Evidence basis for screening recommendations

Screening Metrics - Numbers Don't Lie

• Sensitivity & Specificity (Intrinsic to the test)

  • Sensitivity: $TP / (TP + FN)$. Probability of a positive test in a diseased person.
    • High sensitivity is used to Screen and Negate a disease (SNOUT).
  • Specificity: $TN / (TN + FP)$. Probability of a negative test in a healthy person.
    • High specificity is used to SPin and IN a diagnosis (SPIN).

• Predictive Values (Vary with disease prevalence)

  • Positive Predictive Value (PPV): $TP / (TP + FP)$.
  • Negative Predictive Value (NPV): $TN / (TN + FN)$.

• Likelihood Ratios (LR)

  • Positive LR (LR+): $Sensitivity / (1 - Specificity)$. For a positive test result.
    • LR+ >10 indicates a large and often conclusive increase in the likelihood of disease.
  • Negative LR (LR-): $(1 - Sensitivity) / Specificity$. For a negative test result.
    • LR- <0.1 indicates a large and often conclusive decrease in the likelihood of disease.

⭐ Prevalence Effect: Sensitivity and Specificity are intrinsic to the test and do not change with prevalence. However, PPV is directly proportional to prevalence, while NPV is inversely proportional.

Screening Biases - Tricky Data Traps

• Lead-Time Bias: Apparent increase in survival time just from detecting a disease earlier. The disease's natural history and outcome are not actually changed. Think of it as starting the survival clock sooner.

• Length-Time Bias: Screening is more likely to detect slow-growing, less aggressive diseases. Fast-growing, aggressive diseases often cause symptoms between screening intervals. This makes the screening program look better than it is.

• Selection (Volunteer) Bias: People who choose to get screened are often healthier and more health-conscious than those who don't. Their better outcomes might be due to their healthier lifestyle, not the screening.

⭐ The gold standard to overcome these biases is a Randomized Controlled Trial (RCT) demonstrating a reduction in disease-specific mortality.

Evidence Grading - The USPSTF Report Card

The USPSTF assigns a letter grade based on the strength of evidence and the balance of benefits and harms of a preventive service.

• Grade A: High certainty of substantial net benefit.
  • Action: Offer or provide this service.
• Grade B: High certainty of moderate or moderate certainty of substantial net benefit.
  • Action: Offer or provide this service.
• Grade C: Moderate certainty of small net benefit.
  • Action: Offer selectively to individual patients.
• Grade D: Moderate/High certainty of no net benefit or that harms outweigh benefits.
  • Action: Discourage use.
• I Statement: Evidence is insufficient, conflicting, or poor quality.
  • Action: Clinical judgment; shared decision-making.

⭐ For the exam, services with grades A and B are generally recommended and should be offered.

High‑Yield Points - ⚡ Biggest Takeaways

• USPSTF Grades are crucial: Grade A/B means offer the service. Grade D means discourage use. Grade I indicates insufficient evidence.
• Screening studies are prone to lead-time bias (earlier detection without changing outcome) and length-time bias (detecting more indolent cases).
• Overdiagnosis is a critical harm of screening-detecting disease that would never have become clinically significant.
• The Number Needed to Screen (NNS) is a key metric for evaluating the efficiency of a screening program.