Diagnosis
Pulp vitality testing (EPT + cold): vital = periodontal origin; non-vital = endodontic origin. This is the definitive differentiator.
Probing: deep pocket with exudate expressed under pressure. Radiograph: bone loss pattern (generalised in periodontal vs local periapical in endodontic). Acute periodontal abscess may show minimal radiographic change.
Parulis (sinus tract): gutta-percha tracing through sinus tract on radiograph — points to periodontal pocket (lateral root surface) vs periapex, confirming origin.