Screening of Disease😊

Screening of Disease

Levels of Prevention

Levels of Prevention	Prevents	Examples
Primordial Prevention/ Health promotion (Ideal)	Risk factor	- Lifestyle modification  - Health education  - Sanitation  - Safe water
1° Prevention / Specific protection/ Presumptive/ Prospective/ Predictive (Best)	Disease	- Specific protection  - Immunization  - Pre-employment checkup  - Pre-exposure prophylaxis  - Chemoprophylaxis  - Food fortification Screening - HIV screening: ◦ In mothers in ANC: Opt-out testing. ◦ During blood donation: Unlinked anonymous - COVID screening in airports - Immigrant screening protocol
2° Prevention / Prescriptive (m/c)	Complication	- disability (or complication) prevention - Early diagnosis  - Prompt treatment  - Screening for disease, - Fecal occult blood test - Mammography - PAP smear - Urinary ALA levels - VDRL
3° Prevention	Disability/death	- Disability treatment/rehabilitation/limitation  - Prosthesis  - Transplants

According to the NACO guidelines, what level of prevention is achieved when an HIV patient
undergoes counselling and screening for TB at an ICTC clinic?
A. Primordial
B. Primary
C. Secondary
D. Primary & secondary

ANS

Counselling for TB here is primary level as a risk factor is present and screening for TB is a secondary level of prevention as we are trying to do early diagnosis and treatment

Q. Hand washing during COVID-19 is what level of prevention?
A. Primordial prevention
B. Primary prevention
C. Secondary prevention
D. Tertiary prevention

ANS

B

Q. A government plans to outline tobacco control laws. What is the level of prevention here?
A. Primordial prevention
B. Primary prevention
C. Secondary prevention
D. Tertiary prevention

ANS

B

Vaccination & Mass chemoprophylaxis represent
A. Health promotion
B. Specific protection
C. Early diagnosis and treatment
D. Rehabilitation

ANS

B

Concept of Lead Time

Key Points in Disease Progression

• A: Point of disease onset.

• B: 1st possible point of diagnosis (no signs or symptoms).

• X: Critical point of diagnosis.

• Y: Usual time of diagnosis (signs & symptoms present).

• B-Y: Lead time (early detection).
• Time gained by early disease detection.

• B-X: Screening time.
• Between the earliest possible diagnosis point (B) and critical diagnosis point (X).

Properties of Screening Tests

Test Outcomes Matrix

Diagnostic Measures

• Pretest probability: Prevalance

• Post-test probability: Positive PV

• Diagnostic power of a screening test: Positive PV

Factors Affecting PPV/Diagnostic Accuracy

1. Prevalence (most important)

2. Sn

3. Sp

Test Utilization Based on Properties

• Sensitivity
• True Positive = identify those with the disease
• Rule out disease
• Mnemonic: Snout
• Screening test
• Confirms disease → if the disease is common.

• Specificity
• True negative = identify those without the disease
• Rule in disease
• Mnemonic: Spin
• Diagnostic test

Likelihood Ratio

• Chance of having disease:
• For positive test: TP/FP ~ Sn / (1-Sp)
• For negative test: FN/TN ~ (1-Sn) / Sp

• “Like → Snip” → Always (Sn / Sp)
• If positive test→ Add “1 -” to Sp
• If negative → Add “1 -” to Sn

Bayes' Formula

• To calculate PPV or NPV.

• Based on Sn, Sp & prevalence.

Combination of Tests

1. In Series:
• T2 only if T1 is positive and so on.
• Series → kanunnath Positive anu → P(PPV), P(Specificity)

2. In Parallel:
• All tests occur simultaneously.

Test Combination Effects