The Wells criteria is a validated clinical decision rule that stratifies the pre-test probability of PE using seven weighted variables: clinical signs/symptoms of DVT (3 points), PE as likely or more likely than alternative diagnosis (3 points), heart rate > 100 (1.5 points), immobilization > 3 days or surgery within 4 weeks (1.5 points), previous PE/DVT (1.5 points), hemoptysis (1 point), and active cancer (1 point). Using a two-tier model: score ≤ 4 = PE unlikely (proceed to D-dimer), score > 4 = PE likely (proceed directly to CTPA). D-dimer is a fibrin degradation product with high sensitivity (>95%) but low specificity — it reliably excludes PE when negative in low-probability patients but is elevated in many other conditions (infection, surgery, pregnancy, malignancy, aging).