Objective To explore the application value and related factors of serum carbonic anhydrase Ⅲ (CAⅢ) in the clinical diagnosis of mild to moderate Alzheimer disease (AD).

Methods A total of 106 elderly patients initially diagnosed with mild to moderate AD at Huashan Hospital, Fudan University from October 2020 to November 2022 were enrolled as the AD group, and 89 healthy elderly people during the same period were enrolled as the control group. The serum biochemical indicators including liver and kidney function, blood lipids, blood glucose, folic acid and homocysteine were detected in both groups. Cognitive function was assessed by Mini-mental State Examination (MMSE). Patient Health Questionnaire-9 (PHQ-9) and Generalized Anxiety Disorder-7 (GAD-7) were used to assess psychological status. The activities of daily living (ADL) were assessed by modified Barthel Index (BI). Serum CAⅢ levels were measured by enzyme-linked immunosorbent assay (ELISA). Correlation analysis and multivariate linear regression analysis were used to identify factors influencing serum CAⅢ levels, and receiver operating characteristic (ROC) curve analysis was performed to evaluate the diagnostic value of serum CAⅢ levels in elderly patients with mild to moderate AD.

Results The MMSE score of the AD group was significantly lower than that of the control group (P＜0.001), and the PHQ-9 and GAD-7 scores were significantly higher than those of the control group (P＜0.001). The serum CAⅢ level in the AD group was significantly lower than that in the control group (P＜0.000 1). In patients with AD, serum CAⅢ levels in patients with a disease course ＞3 years, accompanied by depression or anxiety, moderate AD, and serum creatinine≤111 μmol/L were significantly lower than those in patients with a disease course ≤3 years, normal emotions, mild AD, and serum creatinine ＞111 μmol/L (P＜0.05). Correlation analysis and multivariate linear regression analysis showed that serum CAⅢ levels were negatively correlated with disease duration, PHQ-9 score, GAD-7 scores and severity degree, positively correlated with serum creatinine level (P＜0.05). The PHQ-9 score, severity degree, and serum creatinine level were independent related factors for serum CAⅢ level in mild to moderate AD elderly patients (P＜0.05). ROC curve result showed that the area under the curve (AUC) of serum CAⅢ in diagnosing mild to moderate AD in elderly patients was 0.946, with sensitivity and specificity of 88.79% and 96.74%, respectively.

Conclusions Serum CAⅢ levels in elderly patients with mild to moderate AD are higher than those in healthy individuals. Mild AD, without depressive mood, and elevated serum creatinine levels are related factors for elevated serum CAⅢ levels in elderly AD patients. Serum CAⅢ may serve as a novel biological marker for the diagnosis of mild to moderate AD in the elderly.