Screening Programs

Screening Basics - Spotting Trouble Early

Identifying unrecognized disease/risk factors in apparently healthy individuals using simple tests.

• Aim: Early detection to ↓ morbidity & mortality.
• Principles (WHO/Wilson & Jungner):
  • Disease: Important, common.
  • Natural History: Recognizable latent/early symptomatic stage.
  • Test: Suitable, acceptable, valid & reliable.
  • Treatment: Effective, available.
  • Policy: Agreed on whom to treat.
  • Facilities: For diagnosis & treatment.
  • Cost: Cost-effective.
• Types:
  • Mass: Whole population.
  • Selective/High-risk: Specific, targeted at-risk groups.
  • Multiphasic: Multiple tests, one occasion.
  • Opportunistic: During other health encounters.

⭐ Screening is intended for apparently healthy individuals to detect unrecognized disease or risk factors.

Ideal Screening Tests - The Right Tools

Essential qualities of a good screening test:

• Validity: Test's accuracy.
  • Sensitivity: Detects true positives. $Sensitivity = TP / (TP + FN)$
  • Specificity: Detects true negatives. $Specificity = TN / (TN + FP)$
• Reliability: Consistent results (precision).
• Yield: New cases found & treated.
• Acceptability: Patient willingness.
• Simplicity: Easy to perform & interpret.
• Safety: Minimal risk.
• Cost-effectiveness: Benefits > costs.

⭐ Sensitivity correctly identifies those WITH the disease (True Positives).

📌 SNOUT: Highly Sensitive test, Negative result rules OUT disease. SPIN: Highly SPecific test, Positive result rules IN disease.

Eye Spy India - Common Targets

Key conditions screened in India under NPCBVI (National Programme for Control of Blindness and Visual Impairment):

Program Pitfalls & Perks - Gauging Success

• Evaluation:
  • PPV (Positive Predictive Value): $PPV = TP / (TP + FP)$; ↑ with ↑ prevalence.
  • NPV (Negative Predictive Value): $NPV = TN / (TN + FN)$; ↓ with ↑ prevalence.
• Biases:
  • Lead Time Bias: Apparent ↑ survival from early detection.

    ⭐ Lead time bias refers to the apparent increase in survival time among screened individuals merely because the disease is detected earlier, without necessarily affecting the actual course of the disease.

  • Length Time Bias: Favors slow-growing, less aggressive cases.
  • Volunteer Bias: Participants healthier/more health-conscious.
  • Overdiagnosis: Detecting disease unlikely to cause harm.
• Benefits:
  • Early detection → improved prognosis.
  • ↓ costs for advanced disease treatment.

High‑Yield Points - ⚡ Biggest Takeaways

• Screening aims for early detection of disease in asymptomatic individuals.
• Wilson-Jungner criteria are crucial for justifying a screening program.
• Common targets: Diabetic Retinopathy, Glaucoma, Cataract, Refractive errors, ROP.
• Sensitivity (detects disease) and Specificity (detects health) are vital test properties.
• Beware of Lead time bias (apparent increased survival) and Length time bias.
• NPCBVI (National Programme for Control of Blindness and Visual Impairment) is key in India for mass screening and eye care delivery.