Value (NPV) = TN / (TN + FN) × 100

Example from Park's (EEG for brain tumours):

Sensitivity = 36/40 × 100 = 90%
Specificity = 306,000/360,000 × 100 = 85%

Key rule: When sensitivity ↑, specificity tends to ↓ (trade-off determined by the cut-off point chosen). A lower cut-off → more sensitive but less specific. This is illustrated by the ROC (Receiver Operating Characteristic) curve.

Predictive Value and Prevalence

A critical concept: PPV is directly influenced by the prevalence of disease in the population screened.

When prevalence is high → PPV is high (more true positives among positive tests)
When prevalence is low → PPV falls dramatically (most positives will be false positives)
This is why screening is more effective in high-risk populations rather than the general public for rare conditions

Summary: Where Screening Fits in Natural History

Pre-pathogenesis → Presymptomatic phase → Clinical horizon → Clinical disease
                         ↑
              [SCREENING WINDOW]
             Lead time gained here
             Secondary prevention

Screening targets the presymptomatic phase - after pathological changes have begun but before the clinical horizon is crossed. The goal is to detect disease here, where intervention can still alter the outcome.

Park's Textbook of Preventive and Social Medicine, 24th ed., Block 1 (Natural History, pp. 46-47) and Block 2 (Screening, pp. 155-162)

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