**3. Diagnosis of COPD**

Patients with dyspnea, chronic cough and/or expectoration, a history of recurrent lower respiratory tract infections, and/or a history of exposure to risk factors are considered as COPD. Pulmonary function tests are necessary to confirm the diagnosis of COPD, such as forced expiratory volume in 1 second/forced vital capacity, FEV1/ FVC < 0.70 after inhalation of bronchodilator, which can confirm the presence of persistent airflow limitation. Lung function assessed by forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and the ratio of FEV1 to FVC (FEV1/ FVC) reflect the physiological state of the lung, and these indices can be used to diagnose and monitor COPD. The goal of COPD assessment is to determine the degree of airflow limitation, the impact of the disease on the patient's health, and the risk of long-term adverse outcomes (such as AECOPD, hospitalization, or death) to guide treatment. Spirometer is an important examination instrument for the diagnosis of COPD. Clinically, it is necessary to find indicators that can predict the occurrence and development of airflow limitation and comprehensively evaluate the respiratory physiology of COPD. Alveolar diffusion is the process of gas molecules exchange through the alveolar membrane (alveolar-capillary membrane). DLco was measured by single breath method to reflect the pulmonary diffusion function. Respiratory physiological indicators other than portable pulmonary function instruments can be supplemented to better assess COPD.

Recent studies have suggested that COPD may be caused by a decreased peak and/ or an accelerated decline in lung function in early adulthood. COPD can start early in life and take a long time to manifest itself clinically, so identifying "early" COPD is difficult. In addition, the biological "early" associated with the initial mechanisms that ultimately lead to COPD should be distinguished from the clinical "early," which reflects the initial perception of symptoms, functional limitations, and/or noted structural abnormalities. Pulmonary function tests are poorly correlated with clinical characteristics and lack sufficient sensitivity for early diagnosis. Meanwhile, due to the heterogeneity and phenotypic complexity of COPD, pulmonary function measurements provide limited information on prognosis, predictive outcome, and treatment strategy, which are not sufficient for accurate diagnosis, treatment, and efficacy evaluation. Patients with COPD often suffer from cardiovascular disease, skeletal muscle dysfunction, metabolic syndrome, osteoporosis, depression, anxiety, lung cancer, and other diseases. In view of the fact that these complications are independent risk factors for hospitalization and death, we should actively look for complications and give correct treatment to patients with COPD. The development of high-throughput technologies such as genomics, proteomics, and metabolomics has provided effective tools for elucidating global changes in complex inflammatory diseases such as COPD. Among them, COPD "omics" research mainly focuses on DNA (genetic) and RNA (transcriptome) markers. The advent of mass spectrometry (MS), including gas chromatography/MS and liquid chromatography/MS, has made proteomics and metabolomics more feasible for large-scale population studies. The

multi-omics integration study by similarity network fusion significantly improved the accuracy of grouping COPD patients from healthy nonsmokers to smokers with normal lung capacity, indicating that multi-omics integration data can improve the accuracy of COPD diagnosis and help promote the understanding of its pathogenesis.
