Appropriate test sequencing

Initial Workup - First Things First

• Principle: Start with the simplest, cheapest, and least invasive tests that offer the highest yield. Prioritize ruling out life-threatening conditions.
• Sequence: Follow a logical progression from broad screening to specific confirmation.

• Categories:
  • Screening: Detects potential disease (e.g., Pap smear).
  • Diagnostic: Confirms or rules out disease (e.g., Biopsy).
  • Baseline: Establishes initial values before treatment (e.g., CBC, LFT, RFT).

⭐ Always order baseline investigations before initiating therapies with potential side effects (e.g., chemotherapy, anticoagulants) to monitor for toxicity.

Diagnostic Strategy - The Funnel Approach

Start broad, then narrow down. This cost-effective approach minimizes invasive testing by moving from high-sensitivity to high-specificity tests, ensuring a logical and efficient diagnostic workflow.

• Screening Tests (High Sensitivity):
  • Identifies potential candidates; cheap and widely available.
  • 📌 SNOUT: Sensitive test, when Negative, rules OUT.
• Confirmatory Tests (High Specificity):
  • Confirms the diagnosis in those who screened positive.
  • 📌 SPIN: Specific test, when Positive, rules IN.

⭐ Likelihood Ratios (LR): A positive LR >10 provides strong evidence to rule IN a disease. A negative LR <0.1 provides strong evidence to rule it OUT.

Invasiveness & Cost - The Sensible Ladder

• Principle: Always start with the least invasive & most cost-effective tests.
• Ascend the ladder only when a diagnosis isn't clear from the lower rungs.

⭐ In emergencies or with strong "red flag" signs (e.g., acute head trauma), it is appropriate to jump directly to a higher-level test like a CT scan.

Contextual Sequencing - When Rules Bend

Sometimes, the clinical context demands bypassing the standard "low-cost, non-invasive first" sequence. This is crucial in time-sensitive scenarios.

• High Pre-test Probability (PTP): When clinical suspicion is very high, initial screening tests are skipped to avoid delays.
  • Example: Classic STEMI presentation → bypass serial troponins → direct to cardiac catheterization.
• Hemodynamic Instability: In critical patients, the most rapid diagnostic test (even if invasive) is chosen.
  • Example: Unstable trauma patient → eFAST scan instead of a slower, comprehensive CT scan.
• Combined Diagnostic & Therapeutic Utility: A single, more invasive test is preferred if it can both diagnose and treat.
  • Example: Choledocholithiasis → ERCP.

⭐ In a patient with high clinical probability of Pulmonary Embolism (e.g., high Wells score), guidelines recommend proceeding directly to CT Pulmonary Angiography (CTPA), bypassing the D-dimer test.

High‑Yield Points - ⚡ Biggest Takeaways

• Always start with high-sensitivity screening tests, followed by high-specificity confirmatory tests.
• Prioritize non-invasive or minimally invasive procedures over invasive ones to reduce patient risk.
• Sequence investigations from cheaper and simpler to more expensive and complex.
• First, rule out the most probable and most life-threatening diagnoses.
• The pre-test probability of a disease should guide your initial test choice.
• Each test result must meaningfully inform the next diagnostic step.