Clinical Research Directory

Inclusion Criteria: * Aged 18-65 years. At least 20 natural teeth. For the periodontal health groups: periodontal health on an intact periodontium, with probing depths ≤3 mm, full-mouth bleeding on probing \<10%, and no interdental clinical attachment loss or radiographic bone loss attributable to periodontitis. For the periodontitis groups: Stage III or Stage IV periodontitis according to the 2017 periodontal disease classification. For the smoking groups: current cigarette smoking of at least 10 cigarettes per day for at least 5 years. For the nonsmoking groups: fewer than 100 cigarettes smoked during the lifetime and no current use of tobacco or nicotine products. Provision of written informed consent. Exclusion Criteria: * ormer cigarette smoking. Use of electronic cigarettes, heated tobacco products, nicotine pouches, or nicotine replacement products. Uncontrolled diabetes, immunodeficiency, autoimmune or chronic inflammatory disease, or a serious systemic disease that may affect periodontal or mitochondrial responses. Pregnancy or breastfeeding. Use of antibiotics, systemic corticosteroids, immunosuppressive medications, or regular anti-inflammatory medications within the previous 3 months. Periodontal treatment within the previous 6 months. Acute oral infection, periodontal abscess, or local pathology affecting the sampling sites. Gingival crevicular fluid samples contaminated with blood or saliva, or insufficient for ELISA analysis.