This is a cross-sectional study to investigate ATP level in induced sputum and cough sensitivity to ATP in subjects with refractory/unexplained chronic cough. 60 patients with refractory/unexplained chronic cough and 30 matched healthy controls were recruited. Demographic data, clinical characteristic, cough sacles and medical history records were collected. Cough chanllage(ATP and capsaicin) and ATP measurement in sputum were conducted.The difference of sputum ATP level between patients and control and the correlation between sputum ATP level and cough sensitivity to ATP were analyzed.

Inclusion criteria for patients:

1. ≥18 years old
2. cough as sole symptom lasting ≥6 months
3. cough visual analogue score ≥30mm
4. diagnosed as RCC/UCC according to guidelines

Exclusion criteria for patients:

1. smoking currently or in the past 6 months, or a smoking history of >20 pack-year
2. a forced expiratory volume in 1 second/forced vital capacity ratio <70%;
3. use of inhaled/oral corticosteroid, bronchodilators, anti-allergic medicine, antitussive medicine or anti-reflux treatment in the past 1 week;
4. a history of acute respiratory infection in the past 4 weeks;
5. a history of severe disease (cancer, acute coronary syndrome and so on).

Healthy control aged ≥18 years old are included. Exclusion criteria are as follow:

1. smoking currently or in the past 6 months, or a smoking history of >20 pack-year
2. a forced expiratory volume in 1 second/forced vital capacity ratio <70%
3. a history of acute respiratory infection in the past 4 weeks
4. posstive findings of chest radiography
5. a history of chronic respiratory disease and allergic disease
6. any signs of cough hypersensitivity

Subjects with chronic cough visiting cough clinics at the First Affiliated Hospital of Guangzhou Medical University would all be screened by the investigator. To be specific, RCC/UCC refers to a chronic cough: 1) without identifiable causes after throughout investigation; 2) persistent after empirical treatment to known causes; 3) persistent after treatment for positive conditions known to be associated with chronic cough. Age and gender matched healthy controls would be recruited.

The subject who meets the criteria and provides written informed consent is considered as eligible and enrolled into the study.

This study mainly involves induced sputum test and cough challenge without any interventional design. Induced sputum test and cough challenge are received as safe and widely used in clinics or researches. Therefore, observation of adverse event has not been designed.

All-Cause mortality, serious, and other [Not Including Serious] adverse events were not monitored, so total number of participants at risk for adverse events is zero.