LEARNING OBJECTIVES – CHAPTER 6

1. Relate the different stages of the development of a disease to the phases of prevention.

2. Describe the advantages and disadvantages of population and high-risk prevention strategies.

3. Understand the role and limitations of screening regarding the early detection of disease.

Overhead 6.1

PRIMORDIAL PREVENTION

Prevention of the emergence of living patterns that contribute to increased risk of disease (e.g., maintenance of low fat diet in Asian

countries).

Overhead 6.2

PRIMARY PREVENTION

Prevention of disease by controlling risk factors (e.g., non-smoking promotion).

STRATEGIES :PopulationHigh-risk

Overhead 6.3

PRIMARY PREVENTION :The Population Strategy

Advantages• Radical• Large potential for population• Behaviourally appropriate Disadvantages• Small benefits to individuals• Poor motivation of subject• Poor motivation of physician• Benefit-to-risk ratio may be low

Overhead 6.4

PRIMARY PREVENTION :The High-risk Strategy

Advantages• Appropriate to individuals• Subject motivation• Physician motivation• Benefit-to-risk ratio is favourableDisadvantages• High screening costs• Temporary effect• Limited effect• Behaviourally inappropriate

Overhead 6.5

PREVENTION PARADOX

“A preventive measure which brings much benefit to the population often offers little to each participating

individual.”

(Rose, 1985)

Overhead 6.6

SECONDARY PREVENTION

Reduction in consequences of disease by early diagnosis and treatment (e.g., cervical cancer screening)

TERTIARY PREVENTION

Reduction of complications of disease (e.g., MV crashes and ICU)

Overhead 6.7

SCREENING

The organized attempt to detect, among apparently health people in the community, disorders or risk factors of which they are

unaware.

Table 6.1. Levels of prevention

Level of prevention Phase of disease Target

Primordial Underlying conditions leading to causation

Total population and selected groups

Primary Specific causal factors Total population, selected groups and healthy individuals

Secondary Early stage of disease Patients

Tertiary Late stage of disease (treatment, rehabilitation)

Patients

Table 6.2. Advantages and disadvantages of strategies forprimary prevention

Population strategy High-risk individual strategy

Advantages

• Radical• Large potential for whole population• Behaviourally appropriate

• Appropriate to individuals• Subject motivation• Physician motivation• Favourable benefit-to-risk ratio

Disadvantages

• Small benefit to individuals• Poor motivation of suject• Poor motivation of physician• Benefit-to-risk ratio may be low

• Difficulties identifying high-risk individuals• Temporary effect• Limited effect• Behaviourally inappropriate

Table 6.3. Criteria for instituting a screening programme

Disease SeriousHigh prevalence of preclinical stageNatural history understoodLong period between first signs and overt disease

Diagnostic test Sensitive and specificSimple and cheapSafe and acceptableReliable

Diagnosis and Facilities are adequate Treatment Effective, acceptable, and safe treatment available

Table 6.4. Validity of a screening test

Disease statusPresent Absent Total

Positive

NegativeScreening test

a b

c d

a + b

c + d

a + cTotal b + d a + b + c + d

KEY :

a : no. of true positives c : no.of false negatives

b : no. of false positives d : no. of true negatives

Sensitivity = probability of a positive test in people with the disease

= a/(a + c)

Specificity = probability of a negative test in people without the disease

= d/(b + d)

Positive predictive value = probability of the person having the disease when the test is positive= a/(a + b)

Negative predictive value = probability of the person not having the disease when the test is negative= d/(c + d)