Serial vs parallel testing

Testing Strategies - The Two Paths

• Serial Testing: One test performed after another.

  • Reduces false positives; used to confirm a diagnosis.
  • Net Result: ↑ Specificity (Sp), ↓ Sensitivity (Se).

• Parallel Testing: Two or more tests performed simultaneously.

  • Reduces false negatives; used to screen for a disease.
  • Net Result: ↑ Sensitivity (Se), ↓ Specificity (Sp).

⭐ Use serial testing for confirmatory purposes (high Sp), and parallel testing for screening when a rapid diagnosis is critical (high Se).

Serial Testing - The Confirmer

• A sequential, two-step process: a second, more specific test is performed only after an initial positive test. Used to confirm a diagnosis.
• Primary Effect: Maximizes Specificity (Sp) and Positive Predictive Value (PPV).
  • Reduces the number of false positives.
  • This confirmatory approach leads to a ↓ in overall sensitivity (Sn).
• Net Specificity Calculation:
  • The combined specificity is greater than that of either test alone.
  • $NetSpec = Sp_A + Sp_B - (Sp_A \times Sp_B)$
• 📌 Serial testing for Specificity.

⭐ To be considered positive in a serial pathway, an individual must test positive on all sequential tests. This "rule-in" strategy is crucial for conditions where a false-positive diagnosis could lead to significant harm (e.g., invasive procedures, social stigma).

Parallel Testing - The Screener

• Two or more tests are performed simultaneously. A positive result on any test is considered a positive overall.
• Goal: Maximize the probability of detecting a disease (↑ sensitivity).
• Effect on test characteristics:
  • Net Sensitivity: Increases. The combined sensitivity is higher than any single test.
    • $NetSens = Se_A + Se_B - (Se_A \times Se_B)$
  • Net Specificity: Decreases. The probability of a false positive increases.
    • $NetSp = Sp_A \times Sp_B$
• Use Case: Ideal for screening tests where missing a case is more dangerous than a false positive (e.g., ruling out a disease).
• 📌 Parallel testing Picks up more Positives.

⭐ Useful in situations demanding high sensitivity, such as screening donated blood for multiple infectious agents (e.g., HIV, HCV) simultaneously. A single positive flags the unit.

High‑Yield Points - ⚡ Biggest Takeaways

• Serial testing is a sequential approach (Test A+, then Test B+), which increases specificity but decreases sensitivity.
• Use serial testing to confirm a diagnosis by reducing false positives (SpIN).
• Parallel testing is a simultaneous approach (Test A+ or Test B+), which increases sensitivity but decreases specificity.
• Use parallel testing to screen for diseases, minimizing false negatives (SnOUT), especially in emergencies.