Both frailty and a reduced preoperative lymphocyte-to-monocyte ratio (LMR) are known risk factors for postoperative delirium (POD). However, whether the relationship between LMR and POD varies by frailty status remains unclear. This study aimed to evaluate the frailty-stratified association between preoperative LMR and POD in older surgical patients.

Inclusion Criteria:

• Aged ≥65 years;
• Underwent elective noncardiac, nonneurosurgical procedures;
• Completed the preoperative health and functional status questionnaire.

Exclusion Criteria:

• Severe dementia, language disorders, significant hearing or visual impairments, or coma;
• Cognitive function below the Mini-Mental State Examination thresholds (<18 for illiterate individuals, <21 for individuals with primary education, and <25 for individuals with secondary education or higher);
• Surgery performed under local anesthesia or monitored anesthesia care;
• Missing lymphocyte or monocyte count data;
• Missing postoperative delirium assessment;
• Incomplete preoperative FRAIL scale evaluation;
• Missing data for more than 20% of covariates.

Patients were grouped according to the preoperative lymphocyte-to-monocyte ratio (LMR) and were further stratified into subgroups based on frailty status, which was assessed one day before surgery using the FRAIL scale. Postoperative delirium was assessed using the 3-Minute Diagnostic Interview for Confusion Assessment Method (3D-CAM) for seven consecutive postoperative days or until hospital discharge, whichever occurred first.